LTC2400
24-Bit µPower
No Latency ∆Σ ADC in SO-8
TM
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FEATURES
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DESCRIPTIO
24-Bit ADC in SO-8 Package
4ppm INL, No Missing Codes
4ppm Full-Scale Error
Single Conversion Settling Time
for Multiplexed Applications
0.5ppm Offset
0.3ppm Noise
Internal Oscillator—No External Components
Required
110dB Min, 50Hz/60Hz Notch Filter
Reference Input Voltage: 0.1V to VCC
Live Zero—Extended Input Range Accommodates
12.5% Overrange and Underrange
Single Supply 2.7V to 5.5V Operation
Low Supply Current (200µA) and Auto Shutdown
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APPLICATIO S
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The LTC®2400 is a 2.7V to 5.5V micropower 24-bit
converter with an integrated oscillator, 4ppm INL and
0.3ppm RMS noise. It uses delta-sigma technology and
provides single cycle settling time for multiplexed applications. Through a single pin the LTC2400 can be configured for better than 110dB rejection at 50Hz or 60Hz ±2%,
or it can be driven by an external oscillator for a user
defined rejection frequency in the range 1Hz to 120Hz.
The internal oscillator requires no external frequency
setting components.
The converter accepts any external reference voltage from
0.1V to VCC. With its extended input conversion range of
–12.5% VREF to 112.5% VREF, the LTC2400 smoothly
resolves the offset and overrange problems of preceding
sensors or signal conditioning circuits.
The LTC2400 communicates through a flexible 3-wire
digital interface which is compatible with SPI and
MICROWIRETM protocols.
Weight Scales
Direct Temperature Measurement
Gas Analyzers
Strain-Gage Transducers
Instrumentation
Data Acquisition
Industrial Process Control
6-Digit DVMs
, LTC and LT are registered trademarks of Linear Technology Corporation.
No Latency ∆Σ is a trademark of Linear Technology Corporation.
MICROWIRE is a trademark of National Semiconductor Corporation.
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TYPICAL APPLICATIO
Total Unadjusted Error vs Output Code
10
2.7V TO 5.5V
VCC
FO
= INTERNAL OSC/50Hz REJECTION
= EXTERNAL CLOCK SOURCE
= INTERNAL OSC/60Hz REJECTION
LTC2400
REFERENCE
VOLTAGE
0.1V TO VCC
ANALOG
INPUT RANGE
–0.12VREF TO 1.12VREF
VREF
SCK
VIN
SDO
GND
3-WIRE
SPI INTERFACE
CS
LINEARITY ERROR (ppm)
1µF
VCC = 5V
VREF = 5V
TA = 25°C
FO = LOW
8
VCC
6
4
2
0
–2
–4
–6
–8
2400 TA01
–10
0
8,338,608
OUTPUT CODE (DECIMAL)
16,777,215
2400 TA02
1
LTC2400
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ABSOLUTE MAXIMUM RATINGS
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PACKAGE/ORDER INFORMATION
(Notes 1, 2)
Supply Voltage (VCC) to GND .......................– 0.3V to 7V
Analog Input Voltage to GND ....... – 0.3V to (VCC + 0.3V)
Reference Input 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
LTC2400C ............................................... 0°C to 70°C
LTC2400I ............................................ – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART NUMBER
TOP VIEW
LTC2400CS8
LTC2400IS8
VCC 1
8
FO
VREF 2
7
SCK
VIN 3
6
SDO
GND 4
5
CS
S8 PART MARKING
S8 PACKAGE
8-LEAD PLASTIC SO
2400
2400I
TJMAX = 125°C, θJA = 130°C/W
Consult factory for Military grade parts.
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CONVERTER CHARACTERISTICS The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Resolution (No Missing Codes)
0.1V ≤ VREF ≤ VCC, (Note 5)
●
Integral Nonlinearity
VREF = 2.5V (Note 6)
VREF = 5V (Note 6)
●
●
2
4
10
15
ppm of VREF
ppm of VREF
Offset Error
2.5V ≤ VREF ≤ VCC
●
0.5
2
ppm of VREF
24
Bits
Offset Error Drift
2.5V ≤ VREF ≤ VCC
Full-Scale Error
2.5V ≤ VREF ≤ VCC
Full-Scale Error Drift
2.5V ≤ VREF ≤ VCC
Total Unadjusted Error
VREF = 2.5V
VREF = 5V
5
10
ppm of VREF
ppm of VREF
Output Noise
VIN = 0V (Note 13)
1.5
µVRMS
Normal Mode Rejection 60Hz ±2%
(Note 7)
●
110
130
dB
Normal Mode Rejection 50Hz ±2%
(Note 8)
●
110
130
dB
Power Supply Rejection, DC
VREF = 2.5V, VIN = 0V
100
dB
Power Supply Rejection, 60Hz ±2%
VREF = 2.5V, VIN = 0V, (Notes 7, 15)
110
dB
Power Supply Rejection, 50Hz ±2%
VREF = 2.5V, VIN = 0V, (Notes 8, 15)
110
dB
0.01
