LTC2413
24-Bit No Latency ∆Σ™ ADC,
with Simultaneous 50Hz/60Hz Rejection
Features
Description
Simultaneous 50Hz/60Hz Rejection
(87dB Minimum)
n Differential Input and Differential Reference with
GND to VCC Common Mode Range
n 2ppm INL and No Missing Codes at 24 Bits
n 0.1ppm Offset and 2.5ppm Full-Scale Error
n 0.16ppm Noise
n No Latency: Digital Filter Settles in a Single Cycle
n Internal Oscillator—No External Components
Required
n 24-Bit ADC in Narrow SSOP-16 Package
(SO-8 Footprint)
n Single Supply 2.7V to 5.5V Operation
n Low Supply Current (200µA) and Auto Shutdown
n Pin Compatible with LTC2410
The LTC®2413 is a 2.7V to 5.5V simultaneous 50Hz/60Hz
rejection micropower 24-bit differential ∆Σ analog to
digital converter with an integrated oscillator, 2ppm INL
and 0.16ppm RMS noise. It uses delta-sigma technology
and provides single cycle settling time for multiplexed
applications. Through a single pin, the LTC2413 can be
configured for better than 87dB input differential mode
rejection over the range of 49Hz to 61.2Hz (50Hz and
60Hz ± 2% simultaneously), or it can be driven by an
external oscillator for a user defined rejection frequency.
The internal oscillator requires no external frequency
setting components.
n
Applications
Direct Sensor Digitizer
Weight Scales
n Direct Temperature Measurement
n Gas Analyzers
n Strain-Gauge Transducers
n Instrumentation
n Data Acquisition
n Industrial Process Control
n 6-Digit DVMs
n Products for International Markets
n
n
The converter accepts any external differential reference
voltage from 0.1V to VCC for flexible ratiometric and remote sensing measurement configurations. The full-scale
differential input range is from –0.5VREF to 0.5VREF. The
reference common mode voltage, VREFCM, and the input
common mode voltage, VINCM, may be independently set
anywhere within the GND to VCC range of the LTC2413. The
DC common mode input rejection is better than 140dB.
The LTC2413 communicates through a flexible 3-wire
digital interface which is compatible with SPI and
MICROWIRE protocols.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and No
Latency ∆∑ is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
Typical Application
Measured Noise Rejection from 48Hz to 62.5Hz
–80
2.7V TO 5.5V
MEASURED DATA
CALCULATED DATA
2
VCC
FO
14
LTC2413
REFERENCE
VOLTAGE
0.1V TO VCC
3
4
REF –
ANALOG INPUT RANGE
–0.5VREF TO 0.5VREF
5
+
SDO
IN –
CS
6
1, 7, 8, 9, 10, 15, 16
REF
IN
+
GND
SCK
= EXTERNAL CLOCK SOURCE
= INTERNAL OSC/
SIMULTANEOUS 50Hz/60Hz REJECTION
13
12
3-WIRE
SPI INTERFACE
11
NORMAL MODE REECTION (dB)
–85
1µF
VCC = 5V
REF + = 5V
REF – = GND
VINCM = 2.5V
VIN(P-P) = 5V
TA = 25°C
–90
–95
–100
–105
–110
–115
2413 TA01
–120
48
50
52
54
56
58
60
INPUT FREQUENCY (Hz)
62
2413 TA02
2413fa
For more information www.linear.com/LTC2413
1
LTC2413
Absolute Maximum Ratings
Pin Configuration
(Notes 1, 2)
TOP VIEW
Supply Voltage (VCC) to GND........................– 0.3V to 7V
Analog Input Pins Voltage
to GND.......................................–0.3V to (VCC + 0.3V)
Reference Input Pins Voltage
to GND.......................................–0.3V to (VCC + 0.3V)
Digital Input Voltage to GND..........–0.3V to (VCC + 0.3V)
Digital Output Voltage to GND........–0.3V to (VCC + 0.3V)
Operating Temperature Range
LTC2413C ................................................. 0°C to 70°C
LTC2413I ..............................................–40°C to 85°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................... 300°C
GND
1
16 GND
VCC
2
15 GND
REF +
3
14 FO
REF –
4
13 SCK
IN +
5
12 SDO
IN –
6
11 CS
GND
7
10 GND
GND
8
9
GND
GN PACKAGE
16-LEAD PLASTIC SSOP
TJMAX = 125°C, θJA = 95°C/W
Order Information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC2413CGN#PBF
LTC2413CGN#TRPBF
2413
16-Lead Plastic SSOP
0°C to 70°C
LTC2413IGN#PBF
LTC2413IGN#TRPBF
2413I
16-Lead Plastic SSOP
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on nonstandard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
2
2413fa
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LTC2413
Electrical Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4)
PARAMETER
CONDITIONS
MIN
Resolution (No Missing Codes) 0.1V ≤ VREF ≤ VCC, –0.5 • VREF ≤ VIN ≤ 0.5 • VREF, (Note 5)
Integral Nonlinearity
4.5V ≤ VCC ≤ 5.5V, REF+ = 2.5V, REF– = GND, VINCM = 1.25V, (Note 6)
5V ≤ VCC ≤ 5.5V, REF+ = 5V, REF– = GND, VINCM = 2.5V, (Note 6)
REF+ = 2.5V, REF– = GND, VINCM = 1.25V, (Note 6)
Offset Error
2.5V ≤ REF+ ≤ VCC, REF– = GND,
GND ≤ IN+ = IN– ≤ VCC, (Note 13)
Offset Error Drift
2.5V ≤ REF+ ≤ VCC, REF– = GND,
GND ≤ IN+ = IN– ≤ VCC
Positive Full-Scale Error
2.5V ≤ REF+ ≤ VCC, REF– = GND,
IN+ = 0.75 • REF+, IN– = 0.25 • REF+
Positive Full-Scale Error Drift
2.5V ≤ REF+ ≤ VCC, REF– = GND,
IN+ = 0.75 • REF+, IN– = 0.25 • REF+
Negative Full-Scale Error
2.5V ≤ REF+ ≤ VCC, REF– = GND,
IN+ = 0.25 • REF+, IN– = 0.75 • REF+
Negative Full-Scale Error Drift
2.5V ≤ REF+ ≤ VCC, REF– = GND,
IN+ = 0.25 • REF+, IN– = 0.75 • REF+
Total Unadjusted Error
4.5V ≤ VCC ≤ 5.5V, REF+ = 2.5V, REF– = GND, VINCM = 1.25V
5V ≤ VCC ≤ 5.5V, REF+ = 5V, REF– = GND, VINCM = 2.5V
REF+ = 2.5V, REF– = GND, VINCM = 1.25V
Output Noise
5V ≤ VCC ≤ 5.5V, REF+ = 5V, VREF– = GND,
GND ≤ IN– = IN+ ≤ 5V, (Note 12)
l
TYP
MAX
24
UNITS
Bits
l
1
2
5
14
ppm of VREF
ppm of VREF
ppm of VREF
l
0.5
2.5
µV
10
l
2.5
nV/°C
12
0.03
l
2.5
0.03
3
3
4
0.8
ppm of VREF
ppm of VREF/°C
12
ppm of VREF
ppm of VREF/°C
ppm of VREF
ppm of VREF
ppm of VREF
µVRMS
2413fa
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3
LTC2413
converter Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4)
PARAMETER
CONDITIONS
Input Common Mode Rejection DC
2.5V ≤ REF+ ≤ VCC, REF– = GND,
GND ≤ IN– = IN+ ≤ VCC
2.5V ≤ REF+ ≤ VCC, REF– = GND,
GND ≤ IN– = IN+ ≤ VCC, (Note 7)
Input Common Mode Rejection
49Hz to 61.2Hz
MIN
TYP
l
MAX
UNITS
130
140
l
140
dB
dB
Input Normal Mode Rejection
49Hz to 61.2Hz
(Note 7)
l
87
dB
Input Normal Mode Rejection
External Clock fEOSC/2560 ±14%
External Oscillator
l
87
dB
Input Normal Mode Rejection
External Clock fEOSC/2560 ±4%
External Oscillator
l
110
140
dB
Reference Common Mode
Rejection DC
2.5V ≤ REF+ ≤ VCC, GND ≤ REF– ≤ 2.5V,
VREF = 2.5V, IN– = IN+ = GND
l
130
140
dB
Power Supply Rejection, DC
REF+ = 2.5V, REF– = GND, IN– = IN+ = GND
120
dB
Power Supply Rejection
Simultaneous 50Hz/60Hz ±2%
REF+ = 2.5V, REF– = GND,
IN– = IN+ = GND, (Note 7)
120
dB
analog input and reference
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
IN+
Absolute/Common Mode IN+ Voltage
l
GND – 0.3V
VCC + 0.3V
V
IN–
Absolute/Common Mode IN– Voltage
l
GND – 0.3V
VCC + 0.3V
V
VIN
Input Differential Voltage Range
(IN+ – IN–)
l
–VREF/2
–VREF/2
V
REF+
Absolute/Common Mode REF+ Voltage
l
0.1
VCC
V
REF–
Absolute/Common Mode REF– Voltage
l
GND
VCC – 0.1V
V
VREF
Reference Differential Voltage Range
(REF+ – REF–)
l
0.1
VCC
V
CS (IN+)
IN+ Sampling Capacitance
18
pF
CS
(IN–)
IN– Sampling Capacitance
18
pF
CS
(REF+)
REF+ Sampling Capacitance
18
pF
CS (REF–)
REF– Sampling Capacitance
18
pF
IDC_LEAK
(IN+)
IN+ DC Leakage Current
IDC_LEAK
(IN–)
IN– DC Leakage Current
IDC_LEAK (REF+)
REF+ DC Leakage Current
(REF–)