4
●
ppm of VREF/°C
10
0.02
ppm of VREF
ppm of VREF/°C
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A ALOG I PUT A D REFERE CE
The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
CONDITIONS
VIN
Input Voltage Range
(Note 14)
VREF
Reference Voltage Range
CS(IN)
Input Sampling Capacitance
CS(REF)
Reference Sampling Capacitance
IIN(LEAK)
Input Leakage Current
CS = VCC
●
–10
1
10
nA
IREF(LEAK)
Reference Leakage Current
VREF = 2.5V, CS = VCC
●
– 10
1
10
nA
2
MIN
TYP
MAX
UNITS
●
– 0.125 • VREF
1.125 • VREF
V
●
0.1
VCC
V
10
pF
15
pF
LTC2400
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DIGITAL I PUTS A D DIGITAL OUTPUTS
The ● denotes specifications which apply over the full
operating temperature range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
CONDITIONS
MIN
VIH
High Level Input Voltage
CS, FO
2.7V ≤ VCC ≤ 5.5V
2.7V ≤ VCC ≤ 3.3V
●
VIL
Low Level Input Voltage
CS, FO
4.5V ≤ VCC ≤ 5.5V
2.7V ≤ VCC ≤ 5.5V
●
VIH
High Level Input Voltage
SCK
2.7V ≤ VCC ≤ 5.5V (Note 9)
2.7V ≤ VCC ≤ 3.3V (Note 9)
●
VIL
Low Level Input Voltage
SCK
4.5V ≤ VCC ≤ 5.5V (Note 9)
2.7V ≤ VCC ≤ 5.5V (Note 9)
●
IIN
Digital Input Current
CS, FO
0V ≤ VIN ≤ VCC
●
IIN
Digital Input Current
SCK
0V ≤ VIN ≤ VCC (Note 9)
●
CIN
Digital Input Capacitance
CS, FO
CIN
Digital Input Capacitance
SCK
(Note 9)
VOH
High Level Output Voltage
SDO
IO = – 800µA
●
VOL
Low Level Output Voltage
SDO
IO = 1.6mA
●
VOH
High Level Output Voltage
SCK
IO = – 800µA (Note 10)
●
VOL
Low Level Output Voltage
SCK
IO = 1.6mA (Note 10)
●
IOZ
High-Z Output Leakage
SDO
●
TYP
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
10
pF
10
pF
VCC – 0.5V
V
0.4V
V
VCC – 0.5V
V
–10
0.4V
V
10
µA
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POWER REQUIRE E TS
The ● denotes 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
●
CS = 0V (Note 12)
CS = VCC (Note 12)
●
●
TYP
2.7
200
20
MAX
UNITS
5.5
V
300
30
µA
µA
3
LTC2400
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TI I G CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL
PARAMETER
CONDITIONS
fEOSC
External Oscillator Frequency Range
●
tHEO
External Oscillator High Period
●
tLEO
External Oscillator Low Period
●
tCONV
Conversion Time
FO = 0V
FO = VCC
External Oscillator (Note 11)
fISCK
Internal SCK Frequency
Internal Oscillator (Note 10)
External Oscillator (Notes 10, 11)
DISCK
Internal SCK Duty Cycle
(Note 10)
●
fESCK
External SCK Frequency Range
(Note 9)
●
tLESCK
External SCK Low Period
(Note 9)
●
250
ns
tHESCK
External SCK High Period
(Note 9)
●
250
ns
tDOUT_ISCK
Internal SCK 32-Bit Data Output Time
Internal Oscillator (Notes 10, 12)
External Oscillator (Notes 10, 11)
●
●
1.64
tDOUT_ESCK
External SCK 32-Bit Data Output Time
(Note 9)
●
t1
CS ↓ to SDO Low Z
t2
CS ↑ to SDO High Z
t3
CS ↓ to SCK ↓
(Note 10)
t4
CS ↓ to SCK ↑
(Note 9)
tKQMAX
SCK ↓ to SDO Valid
tKQMIN
SDO Hold After SCK ↓
t5
t6
●
●
●
MAX
UNITS
2.56
307.2
kHz
0.5
390
µs
0.5
390
µs
TYP
130.66
133.33
136
156.80
160
163.20
20480/fEOSC (in kHz)
19.2
fEOSC/8
45
ms
ms
ms
kHz
kHz
55
%
2000
kHz
1.67
1.70
256/fEOSC (in kHz)
ms
ms
32/fESCK (in kHz)
ms
●
0
150
ns
●
0
150
ns
●
0
150
ns
●
50
ns
200
●
(Note 5)
ns
●
15
ns
SCK Set-Up Before CS ↓
●
50
ns
SCK Hold After CS ↓
●
Note 1: Absolute Maximum Ratings are those values beyond which the
life of the device may be impaired.
Note 2: All voltage values are with respect to GND.
Note 3: VCC = 2.7 to 5.5V unless otherwise specified.
Note 4: Internal Conversion Clock source with the FO pin tied
to GND or to VCC or to external conversion clock source with
fEOSC = 153600Hz 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.
Note 7: FO = 0V (internal oscillator) or fEOSC = 153600Hz ±2%
(external oscillator).
Note 8: FO = VCC (internal oscillator) or fEOSC = 128000Hz ±2%
(external oscillator).
Note 9: 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.
4
MIN
50
ns
Note 10: The converter is in internal SCK mode of operation such that
the SCK pin is used as digital output. In this mode of operation the
SCK pin has a total equivalent load capacitance CLOAD = 20pF.
Note 11: The external oscillator is connected to the FO pin. The external
oscillator frequency, fEOSC, is expressed in kHz.