REF– DC Leakage Current
IDC_LEAK
4
CONDITIONS
CS = VCC, IN+ = GND
CS = VCC, IN– = GND
CS = VCC, REF+ = 5V
CS = VCC, REF– = GND
MIN
TYP
MAX
UNITS
l
–10
1
10
nA
l
–10
1
10
nA
l
–10
1
10
nA
l
–10
1
10
nA
2413fa
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LTC2413
digital inputs and outputs
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
CONDITIONS
VIH
High Level Input Voltage
CS, FO
2.7V ≤ VCC ≤ 5.5V
2.7V ≤ VCC ≤ 3.3V
l
VIL
Low Level Input Voltage
CS, FO
4.5V ≤ VCC ≤ 5.5V
2.7V ≤ VCC ≤ 5.5V
l
VIH
High Level Input Voltage
SCK
2.7V ≤ VCC ≤ 5.5V (Note 8)
2.7V ≤ VCC ≤ 3.3V (Note 8)
l
VIL
Low Level Input Voltage
SCK
4.5V ≤ VCC ≤ 5.5V (Note 8)
2.7V ≤ VCC ≤ 5.5V (Note 8)
l
IIN
Digital Input Current
CS, FO
0V ≤ VIN ≤ VCC
l
Digital Input Current
SCK
0V ≤ VIN ≤ VCC (Note 8)
l
CIN
MIN
(Note 8)
VOH
High Level Output Voltage
SDO
IO = –800µA
l
VOL
Low Level Output Voltage
SDO
IO = 1.6mA
l
VOH
High Level Output Voltage
SCK
IO = –800µA (Note 9)
l
VOL
Low Level Output Voltage
SCK
IO = 1.6mA (Note 9)
l
IOZ
Hi-Z Output Leakage
SDO
l
MAX
UNITS
2.5
2.0
V
V
0.8
0.6
V
V
2.5
2.0
V
V
0.8
0.6
V
V
–10
10
µA
–10
10
µA
Digital Input Capacitance
CS, FO
Digital Input Capacitance
SCK
TYP
10
pF
10
pF
VCC – 0.5V
V
0.4
V
VCC – 0.5V
V
–10
0.4
V
10
µA
power requirements
The l denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
VCC
Supply Voltage
ICC
Supply Current
Conversion Mode
Sleep Mode
CONDITIONS
MIN
l
CS = 0V (Note 11)
CS = VCC (Note 11)
l
l
TYP
2.7
200
20
MAX
UNITS
5.5
V
300
30
µA
µA
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5
LTC2413
timing characteristics
The l denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
fEOSC
tHEO
CONDITIONS
MIN
External Oscillator Frequency Range
l
External Oscillator High Period
l
tLEO
External Oscillator Low Period
l
tCONV
Conversion Time
FO = 0V
External Oscillator (Note 10)
fISCK
Internal SCK Frequency
Internal Oscillator (Note 9)
External Oscillator (Notes 9, 10)
DISCK
Internal SCK Duty Cycle
(Note 9)
l
TYP
MAX
UNITS
2.56
500
kHz
0.25
390
µs
390
µs
146.71
20510/fEOSC (in kHz)
l
l
ms
ms
17.5
fEOSC/8
45
kHz
kHz
55
%
2000
kHz
fESCK
External SCK Frequency Range
(Note 8)
l
tLESCK
External SCK Low Period
(Note 8)
l
tHESCK
External SCK High Period
(Note 8)
l
250
tDOUT_ISCK
Internal SCK 32-Bit Data Output Time
Internal Oscillator (Notes 9, 11)
External Oscillator (Notes 9, 10)
l
l
1.80
tDOUT_ESCK
External SCK 32-Bit Data Output Time
(Note 8)
l
t1
CS ↓ to SDO Low Z
l
0
200
ns
t2
CS ↑ to SDO Hi-Z
l
t3
CS ↓ to SCK ↓
t4
CS ↓ to SCK ↑
tKQMAX
SCK ↓ to SDO Valid
tKQMIN
SDO Hold After SCK ↓
l
15
ns
t5
SCK Set-Up Before CS ↓
l
50
ns
t6
SCK Hold After CS ↓
l
ns
ns
1.83
1.86
20510/fEOSC (in kHz)
32/fESCK (in kHz)
ms
ms
ms
0
200
ns
(Note 9)
l
0
200
ns
(Note 8)
l
50
(Note 5)
ns
220
l
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: All voltage values are with respect to GND.
Note 3: VCC = 2.7V to 5.5V unless otherwise specified.
VREF = REF + – REF–, VREFCM = (REF+ + REF–)/2;
VIN = IN+ – IN –, VINCM = (IN + + IN –)/2.
Note 4: FO pin tied to GND or to external conversion clock source with
fEOSC = 139800Hz unless otherwise specified.
Note 5: Guaranteed by design, not subject to test.
Note 6: Integral nonlinearity is defined as the deviation of a code from a
straight line passing through the actual endpoints of the transfer curve.
The deviation is measured from the center of the quantization band.
6
250
50
ns
ns
Note 7: FO = 0V (internal oscillator) or fEOSC = 139800Hz ± 2%
(external oscillator).
Note 8: The converter is in external SCK mode of operation such that the
SCK pin is used as digital input. The frequency of the clock signal driving
SCK during the data output is fESCK and is expressed in kHz.
Note 9: The converter is in internal SCK mode of operation such that the
SCK pin is used as digital output.
Note 10: The external oscillator is connected to the FO pin. The external
oscillator frequency, fEOSC, is expressed in kHz.
Note 11: The converter uses the internal oscillator. FO = 0V.
Note 12: The output noise includes the contribution of the internal
calibration operations.
Note 13: Guaranteed by design and test correlation.
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LTC2413
Typical Performance Characteristics
Total Unadjusted Error vs
Temperature (VCC = 5V,
VREF = 5V)
Total Unadjusted Error vs
Temperature (VCC = 5V,
VREF = 2.5V)
1.5
1.0
TUE (ppm OF VREF)
0.5
0
–1.0
VCC = 5V
REF + = 5V
REF – = GND
VREF = 5V
VINCM = 2.5V
FO = GND
TA = 90°C
0
TA = –45°C
TA = 90°C
TA = 25°C
–0.5
TA = 25°C
TA = –45°C
1
1.5
2
2.5
–1.5
–1
–0.5
0
VIN (V)
0.5
INL ERROR (ppm OF VREF)
INL ERROR (ppm OF VREF)
TA = 90°C
1
1.5
2
2.5
TA = 25°C
TA = 90°C
0
–1.0
–1.5
–2.5 –2 –1.5 –1 –0.5 0 0.5
VIN (V)
TA = –45°C
0.5
–0.5
–1.0
–1.5
VCC = 5V
REF + = 2.5V
REF – = GND
VREF = 2.5V
VINCM = 1.25V
FO = GND
–1
–0.5
4
0
VIN (V)
0.5
12
10
8
6
4
10,000 CONSECUTIVE
READINGS
VCC = 5V
VREF = 5V
VIN = 0V
REF + = 5V
REF – = GND
IN + = 2.5V
IN – = 2.5V
FO = 460800Hz
TA = 25°C
0.8
2413 G07
1
4
2
0
–2
TA = 90°C
–4
TA = 25°C
–6
TA = –45°C
–10
1
–1
–0.5
0
VIN (V)
0.5
1
2413 G06
Noise Histogram (Output Rate =
6.83Hz VCC = 5V, VREF = 2.5V)
GAUSSIAN
DISTRIBUTION
m = 0.067ppm
σ = 0.151ppm
2
2
0
–0.8 –0.6 –0.4 –0.2 0 0.2 0.4 0.6
OUTPUT CODE (ppm OF VREF)
6
Noise Histogram (Output Rate =
22.5Hz, VCC = 5V, VREF = 5V)
NUMBER OF READINGS (%)
NUMBER OF READINGS (%)
6
0.5
2413 G05
Noise Histogram (Output Rate =
6.83Hz, VCC = 5V, VREF = 5V)
GAUSSIAN
DISTRIBUTION
m = 0.105ppm
σ = 0.153ppm
0
VIN (V)
VCC = 2.7V VREF = 2.5V
REF + = 2.5V VINCM = 1.25V
REF – = GND FO = GND
–8
2413 G04
10,000 CONSECUTIVE
READINGS
VCC = 5V
VREF = 5V
VIN = 0V
REF + = 5V
REF – = GND
IN + = 2.5V
IN – = 2.5V
FO = GND
TA = 25°C
–0.5
Integral Nonlinearity vs
Temperature (VCC = 2.7V,
VREF = 2.5V)
10
1.0
TA = 25°C
–1
TA = –45°C
2413 G03
8
–0.5
8
VCC = 2.7V
REF + = 2.5V
REF – = GND
VREF = 2.5V
VINCM = 1.25V
FO = GND
–4
–10
1
1.5
TA = –45°C
0
10
–2
Integral Nonlinearity vs
Temperature (VCC = 5V,
VREF = 2.5V)
1.5
12
TA = 25°C
0
2413 G02
Integral Nonlinearity vs
Temperature (VCC = 5V,
VREF = 5V)
0.5
TA = 90°C
2
–6
–1.0
2413 G01
VCC = 5V
REF + = 5V
REF – = GND
VREF = 5V
VINCM = 2.5V
FO = GND
4
–8
–1.5
–2.5 –2 –1.5 –1 –0.5 0 0.5
VIN (V)
1.0
6
INL ERROR (ppm OF VREF)
–0.5
0.5
8
12
NUMBER OF READINGS (%)
TUE (ppm OF VREF)
1.0
10
VCC = 5V
REF + = 2.5V
REF – = GND
VREF = 2.5V
VINCM = 1.25V
FO = GND
TUE (ppm OF VREF)
1.5
Total Unadjusted Error vs
Temperature (VCC = 2.7V,
VREF = 2.5V)
10
8
6
4
10,000 CONSECUTIVE
READINGS
VCC = 5V
VREF = 2.5V
VIN = 0V
REF + = 2.5V
REF – = GND
IN + = 1.25V
IN – = 1.25V
FO = GND
TA = 25°C
GAUSSIAN
DISTRIBUTION
m = 0.033ppm
σ = 0.293ppm
2
0
–0.8 –0.6 –0.4 –0.2 0 0.2 0.4 0.6
OUTPUT CODE (ppm OF VREF)
0.8
2413 G08
0
–1.6
–0.8
0
0.8
OUTPUT CODE (ppm OF VREF)
1.6
2413 G09
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7
LTC2413
Typical Performance Characteristics
6
4
12
2
10,000 CONSECUTIVE
READINGS
VCC = 2.7V
VREF = 2.5V
VIN = 0V
REF + = 2.5V
REF – = GND
IN + = 1.25V
IN – = 1.25V
FO = GND
TA = 25°C
10
8
6
4
GAUSSIAN
DISTRIBUTION
m = 0.079ppm
σ = 0.298ppm
2
0
–1.6 –1.2 –0.8 –0.4 0 0.4 0.8 1.2
OUTPUT CODE (ppm OF VREF)
2413 G10
8