Note 12: The converter uses the internal oscillator.
FO = 0V or FO = VCC.
Note 13: The output noise includes the contribution of the internal
calibration operations.
Note 14: For reference voltage values VREF > 2.5V the extended input
of – 0.125 • VREF to 1.125 • VREF is limited by the absolute maximum
rating of the Analog Input Voltage pin (Pin 3). For 2.5V < VREF ≤
0.267V + 0.89 • VCC the input voltage range is – 0.3V to 1.125 • VREF.
For 0.267V + 0.89 • VCC < VREF ≤ VCC the input voltage range is – 0.3V
to VCC + 0.3V.
Note 15: The DC voltage at VCC = 4.1V, and the AC voltage applied to
VCC is 2.8VP-P
LTC2400
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TYPICAL PERFOR A CE CHARACTERISTICS
Total Unadjusted Error
(3V Supply)
10
10
VCC = 3V
VREF = 3V
10
VCC = 3V
VREF = 3V
5
0
TA = –55°C, –45°C, 25°C, 90°C
–5
TA = –55°C, –45°C, 25°C, 90°C
0
–5
–10
0.5
1.0
1.5
2.0
INPUT VOLTAGE (V)
2.5
3.0
TA = – 45°C
0.5
1.0
1.5
2.0
INPUT VOLTAGE (V)
2.5
3.0
TA = – 55°C
0
– 0.05 – 0.10 – 0.15 – 0.20 – 0.25 – 0.30
INPUT VOLTAGE (V)
2400 G03
Total Unadjusted Error
(5V Supply)
INL (5V Supply)
10
10
10
VCC = 5V
8 VREF = 5V
VCC = 3V
VREF = 3V
6
5
VCC = 5V
VREF = 5V
5
TA = – 45°C
TA = 25°C
2
0
–2
TA = –55°C, –45°C, 25°C, 90°C
–4
–5
ERROR (ppm)
4
TA = – 55°C
ERROR (ppm)
ERROR (ppm)
0
2400 G02
Positive Input Extended Total
Unadjusted Error (3V Supply)
TA = 90°C
TA = 25°C
–10
0
2400 G01
0
TA = 90°C
–5
–10
0
VCC = 3V
VREF = 3V
5
ERROR (ppm)
ERROR (ppm)
5
ERROR (ppm)
Negative Input Extended Total
Unadjusted Error (3V Supply)
INL (3V Supply)
0
TA = –55°C, –45°C, 25°C, 90°C
–5
–6
–8
–10
–10
3.1
3.2
INPUT VOLTAGE (V)
3.3
1
0
3
2
INPUT VOLTAGE (V)
4
VCC = 5V
VREF = 5V
TA = 90°C
ERROR (ppm)
ERROR (ppm)
TA = – 55°C
TA = – 45°C
0
TA = – 45°C
TA = 90°C
TA = 25°C
–5
–5
4
5
VCC = 5V
TA = 25°C
5
5
5
0
3
2
INPUT VOLTAGE (V)
Offset Error vs Reference Voltage
6
10
TA = 25°C
1
2400 G06
Positive Input Extended Total
Unadjusted Error (5V Supply)
Negative Input Extended Total
Unadjusted Error (5V Supply)
VCC = 5V
VREF = 5V
0
2400 G05
2400 G04
10
–10
5
OFFSET ERROR (ppm)
3.0
4
3
2
1
TA = – 55°C
0
–1
–10
–10
0
– 0.05 – 0.10 – 0.15 – 0.20 – 0.25 – 0.30
INPUT VOLTAGE (V)
2400 G07
5.0
5.1
5.2
INPUT VOLTAGE (V)
5.3
2400 G08
0
1
2
3
4
REFERENCE VOLTAGE
5
2400 G09
5
LTC2400
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TYPICAL PERFOR A CE CHARACTERISTICS
RMS Noise vs Reference Voltage
Offset Error vs VCC
5.0
20
10
5
0
1
0
3
4
2
REFERENCE VOLTAGE (V)
– 5.0
2.7
5
3.2
3.7
4.2
4.7
500
0.50
0
1.5
0
10.0
Full-Scale Error vs VCC
6
VCC = 5V
VIN = VREF
FULL-SCALE ERROR (ppm)
7.5
5.0
2.5
4
3
2
1
– 5.0
– 55 – 30
– 5 20 45 70
TEMPERATURE (°C)
95
120
2400 G15
6
120
5
FULL-SCALE ERROR (ppm)
– 2.5
95
2400 G13
Full-Scale Error
vs Reference Voltage
VCC = 5V
VREF = 5V
VIN = 5V
0
– 5 20 45 70
TEMPERATURE (°C)
2400 G18
Full-Scale Error vs Temperature
2.5
0
– 5.0
– 55 – 30
FFFFFF
7FFFFF
CODE OUT (HEX)
2400 G14
5.0
VCC = 5V
VREF = 5V
VIN = 0V
– 2.5
0.25
0
0.5
1.0
OUTPUT CODE (ppm)
5.2
4.7
2.5
OFFSET ERROR (ppm)
VCC = 5V
VREF = 5V
VIN = – 0.3V TO 5.3V
TA = 25°C
0.75
1000
4.2
VCC
Offset Error vs Temperature
5.0
1.00
– 0.5
3.7
2400 G12
RMS Noise vs Code Out
VCC = 5V
VREF = 5V
VIN = 0V
0
–1.0
3.2
2400 G11
RMS NOISE (ppm)
NUMBER OF READINGS
0
2.7
5.2
VCC
Noise Histogram
FULL-SCALE ERROR (ppm)
2.5
– 2.5
2400 G10
1500
VREF = 2.5V
TA = 25°C
2.5
RMS NOISE (ppm)
15
OFFSET ERROR (ppm)
RMS NOISE (ppm OF VREF)
5.0
VREF = 2.5V
TA = 25°C
VCC = 5V
TA = 25°C
0
RMS Noise vs VCC
0
0
1
3
4
2
REFERENCE VOLTAGE (V)
5
2400 G16
VREF = 2.5V
VIN = 2.5V
TA = 25°C
0
2.7
3.2
3.7
4.2
VCC
4.7
5.2
2400 G17
LTC2400
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TYPICAL PERFOR A CE CHARACTERISTICS
Sleep Current vs Temperature
Conversion Current vs Temperature
220
VCC = 5.5V
SUPPLY CURRENT (µA)
SUPPLY CURRENT (µA)
210
VCC = 4.1V
200
190
VCC= 2.7V
180
170
0
25
–20
VCC = 2.7V, 5.5V
20
15
10
150
–55 –30
70
45
20
TEMPERATURE (°C)
–5
95
45
20
70
–5
TEMPERATURE (°C)
95
PSRR vs Frequency at VCC
–70
–90
–110
–130
50
100
150
200
FREQUENCY AT VCC (Hz)
250
Rejection vs Frequency at VIN
VCC = 4.1V
VIN = 0V
TA = 25°C
FO = 0
–60
–80
–40
–60
–80
–100
–120
15200 15250 15300 15350 15400 15450 15500
FREQUENCY AT VCC (Hz)
–120
1
50
100
150
200
FREQUENCY AT VIN (Hz)
Rejection vs Frequency at VIN
–20
REJECTION (dB)
–80
Rejection vs Frequency at VIN
0
VCC = 5V
VREF = 5V
VIN = 2.5V
FO = 0
–20
–40
–40
REJECTION (dB)
–70
250
2400 G24
1635 G22
0
–60
–80
–60
–80
–100
–120
–100
–130
VCC = 5V
VREF = 5V
VIN = 2.5V
FO = 0
–20
–40
Rejection vs Frequency at VIN
–110
100
10k
1M
FREQUENCY AT VCC (Hz)
2400 G23
–100
–60
–100
153,600Hz
0
2400 G21
–90
15,360Hz
1
120
REJECTION (dB)
REJECTION (dB)
REJECTION (dB)
–20
–50
0
–80
PSRR vs Frequency at VCC
0
VCC = 4.1V
VIN = 0V
TA = 25°C
F0 = 0
–30
–60
2400 G20
2400 G19
–10
–40
–120
0
–55 –30
120
VCC = 4.1V
VIN = 0V
TA = 25°C
FO = 0
–100
5
160
REJECTION (dB)
PSRR vs Frequency at VCC
30
REJECTION (dB)
230
–120
SAMPLE RATE = 15.36kHz ± 2%
–140
–120
15100
–12
–8
–4
0
4
8
12
INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%)
2400 G25
15200
15300
15400
FREQUENCY AT VIN (Hz)
15500
–140
0
fS/2
fS
INPUT FREQUENCY
2400 G26
2400 F26
7
LTC2400
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TYPICAL PERFOR A CE CHARACTERISTICS
INL vs Output Rate
VCC = 5V
VREF = 5V
TA = 25°C
F0 = EXTERNAL
22
INL (BITS)
20
18
16
14
24
20
18
16
14
12
12
10
10
8
VCC = 5V
VREF = 5V
TA = 25°C
FO = EXTERNAL
22
RESOLUTION (BITS)*
24
Resolution vs Output Rate
*RESOLUTION =
8
0
5 10 15 20 25 30 35 40 45 50 55 60
OUTPUT RATE (Hz)
2400 G27
0
LOG(VREF/RMS NOISE)
LOG (2)
5 10 15 20 25 30 35 40 45 50 55 60
OUTPUT RATE (Hz)
2400 G28
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PIN FUNCTIONS
VCC (Pin 1): Positive Supply Voltage. Bypass to GND
(Pin␣ 4) with a 10µF tantalum capacitor in parallel with
0.1µF ceramic capacitor as close to the part as possible.
VREF (Pin 2): Reference Input. The reference voltage range
is 0.1V to VCC.
VIN (Pin 3): Analog Input. The input voltage range is
– 0.125 • VREF to 1.125 • VREF. For VREF > 2.5V, the input
voltage range may be limited by the pin absolute maximum rating of – 0.3V to VCC + 0.3V.
GND (Pin 4): Ground. Shared pin for analog ground,
digital ground, reference ground and signal ground. Should
be connected directly to a ground plane through a minimum length trace or it should be the single-point-ground
in a single point grounding system.
CS (Pin 5): 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 on CS wakes up the ADC. A
LOW-to-HIGH transition on this pin disables the SDO
digital output. A LOW-to-HIGH transition on CS during the
Data Output transfer aborts the data transfer and starts a
new conversion.