6
4
2
ADC CONSECUTIVE
READINGS
VCC = 5V
VREF = 5V
VIN = 0V
REF + = 5V
REF – = GND
IN + = 2.5V
IN – = 2.5V
FO = GND
TA = 25°C
0.6
0.4
0.2
0
–0.2
–0.4
IN + = 2.5V
VCC = 5V TA = 25°C
VREF = 5V REF + = 5V IN – = 2.5V
VIN = 0V REF – = GND
FO = GND
–0.6
–1.0
0
0.3
0.2
0.1
0
–2.5 –2 –1.5 –1 –0.5 0 0.5 1 1.5 2
INPUT DIFFERENTIAL VOLTAGE (V)
5 10 15 20 25 30 35 40 45 50 55 60
TIME (HOURS)
800
800
VCC = 5V
REF + = 5V
REF – = GND
VREF = 5V
IN + = VINCM
IN – = VINCM
VIN = 0V
FO = GND
TA = 25°C
650
–0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5
VINCM (V)
2413 G16
775
RMS Noise vs VCC
850
VCC = 5V
REF + = 5V
REF – = GND
IN + = 2.5V
IN – = 2.5V
VIN = 0V
FO = GND
800
725
775
750
725
700
700
675
675
–25
0
25
50
TEMPERATURE (°C)
REF + = 2.5V
REF – = GND
VREF = 2.5V
IN + = GND
IN – = GND
FO = GND
TA = 25°C
825
750
650
–50
75
2.5
2413 G15
RMS NOISE (nV)
825
RMS NOISE (nV)
RMS NOISE (nV)
825
8
0.4
RMS Noise vs Temperature (TA)
850
675
VCC = 5V
VREF = 5V
REF + = 5V
REF – = GND
VINCM = 2.5V
FO = GND
TA = 25°C
2413 G14
RMS Noise vs VINCM
700
RMS Noise vs Input Differential
Voltage
0.5
–0.8
0.8
1.6
2413 G12
0.8
850
725
4
0
–1.6 –1.2 –0.8 –0.4 0 0.4 0.8 1.2
OUTPUT CODE (ppm OF VREF)
1.6
1.0
GAUSSIAN DISTRIBUTION
m = 0.101837ppm
σ = 0.154515ppm
2413 G13
750
6
Consecutive ADC Readings vs
Time
0
–0.8 –0.6 –0.4 –0.2 0 0.2 0.4 0.6
OUTPUT CODE (ppm OF VREF)
775
8
GAUSSIAN
DISTRIBUTION
m = 0.177ppm
σ = 0.297ppm
2413 G11
ADC READING (ppm OF VREF)
NUMBER OF READINGS (%)
10
10
10,000 CONSECUTIVE
READINGS
VCC = 2.7V
VREF = 2.5V
VIN = 0V
REF + = 2.5V
REF – = GND
IN + = 1.25V
IN – = 1.25V
FO = 460800Hz
TA = 25°C
2
0
–1.6 –1.2 –0.8 –0.4 0 0.4 0.8 1.2
OUTPUT CODE (ppm OF VREF)
1.6
Long-Term Noise Histogram
(Time = 60 Hrs, VCC = 5V,
VREF = 5V)
12
12
NUMBER OF READINGS (%)
8
GAUSSIAN
DISTRIBUTION
m = 0.014ppm
σ = 0.292ppm
RMS NOISE (ppm OF VREF)
NUMBER OF READINGS (%)
10
10,000 CONSECUTIVE
READINGS
VCC = 5V
VREF = 2.5V
VIN = 0V
REF + = 2.5V
REF – = GND
IN + = 1.25V
IN – = 1.25V
FO = 460800Hz
TA = 25°C
NUMBER OF READINGS (%)
12
Noise Histogram (Output Rate =
22.5Hz, VCC = 2.7V, VREF = 2.5V)
Noise Histogram (Output Rate =
6.83Hz VCC = 2.7V, VREF = 2.5V)
Noise Histogram (Output Rate =
22.5Hz, VCC = 5V, VREF = 2.5V)
100
2413 G17
650
2.7
3.1
3.5
3.9 4.3
VCC (V)
4.7
5.1
5.5
2413 G18
2413fa
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LTC2413
Typical Performance Characteristics
RMS Noise vs VREF
Offset Error vs VINCM
775
750
725
700
675
650
0
0.5
1
1.5
2 2.5 3
VREF (V)
3.5
4
4.5
0.3
0.2
0.2
0.1
VCC = 5V
REF + = 5V
REF – = GND
VREF = 5V
IN + = VINCM
IN – = VINCM
VIN = 0V
FO = GND
TA = 25°C
0
–0.1
–0.2
–0.3
–0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5
VINCM (V)
5
0.3
0.2
0.2
REF + = 2.5V
REF – = GND
VREF = 2.5V
IN + = GND
IN – = GND
FO = GND
TA = 25°C
–0.2
–0.3
2.7
3.1
3.5
3.9 4.3
VCC (V)
4.7
5.1
5.5
0
VCC = 5V
REF – = GND
IN + = GND
IN – = GND
FO = GND
TA = 25°C
–0.1
–0.2
–0.3
0
0.5
1
1.5
2 2.5 3
VREF (V)
3.5
4
4.5
+FULL-SCALE ERROR (ppm OF VREF)
REF + = 2.5V
REF – = GND
VREF = 2.5V
IN + = 1.25V
IN – = GND
FO = GND
TA = 25°C
–1
–2
2.7
3.1
3.5
3.9 4.3
VCC (V)
4.7
5.1
5.5
0
–1
–2
2413 G25
VCC = 5V
REF + = 5V
REF – = GND
IN + = 2.5V
IN – = GND
FO = GND
0 15 30 45 60
TEMPERATURE (°C)
75
90
2413 G24
– Full-Scale Error vs
Temperature (TA)
3
2
1
0
VCC = 5V
REF + = VREF
REF – = GND
IN + = 0.5 • REF +
IN – = GND
FO = GND
TA = 25°C
–1
–2
–3
100
1
–3
–45 –30 –15
5
3
0
75
2
2413 G23
3
1
0
25
50
TEMPERATURE (°C)
2413 G21
+ Full-Scale Error vs VREF
2
–25
3
0.1
+ Full-Scale Error vs VCC
+FULL-SCALE ERROR (ppm OF VREF)
–0.3
–50
+ Full-Scale Error vs
Temperature (TA)
2413 G22
–3
–0.2
+FULL-SCALE ERROR (ppm OF VREF)
0.3
0.1
VCC = 5V
REF + = 5V
REF – = GND
IN + = 2.5V
IN – = 2.5V
VIN = 0V
FO = GND
–0.1
Offset Error vs VREF
OFFSET ERROR (ppm OF VREF)
OFFSET ERROR (ppm OF VREF)
Offset Error vs VCC
–0.1
0
2413 G20
2413 G19
0
0.1
–FULL-SCALE ERROR (ppm OF VREF)
RMS NOISE (nV)
800
OFFSET ERROR (ppm OF VREF)
VCC = 5V
REF – = GND
IN + = GND
IN – = GND
FO = GND
TA = 25°C
825
Offset Error vs Temperature (TA)
0.3
OFFSET ERROR (ppm OF VREF)
850
0
0.5
1
1.5
2 2.5 3
VREF (V)
3.5
4
4.5
5
2413 G26
2
1
VCC = 5V
REF + = 5V
REF – = GND
IN + = GND
IN – = 2.5V
FO = GND
0
–1
–2
–3
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
75
90
2413 G27
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LTC2413
Typical Performance Characteristics
– Full-Scale Error vs VCC
0
–1
–2
–3
2.7
3.1
3.5
3.9 4.3
VCC (V)
4.7
5.1
1
0
–2
–120
0
0.5
1
1.5
2 2.5 3
VREF (V)
3.5
4
4.5
–140
0.01
5
VCC = 4.1VDC ±1.4V
REF + = 2.5V
REF – = GND
IN + = GND
IN – = GND
FO = GND
TA = 25°C
2413 G30
–40
–60
–20
–40
–80
–60
–100
–100
–120
–120
–120
0 20 40 60 80 100 120 140 160 180 200 220
FREQUENCY AT VCC (Hz)
–140
1
10
100
1k
10k 100k
FREQUENCY AT VCC (Hz)
Conversion Current vs
Output Data Rate
1100
900
VCC = 5.5V
200
190
VCC = 4.1V
180
170
VCC = 2.7V
800
700
600
500
22
400
300
10
0 15 30 45 60
TEMPERATURE (°C)
75
90
2413 G34
100
21
20
FO = GND
CS = VCC
SCK = NC
SDO = NC
VCC = 5.5V
VCC = 4.1V
19
18
VCC = 2.7V
17
200
160
–45 –30 –15
14100
Sleep Current vs Temperature (TA)
23
VCC = 5V
REF + = 5V
REF – = GND
IN + = GND
IN – = GND
TA = 25°C
FO = EXTERNAL OSC
CS = GND
SCK = NC
SDO = NC
1000
SUPPLY CURRENT (µA)
SUPPLY CURRENT (µA)
210
13950
14000
14050
FREQUENCY AT VCC (Hz)
2413 G33
SUPPLY CURRENT (µA)
Conversion Current vs
Temperature (TA)
FO = GND
CS = GND
SCK = NC
SDO = NC
–140
13900
2413 G32
2413 G31
220
1M
VCC = 4.1VDC ±0.7V
REF + = 2.5V
REF – = GND
IN + = GND
IN – = GND
FO = GND
TA = 25°C
–80
–100
–140
100
PSRR vs Frequency at VCC
REF + = 2.5V
REF – = GND
IN + = GND
IN – = GND
FO = GND
TA = 25°C
–20
–80
0.1
1
10
FREQUENCY AT VCC (Hz)
0
0
REJECTION (dB)
REJECTION (dB)
–80
PSRR vs Frequency at VCC
PSRR vs Frequency at VCC
–60
–60
2413 G29
0
–40
–40
–100
2413 G28
–20
VCC = 4.1VDC ±1.4V
REF + = 2.5V
REF – = GND
IN + = GND
IN – = GND
FO = GND
TA = 25°C
–20
–1
–3
5.5
VCC = 5V
REF + = VREF
REF – = GND
IN + = GND
IN – = 0.5 • REF +
FO = GND
TA = 25°C
2
REJECTION (dB)
1
0
REJECTION (dB)
REF + = 2.5V
REF – = GND
VREF = 2.5V
IN + = GND
IN – = 1.25V
FO = GND
TA = 25°C
2
PSRR vs Frequency at VCC
– Full-Scale Error vs VREF
3
–FULL-SCALE ERROR (ppm OF VREF)
–FULL-SCALE ERROR (ppm OF VREF)
3
0
5
10
15
20
OUTPUT DATA RATE (READINGS/SEC)
25
2413 G35
16
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
75
90
2413 G36
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LTC2413
Pin Functions
GND (Pins 1, 7, 8, 9, 10, 15, 16): Ground. Multiple ground
pins internally connected for optimum ground current flow
and VCC decoupling. Connect each one of these pins to a
ground plane through a low impedance connection. All seven
pins must be connected to ground for proper operation.
VCC (Pin 2): Positive Supply Voltage. Bypass to GND
(Pin 1) with a 10µF tantalum capacitor in parallel with
0.1µF ceramic capacitor as close to the part as possible.
REF + (Pin 3), REF– (Pin 4): Differential Reference Input.
The voltage on these pins can have any value between
GND and VCC as long as the reference positive input, REF+,
is maintained more positive than the reference negative
input, REF –, by at least 0.1V.