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SDO (Pin 6): Three-State Digital Output. During the data
output period, this pin is used for 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 can be used as a conversion status output.
The conversion status can be observed by pulling CS LOW.
SCK (Pin 7): 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. A
weak internal pull-up is automatically activated in Internal
Serial Clock Operation mode. The Serial Clock mode is
determined by the level applied to SCK at power up and the
falling edge of CS.
FO (Pin 8): Frequency Control Pin. Digital input that
controls the ADC’s notch frequencies and conversion
time. When the FO pin is connected to VCC (FO = VCC), the
converter uses its internal oscillator and the digital filter
first null is located at 50Hz. When the FO pin is connected
to GND (FO = OV), the converter uses its internal oscillator
and the digital filter first null is located at 60Hz. When FO
is driven by an external clock signal with a frequency fEOSC,
the converter uses this signal as its clock and the digital
filter first null is located at a frequency fEOSC/2560.
LTC2400
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OSCILLATOR
VCC
GND
VIN
AUTOCALIBRATION
AND CONTROL
∫
∫
FO
(INT/EXT)
∫
∑
SDO
SERIAL
INTERFACE
ADC
SCK
CS
VREF
DECIMATING FIR
DAC
2400 FD
TEST CIRCUITS
VCC
3.4k
SDO
3.4k
CLOAD = 20pF
SDO
CLOAD = 20pF
HI-Z TO VOL
VOH TO VOL
VOL TO HI-Z
2400 TA03
2400 TA04
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VOH TO HI-Z
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Converter Operation Cycle
The LTC2400 is a low power, delta-sigma analog-todigital converter with an easy to use 3-wire serial interface.
Its operation is simple and made up of three states. The
converter operating cycle begins with the conversion,
followed by a low power sleep state and concluded with
the data output (see Figure 1). The 3-wire interface consists of serial data output (SDO), a serial clock (SCK) and
a chip select (CS).
Initially, the LTC2400 performs a conversion. Once the
conversion is complete, the device enters the sleep state.
While in this sleep state, power consumption is reduced by
CONVERT
SLEEP
1
CS AND
SCK
0
DATA OUTPUT
2400 F01
Figure 1. LTC2400 State Transition Diagram
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an order of magnitude. The part remains in the sleep state
as long as CS is logic HIGH. The conversion result is held
indefinitely in a static shift register while the converter is
in the sleep state.
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 cycle and the
cycle repeats.
Through timing control of the CS and SCK pins, the
LTC2400 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 delta-sigma converters offer over
conventional type converters is an on-chip digital filter
(commonly known as Sinc or Comb filter). For high
resolution, low frequency applications, this filter is typically designed to reject line frequencies of 50 or 60Hz plus
their harmonics. In order to reject these frequencies in
excess of 110dB, a highly accurate conversion clock is
required. The LTC2400 incorporates an on-chip highly
accurate oscillator. This eliminates the need for external
frequency setting components such as crystals or oscillators. Clocked by the on-chip oscillator, the LTC2400
rejects line frequencies (50 or 60Hz ±2%) a minimum of
110dB.
Ease of Use
The LTC2400 data output has no latency, filter settling or
redundant data associated with the conversion cycle.
There is a one-to-one correspondence between the
10
conversion and the output data. Therefore, multiplexing
an analog input voltage is easy.
The LTC2400 performs offset and full-scale calibrations
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 LTC2400 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 which is performed at the initial power-up. (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 duration of approximately 0.5ms. The POR
signal clears all internal registers. Following the POR
signal, the LTC2400 starts a normal conversion cycle and
follows the normal succession of states described above.
The first conversion result following POR is accurate
within the specifications of the device.
Reference Voltage Range
The LTC2400 can accept a reference voltage from 0V to
VCC. The converter output noise is determined by the
thermal noise of the front-end circuits, and as such, its
value in microvolts 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
overall converter INL performance. The recommended
range for the LTC2400 voltage reference is 100mV to VCC.
Input Voltage Range
The converter is able to accommodate system level offset
and gain errors as well as system level overrange situations due to its extended input range, see Figure 2. The
LTC2400 converts input signals within the extended input
range of – 0.125 • VREF to 1.125 • VREF.
LTC2400
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VCC + 0.3V
9/8VREF
VREF
1/2VREF
NORMAL
INPUT
RANGE
EXTENDED
INPUT
RANGE
ABSOLUTE
MAXIMUM
INPUT
RANGE
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.
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.
0
–1/8VREF
–0.3V
2400 F02
Figure 2. LTC2400 Input Range
For large values of VREF this range is limited by the
absolute maximum voltage range of – 0.3V to (VCC + 0.3V).
Beyond this range the input ESD protection devices begin
to turn on and the errors due to the input leakage current
increase rapidly.
Bit 29 (third output bit) is the conversion result sign indicator (SIG). If VIN is >0, this bit is HIGH. If VIN is VREF or VIN < 0, this bit is HIGH.
The function of these bits is summarized in Table 1.
Table 1. LTC2400 Status Bits
Bit 31
EOC
Bit 30
DMY
Bit 29
SIG
Bit 28
EXR
VIN > VREF
0
0
1
1
0 < VIN ≤ VREF
0
0
1
0
0
0
1/0
0
0
0
0
1
Input Range
Input signals applied to VIN may extend below ground by
– 300mV and above VCC by 300mV. In order to limit any
fault current, a resistor of up to 5k may be added in series
with the VIN pin without affecting the performance of the
device. In the physical layout, it is important to maintain
the parasitic capacitance of the connection between this
series resistance and the VIN pin as low as possible;
therefore, the resistor should be located as close as
practical to the VIN pin. The effect of the series resistance
on the converter accuracy can be evaluated from the
curves presented in the Analog Input/Reference Current
section. In addition a series resistor 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 LTC2400 serial output data stream is 32 bits long. The
first 4 bits represent status information indicating the
sign, input range and conversion state. The next 24 bits are
the conversion result, MSB first. The remaining 4 bits are
sub LSBs beyond the 24-bit level that may be included in
averaging or discarded without loss of resolution.
VIN =
0+/0 –
VIN < 0
Bit 27 (fifth output bit) is the most significant bit (MSB).
Bits 27-4 are the 24-bit conversion result MSB first.
Bit 4 is the least significant bit (LSB).
Bits 3-0 are sub LSBs below the 24-bit level. Bits 3-0 may
be included in averaging or discarded without loss of
resolution.
Data is shifted out of the SDO pin under control of the serial
clock (SCK), see Figure 3. Whenever CS is HIGH, SDO
remains high impedance and any SCK clock pulses are
ignored by the internal data out shift register.
In order to shift the conversion result out of the device, CS
must first be driven LOW. EOC is seen at the SDO pin of the
device once CS is pulled LOW. EOC changes real time from
HIGH to LOW at the completion of a conversion. This
signal may be used as an interrupt for an external
microcontroller. Bit 31 (EOC) can be captured on the first
rising edge of SCK. Bit 30 is shifted out of the device on the
first falling edge of SCK. The final data bit (Bit 0) is shifted
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out on the falling edge of the 31st SCK and may be latched
on the rising edge of the 32nd SCK pulse. On the falling
edge of the 32nd SCK pulse, SDO goes HIGH indicating a
new conversion cycle has been initiated. This bit serves as
EOC (Bit 31) for the next conversion cycle. Table 2 summarizes the output data format.
As long as the voltage on the VIN pin is maintained within
the – 0.3V to (VCC + 0.3V) absolute maximum operating
range, a conversion result is generated for any input value
from – 0.125 • VREF to 1.125 • VREF. For input voltages
greater than 1.125 • VREF, the conversion result is clamped
to the value corresponding to 1.125 • VREF. For input
voltages below – 0.125 • VREF, the conversion result is
clamped to the value corresponding to – 0.125 • VREF.
Frequency Rejection Selection (FO Pin Connection)
The LTC2400 internal oscillator provides better than 110dB
normal mode rejection at the line frequency and all its
harmonics for 50Hz ±2% or 60Hz ±2%. For 60Hz rejection, FO (Pin 8) should be connected to GND (Pin 4) while
for 50Hz rejection the FO pin should be connected to VCC
(Pin␣ 1).
CS
SDO
BIT 31
BIT 30
BIT 29
BIT 28
BIT 27
BIT 4
EOC
“0”
SIG
EXT
MSB
LSB24
BIT 0
Hi-Z
SCK
1
2
3
4
SLEEP
5
27
28
32
DATA OUTPUT
CONVERSION
2400 F03
Figure 3. Output Data Timing
Table 2. LTC2400 Output Data Format
Bit 31
EOC
Bit 30
DMY
Bit 29
SIG
Bit 28
EXR
Bit 27
MSB
Bit 26
Bit 25
Bit 24
Bit 23
…
Bit 4
LSB
Bit 3-0
SUB LSBs*
VIN > 9/8 • VREF
0
0
1
1
0
0
0
1
1
...
1
X
9/8 • VREF
0
0
1
1
0
0
0
1
1
...
1
X
VREF + 1LSB
0
0
1
1
0
0
0
0
0
...
0
X
Input Voltage
VREF
0
0
1
0
1
1
1
1
1
...
1
X
3/4VREF + 1LSB
0
0
1
0
1
1
0
0
0
...
0
X
3/4VREF
0
0
1
0
1
0
1
1
1
...
1
X
1/2VREF + 1LSB
0
0
1
0
1
0
0
0
0
...
0
X
1/2VREF
0
0
1
0
0
1
1
1
1
...
1
X
1/4VREF + 1LSB
0
0
1
0
0
1
0
0
0
...
0
X
1/4VREF
0
0
1
0
0
0
1
1
1
...
1
X
0+/0 –
0
0
1/0**
0
0
0
0
0
0
...
0
X
–1LSB
0
0
0
1
1
1
1
1
1
...
1
X
–1/8 • VREF
0
0
0
1
1
1
1
0
0
...
0
X
VIN < –1/8 • VREF
0
0
0
1
1
1
1
0
0
...
0
X
*The sub LSBs are valid conversion results beyond the 24-bit level that may be included in averaging or discarded without loss of resolution.
**The sign bit changes state during the 0 code.
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–60
The selection of 50Hz or 60Hz rejection can also be made
by driving FO to an appropriate logic level. A selection
change during the sleep or data output states will not
disturb the converter operation. If the selection is made
during the conversion state, the result of the conversion in
progress may be outside specifications but the following
conversions will not be affected.