IN+ (Pin 5), IN– (Pin 6): Differential Analog Input. The
voltage on these pins can have any value between
GND – 0.3V and VCC + 0.3V. Within these limits, the converter
bipolar input range (VIN = IN+ – IN–) extends from –0.5 •
(VREF) to 0.5 • (VREF). Outside this input range, the converter
produces unique overrange and underrange output codes.
CS (Pin 11): Active LOW Digital Input. A LOW on this pin
enables the SDO digital output and wakes up the ADC.
Following each conversion, the ADC automatically enters
the Sleep mode and remains in this low power state as
long as CS is HIGH. A LOW-to-HIGH transition on CS
during the Data Output transfer aborts the data transfer
and starts a new conversion.
SDO (Pin 12): Three-State Digital Output. During the Data
Output period, this pin is used as serial data output. When
the chip select CS is HIGH (CS = VCC), the SDO pin is in a
high impedance state. During the Conversion and Sleep
periods, this pin is used as the conversion status output.
The conversion status can be observed by pulling CS LOW.
SCK (Pin 13): Bidirectional Digital Clock Pin. In Internal
Serial Clock Operation mode, SCK is used as digital output
for the internal serial interface clock during the Data Output
period. In External Serial Clock Operation mode, SCK is
used as digital input for the external serial interface clock
during the Data Output period. A weak internal pull-up is
automatically activated in Internal Serial Clock Operation
mode. The Serial Clock Operation mode is determined by
the logic level applied to the SCK pin at power up or during
the most recent falling edge of CS.
FO (Pin 14): Frequency Control Pin. Digital input that
controls the ADC’s notch frequencies and conversion
time. When the FO pin is connected to GND (FO = 0V),
the converter uses its internal oscillator and the digital
filter rejects 50Hz and 60Hz simultaneously. When the FO
pin is driven by an external clock signal with a frequency
fEOSC, the converter uses this signal as its system clock
and the digital filter has 87dB minimum rejection in the
range fEOSC/2560 ±14% and 110dB minimum rejection at
fEOSC/2560 ± 4%.
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LTC2413
functional Block Diagram
INTERNAL
OSCILLATOR
VCC
GND
IN +
IN –
AUTOCALIBRATION
AND CONTROL
+
–∫
∫
FO
(INT/EXT)
∫
SDO
∑
SERIAL
INTERFACE
ADC
SCK
CS
REF +
REF –
DECIMATING FIR
– +
DAC
2413 FD
Figure 1. Functional Block Diagram
Test Circuit
VCC
1.69k
SDO
1.69k
SDO
CLOAD = 20pF
CLOAD = 20pF
Hi-Z TO VOH
VOL TO VOH
VOH TO Hi-Z
Hi-Z TO VOL
VOH TO VOL
VOL TO Hi-Z
2413 TA03
2413 TA04
Applications Information
Converter Operation
CONVERT
Converter Operation Cycle
SLEEP
The LTC2413 is a low power, delta-sigma analog-to-digital
converter with an easy to use 3-wire serial interface.
Its operation is made up of three states. The converter
operating cycle begins with the conversion, followed by
the low power sleep state and ends with the data output
(see Figure 2). The 3-wire interface consists of serial data
output (SDO), serial clock (SCK) and chip select (CS).
Initially, the LTC2413 performs a conversion. Once the
conversion is complete, the device enters the sleep state.
While in this sleep state, power consumption is reduced
by an order of magnitude. The part remains in the sleep
state as long as CS is HIGH. The conversion result is held
indefinitely in a static shift register while the converter is
in the sleep state.
12
FALSE
CS = LOW
AND
SCK
TRUE
DATA OUTPUT
2413 F02
Figure 2. LTC2413 State Transition Diagram
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LTC2413
Applications Information
Once CS is pulled LOW, the device begins outputting the
conversion result. There is no latency in the conversion
result. The data output corresponds to the conversion
just performed. This result is shifted out on the serial data
out pin (SDO) under the control of the serial clock (SCK).
Data is updated on the falling edge of SCK allowing the
user to reliably latch data on the rising edge of SCK (see
Figure 3). The data output state is concluded once 32 bits
are read out of the ADC or when CS is brought HIGH. The
device automatically initiates a new conversion and the
cycle repeats.
Through timing control of the CS and SCK pins, the
LTC2413 offers several flexible modes of operation
(internal or external SCK and free-running conversion
modes). These various modes do not require programming
configuration registers; moreover, they do not disturb the
cyclic operation described above. These modes of operation are described in detail in the Serial Interface Timing
Modes section.
Conversion Clock
A major advantage the delta-sigma converter offers over
conventional type converters is an on-chip digital filter
(commonly implemented as a sinc or comb filter). For
high resolution, low frequency applications, this filter
is designed to simultaneously reject line frequencies of
50Hz and 60Hz plus their harmonics. The filter rejection
performance is directly related to the accuracy of the converter system clock. The LTC2413 incorporates a highly
accurate on-chip oscillator. This eliminates the need for
external frequency setting components such as crystals
or oscillators. The LTC2413 achieves a minimum of 87dB
over the range of 49Hz to 61.2Hz.
Ease of Use
The LTC2413 data output has no latency, filter settling
delay or redundant data associated with the conversion
cycle. There is a one-to-one correspondence between the
conversion and the output data. Therefore, multiplexing
multiple analog voltages is easy.
The LTC2413 performs offset and full-scale calibrations in
every conversion cycle. This calibration is transparent to
the user and has no effect on the cyclic operation described
above. The advantage of continuous calibration is extreme
stability of offset and full-scale readings with respect to
time, supply voltage change and temperature drift.
Power-Up Sequence
The LTC2413 automatically enters an internal reset state
when the power supply voltage VCC drops below approximately 2.2V. This feature guarantees the integrity of the
conversion result and of the serial interface mode selection. (See the 2-wire I/O sections in the Serial Interface
Timing Modes section.)
When the VCC voltage rises above this critical threshold,
the converter creates an internal power-on-reset (POR)
signal with a duration of approximately 0.5ms. The POR
signal clears all internal registers. Following the POR
signal, the LTC2413 starts a normal conversion cycle and
follows the succession of states described above. The first
conversion result following POR is accurate within the
specifications of the device if the power supply voltage is
restored within the operating range (2.7V to 5.5V) before
the end of the POR time interval.
Reference Voltage Range
This converter accepts a truly differential external reference
voltage. The absolute/common mode voltage specification
for the REF + and REF – pins covers the entire range from
GND to VCC. For correct converter operation, the REF + pin
must always be more positive than the REF – pin.
The LTC2413 can accept a differential reference voltage
from 0.1V to VCC. The converter output noise is determined
by the thermal noise of the front-end circuits, and as such,
its value in nanovolts is nearly constant with reference
voltage. A decrease in reference voltage will not significantly improve the converter’s effective resolution. On the
other hand, a reduced reference voltage will improve the
converter’s overall INL performance. A reduced reference
voltage will also improve the converter performance when
operated with an external conversion clock (external FO
signal) at substantially higher output data rates (see the
Output Data Rate section).
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13
LTC2413
Applications Information
Input Voltage Range
The analog input is truly differential with an absolute/
common mode range for the IN+ and IN– input pins
extending from GND – 0.3V to VCC + 0.3V. Outside
these limits, the ESD protection devices begin to turn
on and the errors due to input leakage current increase
rapidly. Within these limits, the LTC2413 converts the
bipolar differential input signal, VIN = IN+ – IN–, from
– FS = –0.5 • VREF to +FS = 0.5 • VREF where VREF =
REF+ – REF–. Outside this range, the converter indicates
the overrange or the underrange condition using distinct
output codes.
Input signals applied to IN+ and IN– pins may extend by
300mV below ground and above VCC. In order to limit any
fault current, resistors of up to 5k may be added in series
with the IN+ and IN– pins without affecting the performance of the device. In the physical layout, it is important
to maintain the parasitic capacitance of the connection
between these series resistors and the corresponding
pins as low as possible; therefore, the resistors should
be located as close as practical to the pins. The effect of
the series resistance on the converter accuracy can be
evaluated from the curves presented in the Input Current/
Reference Current sections. In addition, series resistors
will introduce a temperature dependent offset error due to
the input leakage current. A 1nA input leakage current will
develop a 1ppm offset error on a 5k resistor if VREF = 5V.
This error has a very strong temperature dependency.
Output Data Format
The LTC2413 serial output data stream is 32 bits long.
The first 3 bits represent status information indicating
the sign and conversion state. The next 24 bits are the
conversion result, MSB first. The remaining 5 bits are
sub LSBs beyond the 24-bit level that may be included
in averaging or discarded without loss of resolution. The
third and fourth bits together are also used to indicate
an underrange condition (the differential input voltage
is below –FS) or an overrange condition (the differential
input voltage is above +FS).
Bit 31 (first output bit) is the end of conversion (EOC)
indicator. This bit is available at the SDO pin during the
conversion and sleep states whenever the CS pin is LOW.
14
This bit is HIGH during the conversion and goes LOW
when the conversion is complete.
Bit 30 (second output bit) is a dummy bit (DMY) and is
always LOW.
Bit 29 (third output bit) is the conversion result sign
indicator (SIG). If VIN is >0, this bit is HIGH. If VIN is 0.01µF) may be
required in certain configurations for antialiasing or general input signal filtering. Such capacitors will average the
input sampling charge and the external source resistance
will see a quasi constant input differential impedance.
When internal oscillator is used (FO= LOW), the typical
differential input resistance is 2MΩ which will generate
a gain error of approximately 0.25ppm for each ohm of
source resistance driving IN+ or IN–. When FO is driven
by an external oscillator with a frequency fEOSC (external conversion clock operation), the typical differential
input resistance is 0.28 • 1012/fEOSCΩ and each ohm of
source resistance driving IN+ or IN– will result in
1.78 • 10–6 • fEOSCppm gain error. The effect of the source
resistance on the two input pins is additive with respect to
this gain error. The typical +FS and –FS errors as a function of the sum of the source resistance seen by IN+ and
IN– for large values of CIN are shown in Figures 20 and 21.
In addition to this gain error, an offset error term may
also appear. The offset error is proportional with the
mismatch between the source impedance driving the two
input pins IN+ and IN– and with the difference between the
input and reference common mode voltages. While the
input drive circuit nonzero source impedance combined
with the converter average input current will not degrade
the INL performance, indirect distortion may result from
the modulation of the offset error by the common mode
component of the input signal. Thus, when using large
CIN capacitor values, it is advisable to carefully match the
source impedance seen by the IN+ and IN– pins. When
internal oscillator is used (FO = LOW), every 1Ω mismatch
in source impedance transforms a full-scale common
mode input signal into a differential mode input signal
of 0.25ppm. When FO is driven by an external oscillator
with a frequency fEOSC, every 1Ω mismatch in source
impedance transforms a full-scale common mode input
signal into a differential mode input signal of 1.78 • 10–6 •
fEOSCppm. Figure 22 shows the typical offset error due to
input common mode voltage for various values of source
resistance imbalance between the IN+ and IN– pins when
large CIN values are used.