–70
REJECTION (dB)
–80
–90
–100
–110
–120
When a fundamental rejection frequency different from
50Hz or 60Hz is required or when the converter must be
synchronized with an outside source, the LTC2400 can
operate with an external conversion clock. The converter
automatically detects the presence of an external clock
signal at the FO pin and turns off the internal oscillator. The
frequency fEOSC of the external signal must be at least
2560Hz (1Hz notch frequency) to be detected. The external clock signal duty cycle is not significant as long as the
minimum and maximum specifications for the high and
low periods tHEO and tLEO are observed.
While operating with an external conversion clock of a
frequency fEOSC, the LTC2400 provides better than 110dB
normal mode rejection in a frequency range fEOSC/2560
±4% and its harmonics. The normal mode rejection as a
function of the input frequency deviation from fEOSC/2560
is shown in Figure 4.
Whenever an external clock is not present at the FO pin, the
converter automatically activates its internal oscillator and
enters the Internal Conversion Clock mode. The LTC2400
–130
–140
–12
–8
–4
0
4
8
12
INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%)
2400 G25
Figure 4. LTC2400 Normal Mode Rejection When
Using an External Oscillator of Frequency fEOSC
operation will not be disturbed if the change of conversion
clock source occurs during the sleep state or during the
data output state while the converter uses an external
serial clock. If the change occurs during the conversion
state, the result of the conversion in progress may be
outside specifications but the following conversions will
not be affected. If the change occurs during the data output
state and the converter is in the Internal SCK mode, the
serial clock duty cycle may be affected but the serial data
stream will remain valid.
Table 3 summarizes the duration of each state as a
function of FO.
Table 3. LTC2400 State Duration
State
Operating Mode
CONVERT
Internal Oscillator
External Oscillator
Duration
FO = LOW
(60Hz Rejection)
133ms
FO = HIGH
(50Hz Rejection)
160ms
FO = External Oscillator
with Frequency fEOSC kHz
(fEOSC/2560 Rejection)
20480/fEOSCs
SLEEP
DATA OUTPUT
As Long As CS = HIGH Until CS = 0 and SCK
Internal Serial Clock
External Serial Clock with
Frequency fSCK kHz
FO = LOW/HIGH
(Internal Oscillator)
As Long As CS = LOW But Not Longer Than 1.67ms
(32 SCK cycles)
FO = External Oscillator with
Frequency fEOSC kHz
As Long As CS = LOW But Not Longer Than 256/fEOSCms
(32 SCK cycles)
As Long As CS = LOW But Not Longer Than 32/fSCKms
(32 SCK cycles)
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SERIAL INTERFACE
The LTC2400 transmits the conversion results and receives the start of conversion command through a synchronous 3-wire interface. During the conversion and
sleep states, this interface can be used to assess the
converter status and during the data output state it is used
to read the conversion result.
Serial Clock Input/Output (SCK)
The serial clock signal present on SCK (Pin 7) is used to
synchronize the data transfer. Each bit of data is shifted out
the SDO pin on the falling edge of the serial clock.
In the Internal SCK mode of operation, the SCK pin is an
output and the LTC2400 creates its own serial clock by
dividing the internal conversion clock by 8. In the External
SCK mode of operation, the SCK pin is used as input. The
internal or external SCK mode is selected on power-up and
then reselected every time a HIGH-to-LOW transition is
detected at the CS pin. If SCK is HIGH or floating at powerup or during this transition, the converter enters the internal SCK mode. If SCK is LOW at power-up or during this
transition, the converter enters the external SCK mode.
Serial Data Output (SDO)
the SDO pin. Once the conversion is complete, EOC goes
LOW. The device remains in the sleep state until the first
rising edge of SCK occurs while CS = 0.
Chip Select Input (CS)
The active LOW chip select, CS (Pin 5), is used to test the
conversion status and to enable the data output transfer as
described in the previous sections.
In addition, the CS signal can be used to trigger a new
conversion cycle before the entire serial data transfer has
been completed. The LTC2400 will abort any serial data
transfer in progress and start a new conversion cycle
anytime a LOW-to-HIGH transition is detected at the CS
pin after the converter has entered the data output state
(i.e., after the first rising edge of SCK occurs with CS = 0).
Finally, CS can be used to control the free-running modes
of operation, see Serial Interface Timing Modes section.
Grounding CS will force the ADC to continuously convert
at the maximum output rate selected by FO. Tying a
capacitor to CS will reduce the output rate and power
dissipation by a factor proportional to the capacitor’s
value, see Figures 12 to 14.
SERIAL INTERFACE TIMING MODES
The serial data output pin, SDO (Pin 6), drives the serial
data during the data output state. In addition, the SDO pin
is used as an end of conversion indicator during the
conversion and sleep states.
When CS (Pin 5) is HIGH, the SDO driver is switched to a
high impedance state. This allows sharing the serial
interface with other devices. If CS is LOW during the
convert or sleep state, SDO will output EOC. If CS is LOW
during the conversion phase, the EOC bit appears HIGH on
The LTC2400’s 3-wire interface is SPI and MICROWIRE
compatible. This interface offers several flexible modes of
operation. These include internal/external serial clock,
2- or 3-wire I/O, single cycle conversion and autostart. The
following sections describe each of these serial interface
timing modes in detail. In all these cases, the converter
can use the internal oscillator (FO = LOW or FO = HIGH) or
an external oscillator connected to the FO pin. Refer to
Table 4 for a summary.