If possible, it is desirable to operate with the input signal
common mode voltage very close to the reference signal
common mode voltage as is the case in the ratiometric
measurement of a symmetric bridge. This configuration
eliminates the offset error caused by mismatched source
impedances.
The magnitude of the dynamic input current depends upon
the size of the very stable internal sampling capacitors and
upon the accuracy of the converter sampling clock. The
accuracy of the internal clock over the entire temperature
and power supply range is typical better than 0.5%. Such
a specification can also be easily achieved by an external
clock. When relatively stable resistors (50ppm/°C) are used
for the external source impedance seen by IN+ and IN–,
the expected drift of the dynamic current, offset and gain
errors will be insignificant (about 1% of their respective
values over the entire temperature and voltage range). Even
for the most stringent applications a one-time calibration
operation may be sufficient.
In addition to the input sampling charge, the input ESD
protection diodes have a temperature dependent leakage
current. This current, nominally 1nA (±10nA max), results
in a small offset shift. A 100Ω source resistance will create
a 0.1µV typical and 1µV maximum offset voltage.
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LTC2413
Applications Information
Reference Current
+FS ERROR (ppm OF VREF)
300
VCC = 5V
REF + = 5V
REF – = GND
IN + = 3.75V
IN – = 1.25V
FO = GND
TA = 25°C
240
180
CIN = 0.1µF
120
CIN = 0.01µF
60
0
In a similar fashion, the LTC2413 samples the differential
reference pins REF+ and REF– transfering small amount
of charge to and from the external driving circuits, thus
produces a dynamic reference current. This current does
not change the converter offset but it may degrade the
gain and INL performance. The effect of this current can
be analyzed in the same two distinct situations.
CIN = 1µF, 10µF
0 100 200 300 400 500 600 700 800 900 1000
RSOURCE (Ω)
2413 F19
Figure 20. +FS Error vs RSOURCE at IN+ or IN– (Large CIN)
0
–FS ERROR (ppm OF VREF)
CIN = 0.01µF
–60
–120
–180
–240
–300
CIN = 0.1µF
VCC = 5V
REF + = 5V
REF – = GND
IN + = 1.25V
IN – = 3.75V
FO = GND
TA = 25°C
CIN = 1µF, 10µF
0 100 200 300 400 500 600 700 800 900 1000
RSOURCE (Ω)
2413 F21
Figure 21. –FS Error vs RSOURCE at IN+ or IN– (Large CIN)
120
OFFSET ERROR (ppm OF VREF)
100
VCC = 5V
REF + = 5V
REF – = GND
IN + = IN – = VINCM
A
80
60
B
40
C
D
E
F
20
0
–20
–40
G
–60
FO = GND
TA = 25°C
RSOURCEIN – = 500Ω
CIN = 10µF
–80
–100
–120
0
0.5
1
1.5
A: ∆RIN = +400Ω
B: ∆RIN = +200Ω
C: ∆RIN = +100Ω
D: ∆RIN = 0Ω
2 2.5 3
VINCM (V)
3.5
4
4.5
5
E: ∆RIN = –100Ω
F: ∆RIN = –200Ω
G: ∆RIN = –400Ω
2413 F22
Figure 22. Offset Error vs Common Mode Voltage
(VINCM = IN+ = IN–) and Input Source Resistance Imbalance
(∆RIN = RSOURCEIN+ – RSOURCEIN–) for Large CIN Values (CIN ≥ 1µF)
28
For relatively small values of the external reference capacitors (CREF < 0.01µF), the voltage on the sampling capacitor
settles almost completely and relatively large values for
the source impedance result in only small errors. Such
values for CREF will deteriorate the converter offset and
gain performance without significant benefits of reference
filtering and the user is advised to avoid them.
Larger values of reference capacitors (CREF > 0.01µF)
may be required as reference filters in certain configurations. Such capacitors will average the reference sampling
charge and the external source resistance will see a quasi
constant reference differential impedance. When internal
oscillator is used (FO = LOW), the typical differential
input resistance is 1.43MΩ which will generate a gain
error of approximately 0.35ppm for each ohm of source
resistance driving REF+ or REF–. When FO is driven by
an external oscillator with a frequency fEOSC (external
conversion clock operation), the typical differential reference resistance is 0.20 • 1012/fEOSCΩ and each ohm
of source resistance drving REF+ or REF– will result in
2.47 • 10–6 • fEOSCppm gain error. The effect of the source
resistance on the two reference pins is additive with respect
to this gain error. The typical +FS and –FS errors for various combinations of source resistance seen by the REF+
and REF– pins and external capacitance CREF connected
to these pins are shown in Figures 23, 24, 25 and 26.
In addition to this gain error, the converter INL performance is degraded by the reference source impedance.
When internal oscillator is used(FO = LOW), every 100Ω
of source impedance driving REF+ or REF– translates into
about 1.2ppm additional INL error. When FO is driven by
an external oscillator with a frequency fEOSC, every 100Ω
of source resistance driving REF+ or REF– translates
into about 8.73 • 10–6 • fEOSCppm additional INL error.
2413fa
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Applications Information
Figure 27 shows the typical INL error due to the source
resistance driving the REF+ or REF– pins when large CREF
values are used. The effect of the source resistance on
the two reference pins is additive with respect to this INL
error. In general, matching of source impedance for the
REF+ and REF– pins does not help the gain or the INL error. The user is thus advised to minimize the combined
source impedance driving the REF+ and REF– pins rather
than to try to match it.
The magnitude of the dynamic reference current depends
upon the size of the very stable internal sampling capacitors
and upon the accuracy of the converter sampling clock. The
accuracy of the internal clock over the entire temperature
and power supply range is typical better than 0.5%. Such
a specification can also be easily achieved by an external
clock. When relatively stable resistors (50ppm/°C) are
used for the external source impedance seen by REF+
and REF–, the expected drift of the dynamic current gain
error will be insignificant (about 1% of its value over the
entire temperature and voltage range). Even for the most
stringent applications, a one-time calibration operation
may be sufficient.
In addition to the reference sampling charge, the reference pins ESD protection diodes have a temperature dependent leakage current. This leakage current, nominally
1nA (±10nA max), results in a small gain error. A 100Ω
source resistance will create a 0.05µV typical and 0.5µV
maximum full-scale error.
50
–10
–20
VCC = 5V
REF + = 5V
REF – = GND
IN + = 5V
IN – = 2.5V
FO = GND
TA = 25°C
–30
–FS ERROR (ppm OF VREF)
+FS ERROR (ppm OF VREF)
0
CREF = 0.01µF
CREF = 0.001µF
–40
–50
1.E+00
CREF = 100pF
CREF = 0pF
1.E+01
1.E+02 1.E+03
RSOURCE (Ω)
1.E+04
CREF = 0.01µF
CREF = 0.001µF
40
CREF = 100pF
CREF = 0pF
30
VCC = 5V
REF + = 5V
REF – = GND
IN + = GND
IN – = 2.5V
FO = GND
TA = 25°C
20
10
0
1.E+00
1.E+05
1.E+01
1.E+02 1.E+03
RSOURCE (Ω)
1.E+04
2413 F24
2413 F23
Figure 23. +FS Error vs RSOURCE at REF+ or REF– (Small CREF)
450
CREF = 0.01µF
–90
–180
–270
–360
–450
Figure 24. –FS Error vs RSOURCE at REF+ or REF– (Small CREF)
–FS ERROR (ppm OF VREF)
+FS ERROR (ppm OF VREF)
0
CREF = 0.1µF
VCC = 5V
REF + = 5V
REF – = GND
IN + = 3.75V
IN – = 1.25V
FO = GND
TA = 25°C
1.E+05
CREF = 1µF, 10µF
0 100 200 300 400 500 600 700 800 900 1000
RSOURCE (Ω)
VCC = 5V
REF + = 5V
REF – = GND
IN + = 1.25V
IN – = 3.75V
FO = GND
TA = 25°C
360
270
CREF = 0.1µF
180
CREF = 0.01µF
90
0
0 100 200 300 400 500 600 700 800 900 1000
RSOURCE (Ω)
2413 F25
Figure 25. +FS Error vs RSOURCE at REF+ or REF– (Large CREF)
CREF = 1µF, 10µF
2413 F26
Figure 26. –FS Error vs RSOURCE at REF+ or REF– (Large CREF)
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29
LTC2413
Applications Information
15
12
RSOURCE = 1000Ω
INL (ppm OF VREF)
9
RSOURCE = 500Ω
6
3
0
–3
RSOURCE = 100Ω
–6
–9
–12
–15
–0.5 –0.4–0.3–0.2–0.1 0 0.1 0.2 0.3 0.4 0.5
VINDIF/VREFDIF
VCC = 5V
FO = GND
REF+ = 5V
CREF = 10µF
TA = 25°C
REF– = GND
2413 F27
VINCM = 0.5 • (IN + + IN –) = 2.5V
Figure 27. INL vs Differential Input Voltage (VIN = IN+ – IN–)
and Reference Source Resistance (RSOURCE at REF+ and REF– for
Large CREF Values (CREF ≥ 1µF)
Output Data Rate
When using its internal oscillator, the LTC2413 can produce
up to 6.8 readings per second. The actual output data rate
will depend upon the length of the sleep and data output
phases which are controlled by the user and which can be
made insignificantly short. When operated with an external
conversion clock (FO connected to an external oscillator),
the LTC2413 output data rate can be increased as desired.
The duration of the conversion phase is 20510/fEOSC. If
fEOSC = 139800Hz, the converter behaves as if the internal
oscillator is used with simultaneous 50Hz/60Hz rejection.
There is no significant difference in the LTC2413 performance between these two operation modes.
An increase in fEOSC over the nominal 139800Hz will
translate into a proportional increase in the maximum
output data rate. This substantial advantage is nevertheless accompanied by three potential effects, which must
be carefully considered.
30
First, a change in fEOSC will result in a proportional change
in the internal notch position and in a reduction of the
converter differential mode rejection at the power-line
frequency. In many applications, the subsequent performance degradation can be substantially reduced by
relying upon the LTC2413’s exceptional common mode
rejection and by carefully eliminating common mode to
differential mode conversion sources in the input circuit.
The user should avoid single-ended input filters and should
maintain a very high degree of matching and symmetry
in the circuits driving the IN+ and IN– pins.
Second, the increase in clock frequency will increase
proportionally the amount of sampling charge transferred
through the input and the reference pins. If large external
input and/or reference capacitors (CIN, CREF) are used,
the previous section provides formulae for evaluating the
effect of the source resistance upon the converter performance for any value of fEOSC. If small external input and/
or reference capacitors (CIN, CREF) are used, the effect of
the external source resistance upon the LTC2413 typical
performance can be inferred from Figures 18, 19, 23 and
24 in which the horizontal axis is scaled by 139800/fEOSC.
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Applications Information
Third, an increase in the frequency of the external oscillator above 460800Hz (a more than 3× increase in the
output data rate) will start to decrease the effectiveness
of the internal autocalibration circuits. This will result in
a progressive degradation in the converter accuracy and
linearity. Typical measured performance curves for output
data rates up to 25 readings per second are shown in
Figures 28 through 35, inclusive. In order to obtain the
highest possible level of accuracy from this converter at
output data rates above 7.5 readings per second, the user
is advised to maximize the power supply voltage used and
to limit the maximum ambient operating temperature.
In certain circumstances, a reduction of the differential
reference voltage may be beneficial.
500
VCC = 5V
REF + = 5V
REF – = GND
VINCM = 2.5V
VIN = 0V
FO = EXTERNAL OSCILLATOR
OFFSET ERROR (ppm OF VREF)
450
400
350
300
250
200
150
TA = 85°C
100
TA = 25°C
50
0
0
5
10
15
20
OUTPUT DATA RATE (READINGS/SEC)
25
2413 F28
Figure 28. Offset Error vs Output Data Rate and Temperature
7000
0
VCC = 5V
REF + = 5V
REF – = GND
IN + = 3.75V
IN – = 1.25V
FO = EXTERNAL OSCILLATOR
5000
4000
–1000
–FS ERROR (ppm OF VREF)
+FS ERROR (ppm OF VREF)
6000
3000
2000
TA = 85°C
1000
0
TA = 25°C
0
TA = 85°C
–3000
–4000
VCC = 5V
REF + = 5V
REF – = GND
IN + = 1.25V
IN – = 3.75V
FO = EXTERNAL OSCILLATOR
–5000
–6000
5
10
15
20
OUTPUT DATA RATE (READINGS/SEC)
25
–7000
2413 F29
Figure 29. +FS Error vs Output Data Rate and Temperature
TA = 25°C
–2000
0
5
10
15
20
OUTPUT DATA RATE (READINGS/SEC)
25
2413 F30
Figure 30. –FS Error vs Output Data Rate and Temperature
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LTC2413
Applications Information
24
22
23
TA = 25°C
TA = 85°C
21
20
19
18
VCC = 5V
REF + = 5V
REF – = GND
VINCM = 2.5V
VIN = 0V
FO = EXTERNAL OSCILLATOR
RESOLUTION = LOG2(VREF/NOISERMS)
17
16
15
14
13
12
0
TA = 25°C
20
RESOLUTION (BITS)
RESOLUTION (BITS)
22
TA = 85°C
18
RESOLUTION = LOG2(VREF/INLMAX)
16
14
VCC = 5V
REF + = 5V
REF – = GND
VINCM = 2.5V
–2.5V < VIN < 2.5V
FO = EXTERNAL OSCILLATOR
12
10
5
10
15
20
OUTPUT DATA RATE (READINGS/SEC)
8
25
0
5
10
15
20
OUTPUT DATA RATE (READINGS/SEC)
2413 F31
2413 F32
Figure 31. Resolution (NoiseRMS ≤ 1LSB)
vs Output Data Rate and Temperature
Figure 32. Resolution (INLRMS ≤ 1LSB)
vs Output Data Rate and Temperature
250
24
175
150
22
RESOLUTION (BITS)
OFFSET ERROR (ppm OF VREF)
200
125
100
75
50
VREF = 2.5V
25
0
VREF = 5V
23
VCC = 5V
REF + = GND
VINCM = 2.5V
VIN = 0V
FO = EXTERNAL OSCILLATOR
TA = 25°C
225
0
25
21
VREF = 2.5V
20
19
18
VCC = 5V
REF – = GND
VINCM = 2.5V
VIN = 0V
FO = EXTERNAL OSCILLATOR
TA = 25°C
RESOLUTION = LOG2(VREF/NOISERMS)
17
16
15
VREF = 5V
14
13
5
10
15
20
OUTPUT DATA RATE (READINGS/SEC)
12
25
0
5
10
15
20
OUTPUT DATA RATE (READINGS/SEC)
2413 F33
2413 F34
Figure 33. Offset Error vs Output
Data Rate and Reference Voltage
Figure 34. Resolution (NoiseRMS ≤ 1LSB) vs
Output Data Rate and Reference Voltage
22
VREF = 5V
20
RESOLUTION (BITS)
25
VREF = 2.5V
18
RESOLUTION =
LOG2(VREF/INLMAX)
16
14
TA = 25°C
VCC = 5V
REF – = GND
VINCM = 0.5 • REF +
–0.5V • VREF < VIN < 0.5 • VREF
FO = EXTERNAL OSCILLATOR
12
10
8
0
5
10
15
20
OUTPUT DATA RATE (READINGS/SEC)
25
2413 F35
Figure 35. Resolution (INLMAX ≤ 1LSB) vs Output Data Rate and Reference Voltage
32
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LTC2413
Applications Information
Input Bandwidth
The combined effect of the internal sinc4 digital filter and
of the analog and digital autocalibration circuits determines
the LTC2413 input bandwidth. When the internal oscillator
is used (FO = LOW), the 3dB input bandwidth is 3.3Hz. If
an external conversion clock generator of frequency fEOSC
is connected to the FO pin, the 3dB input bandwidth is
0.236 • 10–6 • fEOSC.
Due to the complex filtering and calibration algorithms
utilized, the converter input bandwidth is not modeled
very accurately by a first order filter with the pole located
at the 3dB frequency. When the internal oscillator is used,
the shape of the LTC2413 input bandwidth is shown in
Figure 36. When an external oscillator of frequency fEOSC
is used, the shape of the LTC2413 input bandwidth can
be derived from Figure 36, in which the horizontal axis is
scaled by fEOSC/139800.
The conversion noise (800nVRMS typical for VREF = 5V) can
be modeled as a white noise source connected to a noise
free converter. The noise spectral density is 63nV/√Hz for
an infinite bandwidth source and 77nV/√Hz for a single
0.5MHz pole source. From these numbers, it is clear that
particular attention must be given to the design of external
amplification circuits. Such circuits face the simultaneous
requirements of very low bandwidth (just a few Hz) in
order to reduce the output referred noise and relatively
high bandwidth (at least 500kHz) necessary to drive the
input switched-capacitor network. A possible solution is
a high gain, low bandwidth amplifier stage followed by a
high bandwidth unity-gain buffer.
When external amplifiers are driving the LTC2413, the
ADC input referred system noise calculation can be
simplified by Figure 37. The noise of an amplifier driving
the LTC2413 input pin can be modeled as a band limited
white noise source. Its bandwidth can be approximated
by the bandwidth of a single pole lowpass filter with a
corner frequency fi. The amplifier noise spectral density
is ni. From Figure 37, using fi as the x-axis selector, we
can find on the y-axis the noise equivalent bandwidth freqi
of the input driving amplifier. This bandwidth includes
the band limiting effects of the ADC internal calibration
and filtering. The noise of the driving amplifier referred
to the converter input and including all these effects can
be calculated as N = ni • √freqi. The total system noise
(referred to the LTC2413 input) can now be obtained by
summing as square root of sum of squares the three ADC
input referred noise sources: the LTC2413 internal noise
(800nV), the noise of the IN + driving amplifier and the
noise of the IN– driving amplifier.
If the FO pin is driven by an external oscillator of frequency
fEOSC, Figure 37 can still be used for noise calculation if
the x-axis is scaled by fEOSC/139800. For large values of
the ratio fEOSC/139800, the Figure 37 plot accuracy begins
to decrease, but in the same time the LTC2413 noise floor
rises and the noise contribution of the driving amplifiers
lose significance.
1000
0.0
–1.0
INPUT REFERRED NOISE
EQUIVALENT BANDWIDTH (Hz)
INPUT SIGNAL ATTENUATION (dB)
–0.5
–1.5
–2.0
–2.5
–3.0
–3.5
–4.0
–4.5
–5.0
100
10
FO = LOW
1
–5.5
–6.0
0.1
0.1
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
2413 F36
Figure 36. Input Signal Bandwidth Using the Internal Oscillator
1
10 100 1k
10k 100k 1M
INPUT NOISE SOURCE SINGLE POLE
EQUIVALENT BANDWIDTH (Hz) 2413 F37
Figure 37. Input Referred Noise Equivalent Bandwidth
of an Input Connected White Noise Source
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33
LTC2413
Applications Information
Normal Mode Rejection and Antialiasing
One of the advantages delta-sigma ADCs offer over
conventional ADCs is on-chip digital filtering. Combined
with a large oversampling ratio, the LTC2413 significantly
simplifies antialiasing filter requirements.
The sinc4 digital filter provides greater than 120dB normal
mode rejection at all frequencies except DC and integer
multiples of the modulator sampling frequency (fS). The
LTC2413’s autocalibration circuits further simplify the
antialiasing requirements by additional normal mode
signal filtering both in the analog and digital domain.
Independent of the operating mode, fS = 256 • fN = 2048
• fOUTMAX where fN in the notch frequency and fOUTMAX is
the maximum output data rate. In the internal oscillator
mode, fS = 13980Hz. In the external oscillator mode, fS
= fEOSC/10.
The combined normal mode rejection performance is
shown in Figure 38. The regions of low rejection occurring
at integer multiples of fS have a very narrow bandwidth.
Magnified details of the normal mode rejection curves
are shown in Figure 39 (rejection near DC) and Figure 40
(rejection at fS = 256fN) where fN represents the notch
frequency. These curves have been derived for the external oscillator mode but they can be used in all operating
modes by appropriately selecting the fN value.
INPUT NORMAL MODE REJECTION (dB)
0
FO = LOW OR
FO = EXTERNAL OSCILLATOR,
fEOSC = 10 • fS
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
0 fS 2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS 10fS
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
2413 F38
0
0
–10
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
34
INPUT NORMAL MODE REJECTION (dB)
INPUT NORMAL MODE REJECTION (dB)
Figure 38. Input Normal Mode Rejection
0
fN
2fN 3fN 4fN 5fN 6fN 7fN
INPUT SIGNAL FREQUENCY (Hz)
8fN
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
250fN 252fN 254fN 256fN 258fN 260fN 262fN
INPUT SIGNAL FREQUENCY (Hz)
2413 F39
2413 F40
Figure 39. Input Normal Mode Rejection
Figure 40. Input Normal Mode Rejection
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LTC2413
Applications Information
The user can expect to achieve in practice this level of
performance using the internal oscillator, as it is demonstrated by Figure 41. Typical measured values of the
normal mode rejection of the LTC2413 operating with the
internal oscillator are shown in Figure 41 superimposed
over the theoretical calculated curve.
As a result of these remarkable normal mode specifications, minimal (if any) antialias filtering is required in front
of the LTC2413. If passive RC components are placed in
front of the LTC2413, the input dynamic current should
be considered (see Input Current section). In cases where
large effective RC time constants are used, an external
buffer amplifier may be required to minimize the effects
of dynamic input current.
Traditional high order delta-sigma modulators, while
providing very good linearity and resolution, suffer
from potential instabilities at large input signal levels.
The proprietary architecture used for the LTC2413 third
order modulator resolves this problem and guarantees a
predictable stable behavior at input signal levels of up to
150% of full scale. In many industrial applications, it is
not uncommon to have to measure microvolt level signals
superimposed over volt level perturbations and LTC2413
is eminently suited for such tasks. When the perturbation
is differential, the specification of interest is the normal
mode rejection for large input signal levels. With a reference
voltage VREF = 5V, the LTC2413 has a full-scale differential
NORMAL MODE REJECTION (dB)
0
MEASURED DATA
CALCULATED DATA
–20
–40
VCC = 5V
REF + = 5V
REF – = GND
VINCM = 2.5V
VIN(P-P) = 5V
TA = 25°C
– 60
–80
–100
–120
0
20
40
60
80
100
120
140
INPUT FREQUENCY (Hz)
160
180
200
220
2413 F41
Figure 41. Input Normal Mode Rejection vs Input
Frequency with Input Perturbation of 100% of Full Scale
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LTC2413
Applications Information
input range of 5V peak-to-peak. Figure 42 shows measurement results for the LTC2413 normal mode rejection ratio
with a 7.5V peak-to-peak (150% of full scale) input signal
superimposed over the more traditional normal mode rejection ratio results obtained with a 5V peak-to-peak (full
scale) input signal. It is clear that the LTC2413 rejection
performance is maintained with no compromises in this
extreme situation. When operating with large input signal
levels, the user must observe that such signals do not
violate the device’s absolute maximum ratings.
Bridge applications
Typical strain gauge based bridges deliver only 2mV/
Volt of excitation. As the maximum reference voltage of
the LTC2413 is 5V, remote sensing of applied excitation
without additional circuitry requires that excitation be
limited to 5V. This gives only 10mV full scale, which can
be resolved to 1 part in 10000 without averaging. For many
solid state sensors, this is still better than the sensor. For
example, averaging 64 samples however reduces the noise
level by a factor of eight, bringing the resolving power to
1 part in 80000, comparable to better weighing systems.
Hysteresis and creep effects in the load cells are typically
much greater than this. Most applications that require
strain measurements to this level of accuracy are measuring slowly changing phenomena, hence the time required
to average a large number of readings is usually not an
issue. For those systems that require accurate measurement of a small incremental change on a significant tare
weight, the lack of history effects in the LTC2400 family
is of great benefit.
NORMAL MODE REJECTION (dB)
0
VIN(P-P) = 5V
VIN(P-P) = 7.5V
(150% OF FULL SCALE)
–20
–40
VCC = 5V
REF + = 5V
REF – = GND
VINCM = 2.5V
TA = 25°C
– 60
–80
–100
–120
0
20
40
60
80
100
120
140
INPUT FREQUENCY (Hz)
160
180
200
220
2413 F42
Figure 42. Measured Input Normal Mode Rejection vs Input
Frequency with Input Perturbation of 150% of Full Scale
36
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LTC2413
Applications Information
For those applications that cannot be fulfilled by the
LTC2413 alone, compensating for error in external amplification can be done effectively due to the “no latency” feature
of the LTC2413. No latency operation allows samples of the
amplifier offset and gain to be interleaved with weighing
measurements. The use of correlated double sampling
allows suppression of 1/f noise, offset and thermocouple
effects within the bridge. Correlated double sampling involves alternating the polarity of excitation and dealing with
the reversal of input polarity mathematically. Alternatively,
bridge excitation can be increased to as much as ±10V,
if one of several precision attenuation techniques is used
to produce a precision divide operation on the reference
signal. Another option is the use of a reference within the
5V input range of the LTC2413 and developing excitation
via fixed gain, or LTC1043 based voltage multiplication,
along with remote feedback in the excitation amplifiers,
as shown in Figures 48 and 50.
located at each load bearing point, the output of which
can be summed passively prior to the signal processing
circuitry, actively with amplification prior to the ADC, or
can be digitized via multiple ADC channels and summed
mathematically. The mathematical summation of the output of multiple LTC2413’s provides the benefit of a root
square reduction in noise. The low power consumption
of the LTC2413 makes it attractive for multidrop communication schemes where the ADC is located within the
load-cell housing.
A direct connection to a load cell is perhaps best incorporated into the load-cell body, as minimizing the distance
to the sensor largely eliminates the need for protection
devices, RFI suppression and wiring. The LTC2413 exhibits
extremely low temperature dependent drift. As a result,
exposure to external ambient temperature ranges does
not compromise performance. The incorporation of any
amplification considerably complicates thermal stability, as
input offset voltages and currents, temperature coefficient
of gain settling resistors all become factors.
Figure 43 shows an example of a simple bridge connection.
Note that it is suitable for any bridge application where
measurement speed is not of the utmost importance.
For many applications where large vessels are weighed,
the average weight over an extended period of time is of
concern and short term weight is not readily determined
due to movement of contents, or mechanical resonance.
Often, large weighing applications involve load cells
The circuit in Figure 44 shows an example of a simple
amplification scheme. This example produces a differential output with a common mode voltage of 2.5V, as
determined by the bridge. The use of a true three amplifier
instrumentation amplifier is not necessary, as the LTC2413
+
R1
2
3
350Ω
BRIDGE
LT1019
4
5
VREF
REF +
SDO
REF –
SCK
IN +
CS
12
13
11
LTC2413
6
R2
IN –
GND
FO
14
1, 7, 8, 9,
10, 15, 16
2413 F43
R1 AND R2 CAN BE USED TO INCREASE TOLERABLE AC COMPONENT ON REF SIGNALS
Figure 43. Simple Bridge Connection
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37
LTC2413
Applications Information
has common mode rejection far beyond that of most amplifiers. The LTC1051 is a dual autozero amplifier that can
be used to produce a gain of 15 before its input referred
noise dominates the LTC2413 noise. This example shows
a gain of 34, that is determined by a feedback network built
using a resistor array containing 8 individual resistors. The
resistors are organized to optimize temperature tracking in
the presence of thermal gradients. The second LTC1051
buffers the low noise input stage from the transient load
steps produced during conversion.
The gain stability and accuracy of this approach is very
good, due to a statistical improvement in resistor matching
due to individual error contribution being reduced. A gain
of 34 may seem low, when compared to common practice
in earlier generations of load-cell interfaces, however the
accuracy of the LTC2413 changes the rationale. Achieving high gain accuracy and linearity at higher gains may
prove difficult, while providing little benefit in terms of
noise reduction.
At a gain of 100, the gain error that could result from
typical open-loop gain of 160dB is –1ppm, however,
worst-case is at the minimum gain of 116dB, giving a
gain error of –158ppm. Worst-case gain error at a gain
of 34, is –54ppm. The use of the LTC1051A reduces the
worst-case gain error to –33ppm. The advantage of gain
higher than 34, then becomes dubious, as the input referred noise sees little improvement1 and gain accuracy
is potentially compromised.
Note that this 4-amplifier topology has advantages over
the typical integrated 3-amplifier instrumentation amplifier
in that it does not have the high noise level common in
the output stage that usually dominates when an instrumentation amplifier is used at low gain. If this amplifier
is used at a gain of 10, the gain error is only 10ppm and
input referred noise is reduced to 0.1µVRMS. The buffer
stages can also be configured to provide gain of up to 50
with high gain stability and linearity.
Figure 45 shows an example of a single amplifier used to
produce single-ended gain. This topology is best used in
applications where the gain setting resistor can be made
to match the temperature coefficient of the strain gauges.
If the bridge is composed of precision resistors, with only
one or two variable elements, the reference arm of the
bridge can be made to act in conjunction with the feedback
resistor to determine the gain. If the feedback resistor is
incorporated into the design of the load cell, using resistors
which match the temperature coefficient of the load-cell
elements, good results can be achieved without the need
for resistors with a high degree of absolute accuracy. The
common mode voltage in this case, is again a function of
the bridge output. Differential gain as used with a 350Ω
bridge is AV = (R1+ R2)/(R1+175Ω). Common mode gain
is half the differential gain. The maximum differential signal
that can be used is 1/4 VREF, as opposed to 1/2 VREF in
the 2-amplifier topology above.
Remote Half Bridge Interface
As opposed to full bridge applications, typical half bridge
applications must contend with nonlinearity in the bridge
output, as signal swing is often much greater. Applications
include RTD’s, thermistors and other resistive elements
that undergo significant changes over their span. For single
variable element bridges, the nonlinearity of the half bridge
output can be eliminated completely; if the reference arm
of the bridge is used as the reference to the ADC, as shown
in Figure 46. The LTC2413 can accept inputs up to 1/2
VREF. Hence, the reference resistor R1 must be at least
2x the highest value of the variable resistor.
In the case of 100Ω platinum RTD’s, this would suggest
a value of 800Ω for R1. Such a low value for R1 is not
advisable due to self-heating effects. A value of 25.5k is
shown for R1, reducing self-heating effects to acceptable
levels for most sensors.
1Input referred noise for A = 34 for approximately 0.05µV
V
RMS, whereas at a gain of 50, it would
be 0.048µVRMS.
38
2413fa
For more information www.linear.com/LTC2413
LTC2413
Applications Information
The basic circuit shown in Figure 46 shows connections
for a full 4-wire connection to the sensor, which may be
located remotely. The differential input connections will
reject induced or coupled 60Hz interference, however,
the reference inputs do not have the same rejection. If
60Hz or other noise is present on the reference input, a
low pass filter is recommended as shown in Figure 47.
Note that you cannot place a large capacitor directly at
the junction of R1 and R2, as it will store charge from
the sampling process. A better approach is to produce a
low pass filter decoupled from the input lines with a high
value resistor (R3).
introduced at the reference, at least at higher frequencies,
will be reduced. A filter can be introduced into the network,
in the form of one or more capacitors, or ferrite beads,
as long as the sampling pulses are not translated into an
error. The reference voltage is also reduced, but this is
not undesirable, as it will decrease the value of the LSB,
although, not the input referred noise level.
The circuit shown in Figure 47 shows a more rigorous
example of Figure 46, with increased noise suppression
and more protection for remote applications.
Figure 48 shows an example of gain in the excitation circuit and remote feedback from the bridge. The LTC1043’s
provide voltage multiplication, providing ±10V from a 5V
reference with only 1ppm error. The amplifiers are used at
unity-gain and, hence, introduce a very little error due to
gain error or due to offset voltages. A 1µV/°C offset voltage
drift translates into 0.05ppm/°C gain error. Simpler alternatives, with the amplifiers providing gain using resistor
arrays for feedback, can produce results that are similar
to bridge sensing schemes via attenuators. Note that the
amplifiers must have high open-loop gain or gain error
The use of a third resistor in the half bridge, between the
variable and fixed elements gives essentially the same
result as the two resistor version, but has a few benefits.
If, for example, a 25k reference resistor is used to set the
excitation current with a 100Ω RTD, the negative reference
input is sampling the same external node as the positive
input, but may result in errors if used with a long cable.
For short cable applications, the errors may be acceptably
low. If instead the single 25k resistor is replaced with a
10k 5% and a 10k 0.1% reference resistor, the noise level
5VREF
0.1µF
5V
3
2
350Ω
BRIDGE
8
+
1
U1A
–
2
4
15
14
4
1
RN1
16
6
11
7
2
6
5
10
8
3
5
12
3
U1B
U2A
1
3
4
+
4
5
REF +
VCC
REF –
SDO
SCK
IN +
CS
12
13
11
13
7
5
+
2
8
–
9
6
–
0.1µF
0.1µF
5V
LTC2413
–
U2B
7
6
IN –
+
RN1 = 5k × 8 RESISTOR ARRAY
U1A, U1B, U2A, U2B = 1/2 LTC1051
GND
FO
14
1, 7, 8, 9,
10, 15, 16
2413 F44
Figure 44. Using Autozero Amplifiers to Reduce Input Referred Noise
2413fa
For more information www.linear.com/LTC2413
39
LTC2413
Applications Information
The circuits in Figures 48 and 50 could be used where
multiple bridge circuits are involved and bridge output can
be multiplexed onto a single LTC2413, via an inexpensive
multiplexer such as the 74HC4052.
will be a source of error. The fact that input offset voltage
has relatively little effect on overall error may lead one to
use low performance amplifiers for this application. Note
that the gain of a device such as an LF156, (25V/mV over
temperature) will produce a worst-case error of –180ppm
at a noise gain of 3, such as would be encountered in an
inverting gain of 2, to produce –10V from a 5V reference.
Figure 49 shows the use of an LTC2413 with a differential
multiplexer. This is an inexpensive multiplexer that will
contribute some error due to leakage if used directly with
the output from the bridge, or if resistors are inserted as
a protection mechanism from overvoltage. Although the
bridge output may be within the input range of the A/D and
multiplexer in normal operation, some thought should be
given to fault conditions that could result in full excitation
voltage at the inputs to the multiplexer or ADC. The use of
amplification prior to the multiplexer will largely eliminate
errors associated with channel leakage developing error
voltages in the source impedance.
The error associated with the 10V excitation would be
–80ppm. Hence, overall reference error could be as high
as 130ppm, the average of the two.
Figure 50 shows a similar scheme to provide excitation
using resistor arrays to produce precise gain. The circuit
is configured to provide 10V and –5V excitation to the
bridge, producing a common mode voltage at the input to
the LTC2413 of 2.5V, maximizing the AC input range for
applications where induced 60Hz could reach amplitudes
up to 2VRMS.
10µF
5V
+
0.1µF
5V
350Ω
BRIDGE
3
+
2
1µF
+
3
LTC1050S8
–
R1
4.99k
2
0.1µV
7
6
175Ω
1µF
4
4
+
20k
5
20k
6
REF +
VCC
REF –
IN +
R2
46.4k
LTC2413
IN –
GND
1, 7, 8, 9,
10, 15, 16
AV = 9.95 =
(
R1 + R2
R1 + 175Ω
)
2413 F45
Figure 45. Bridge Amplification Using a Single Amplifier
40
2413fa
For more information www.linear.com/LTC2413
LTC2413
Applications Information
VS
2.7V TO 5.5V
2
3
R1
25.5k
0.1%
4
REF +
VCC
REF –
LTC2413
PLATINUM
100Ω
RTD
5
IN +
6
IN –
GND
1, 7, 8, 9,
10, 15, 16
2413 F46
Figure 46. Remote Half Bridge Interface
5V
R2
10k
0.1%
R1
10k, 5%
5V
R3
10k
5%
+
1µF
LTC1050
2
3
560Ω
4
–
PLATINUM
100Ω
RTD
REF +
VCC
REF –
LTC2413
10k
5
IN +
10k
6
IN –
GND
1, 7, 8, 9,
10, 15, 16
2413 F47
Figure 47. Remote Half Bridge Sensing with Noise Suppression on Reference
2413fa
For more information www.linear.com/LTC2413
41
LTC2413
Applications Information
15V
15V
U1
4
LTC1043
15V
7
20Ω
Q1
2N3904
6
+
LTC1150
4
–
10V
3
200Ω
2
1µF
47µF
11
LT1236-5
+
+
10V
0.1µF
12
14
13
10µF
17
350Ω
BRIDGE
+
0.1µF
1k
5V
7
*
–15V
33Ω
8
5V
0.1µF
2
VCC
3
4
33Ω
5
6
U2
LTC1043
15V
Q2
2N3906
20Ω
7
6
+
LTC1150
4
–15V
–
3
REF +
REF –
IN +
IN –
GND
5
1, 7, 8, 9,
10, 15, 16
6
*
2
2
3
–15V
1k
LTC2413
15
18
0.1µF
U2
LTC1043
*FLYING CAPACITORS ARE
1µF FILM (MKP OR EQUIVALENT)
5V
4
8
7
SEE LTC1043 DATA SHEET FOR
DETAILS ON UNUSED HALF OF U1
*
11
1µF
FILM
12
200Ω
13
14
–10V
17
–10V
2413 F48
Figure 48. LTC1043 Provides Precise 4X Reference for Excitation Voltages
42
2413fa
For more information www.linear.com/LTC2413
LTC2413
Applications Information
5V
5V
+
16
12
14
15
11
3
REF +
4
REF –
5
2
13
5
3
6
IN +
IN –
6
4
8
9
2
VCC
LTC2413
74HC4052
1
TO OTHER
DEVICES
47µF
10
GND
1, 7, 8, 9,
10, 15, 16
A0
A1
2413 F49
Figure 49. Use a Differential Multiplexer to Expand Channel Capability
2413fa
For more information www.linear.com/LTC2413
43
LTC2413
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
GN Package
16-Lead Plastic SSOP (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1641 Rev B)
.189 – .196*
(4.801 – 4.978)
.045 ±.005
16 15 14 13 12 11 10 9
.254 MIN
.009
(0.229)
REF
.150 – .165
.229 – .244
(5.817 – 6.198)
.0165 ±.0015
.150 – .157**
(3.810 – 3.988)
.0250 BSC
RECOMMENDED SOLDER PAD LAYOUT
1
.015 ±.004
× 45°
(0.38 ±0.10)
.007 – .0098
(0.178 – 0.249)
.0532 – .0688
(1.35 – 1.75)
2 3
4
5 6
7
8
.004 – .0098
(0.102 – 0.249)
0° – 8° TYP
.016 – .050
(0.406 – 1.270)
.008 – .012
(0.203 – 0.305)
TYP
NOTE:
1. CONTROLLING DIMENSION: INCHES
INCHES
2. DIMENSIONS ARE IN
(MILLIMETERS)
.0250
(0.635)
BSC
GN16 REV B 0212
3. DRAWING NOT TO SCALE
4. PIN 1 CAN BE BEVEL EDGE OR A DIMPLE
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
44
2413fa
For more information www.linear.com/LTC2413
LTC2413
Revision History
REV
DATE
DESCRIPTION
A
08/15
Updated fEOSC maximum to 500KHz and all associated information
PAGE NUMBER
6, 7, 8, 10,
31, 32
2413fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection
of its circuits
as described
herein will not infringe on existing patent rights.
For more
information
www.linear.com/LTC2413
45
LTC2413
Typical Application
15V
20Ω
Q1
2N3904
+
1/2
LT1112
1
–
C1
0.1µF
22Ω
5V
3
+
LT1236-5
C3
47µF
2
C1
0.1µF
RN1
10k
10V
1
2
3
4
350Ω BRIDGE
TWO ELEMENTS
VARYING
5V
2
RN1
10k
VCC
3
4
5
–5V
6
8
RN1
10k
5
7
6
20Ω
7
REF +
REF –
IN +
IN –
GND
1, 7, 8, 9,
10, 15, 16
15V
C2
0.1µF
33Ω
×2
Q2, Q3
2N3906
×2
RN1
10k
LTC2413
8
–
6
+
5
1/2
LT1112
4
RN1 IS CADDOCK T914 10K-010-02
–15V
–15V
2413 F50
Figure 50. Use Resistor Arrays to Provide Precise Matching in Excitation Amplifier
Related Parts
PART NUMBER
DESCRIPTION
COMMENTS
LT1019
Precision Bandgap Reference, 2.5V, 5V
3ppm/°C Drift, 0.05% Max Initial Accuracy
LT1025
Micropower Thermocouple Cold Junction Compensator
80µA Supply Current, 0.5°C Initial Accuracy
LTC1043
Dual Precision Instrumentation Switched Capacitor
Building Block
Precise Charge, Balanced Switching, Low Power
LTC1050
Precision Chopper Stabilized Op Amp
No External Components 5µV Offset, 1.6µVP-P Noise
LT1236A-5
Precision Bandgap Reference, 5V
0.05% Max Initial Accuracy, 5ppm/°C Drift
LT1460
Micropower Series Reference
0.075% Max, 10ppm/°C Max Drift
LTC2400
24-Bit, No Latency ∆∑ ADC in SO-8
0.3ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200µA
LTC2401/LTC2402
1-/2-Channel, 24-Bit, No Latency ∆∑ ADC in MSOP
0.6ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200µA
LTC2404/LTC2408
4-/8-Channel, 24-Bit, No Latency ∆∑ ADC
0.3ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200µA
LTC2410
24-Bit, Fully Differential, No Latency ∆∑ ADC in SSOP-16
0.16ppm Noise, 2ppm INL, 3ppm Total Unadjusted Error, 200µA
LTC2411
24-Bit, Fully Differential, No Latency ∆∑ ADC in MS10
0.29ppm Noise, 2ppm INL, 3ppm Total Unadjusted Error, 200µA
LTC2415
24-Bit, Fully Differential, ∆∑ ADC
15Hz Output Rate at 60Hz Rejection, Pin Compatible with LTC2410
LTC2420
20-Bit, No Latency ∆∑ ADC in SO-8
1.2ppm Noise, 8ppm INL, Pin Compatible with LTC2400
LTC2424/LTC2428
4-/8-Channel, 20-Bit, No Latency ∆∑ ADC
1.2ppm Noise, 8ppm INL, Pin Compatible with LTC2404/LTC2408
46 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LTC2413
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LTC2413
2413fa
LT 0815 REV A • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2000
工商网监
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