Table 4. LTC2400 Interface Timing Modes
Configuration
SCK
Source
Conversion
Cycle
Control
Data
Output
Control
Connection
and
Waveforms
External SCK, Single Cycle Conversion
External
CS and SCK
CS and SCK
Figures 5, 6
External SCK, 2-Wire I/O
External
SCK
SCK
Figure 7
Internal SCK, Single Cycle Conversion
Internal
CS ↓
CS ↓
Figures 8, 9
Internal SCK, 2-Wire I/O, Continuous Conversion
Internal
Continuous
Internal
Figure 10
Internal SCK, Autostart Conversion
Internal
CEXT
Internal
Figure 11
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out the SDO pin on each falling edge of SCK. This enables
external circuitry to latch the output on the rising edge of
SCK. EOC can be latched on the first rising edge of SCK
and the last bit of the conversion result can be latched on
the 32nd rising edge of SCK. On the 32nd falling edge of
SCK, the device begins a new conversion. SDO goes HIGH
(EOC = 1) indicating a conversion is in progress.
External Serial Clock, Single Cycle Operation
(SPI/MICROWIRE Compatible)
This timing mode uses an external serial clock to shift out
the conversion result and a CS signal to monitor and
control the state of the conversion cycle, see Figure 5.
The serial clock mode is selected on the falling edge of CS.
To select the external serial clock mode, the serial clock pin
(SCK) must be LOW during each CS falling edge.
At the conclusion of the data cycle, CS may remain LOW
and EOC monitored as an end-of-conversion interrupt.
Alternatively, CS may be driven HIGH setting SDO to HI-Z.
As described above, CS may be pulled LOW at any time in
order to monitor the conversion status.
The serial data output pin (SDO) is HI-Z as long as CS is
HIGH. At any time during the conversion cycle, CS may be
pulled LOW in order to monitor the state of the converter.
While CS is pulled LOW, EOC is output to the SDO pin. EOC
= 1 while a conversion is in progress and EOC = 0 if the
device is in the sleep state. Independent of CS, the device
automatically enters the low power sleep state once the
conversion is complete.
Typically, CS remains LOW during the data output state.
However, the data output state may be aborted by pulling
CS HIGH anytime between the first rising edge and the
32nd falling edge of SCK, see Figure 6. On the rising edge
of CS, the device aborts the data output state and immediately initiates a new conversion. This is useful for systems not requiring all 32 bits of output data, aborting an
invalid conversion cycle or synchronizing the start of a
conversion.
When the device is in the sleep state (EOC = 0), its
conversion result is held in an internal static shift register. The device remains in the sleep state until the first
rising edge of SCK is seen while CS is LOW. Data is shifted
2.7V TO 5.5V
VCC
1µF
VCC
= 50Hz REJECTION
= EXTERNAL OSCILLATOR
= 60Hz REJECTION
FO
LTC2400
VREF
0.1V TO VCC
VIN
–0.12VREF TO 1.12VREF
VREF
SCK
VIN
SDO
GND
CS
CS
TEST EOC
TEST EOC
SDO
BIT 31
EOC
Hi-Z
BIT 30
BIT 29
BIT 28
BIT 27
SIG
EXR
MSB
BIT 26
Hi-Z
BIT 4
BIT 0
LSB
SUB LSB
TEST EOC
Hi-Z
SCK
(EXTERNAL)
CONVERSION
SLEEP
DATA OUTPUT
CONVERSION
2400 F05
Figure 5. External Serial Clock, Single Cycle Operation
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2.7V TO 5.5V
VCC
1µF
VCC
= 50Hz REJECTION
= EXTERNAL OSCILLATOR
= 60Hz REJECTION
FO
LTC2400
VREF
0.1V TO VCC
VIN
–0.12VREF TO 1.12VREF
VREF
SCK
VIN
SDO
GND
CS
CS
BIT 0
SDO
TEST EOC
TEST EOC
BIT 31
EOC
BIT 30
EOC
Hi-Z
Hi-Z
BIT 29
BIT 28
BIT 27
SIG
EXR
MSB
Hi-Z
BIT 9
TEST EOC
BIT 8
Hi-Z
SCK
(EXTERNAL)
SLEEP
CONVERSION
SLEEP
DATA OUTPUT
CONVERSION
2400 F06
DATA OUTPUT
Figure 6. External Serial Clock, Reduced Data Output Length
External Serial Clock, 2-Wire I/O
This timing mode utilizes a 2-wire serial I/O interface. The
conversion result is shifted out of the device by an externally generated serial clock (SCK) signal, see Figure 7. CS
may be permanently tied to ground (Pin 4), simplifying the
user interface or isolation barrier.
The external serial clock mode is selected at the end of the
power-on reset (POR) cycle. The POR cycle is concluded
approximately 0.5ms after VCC exceeds 2.2V. The level
applied to SCK at this time determines if SCK is internal or
external. SCK must be driven LOW prior to the end of POR
in order to enter the external serial clock timing mode.
Since CS is tied LOW, the end-of-conversion (EOC) can be
continuously monitored at the SDO pin during the convert
and sleep states. EOC may be used as an interrupt to an
external controller indicating the conversion result is
ready. EOC = 1 while the conversion is in progress and EOC
= 0 once the conversion enters the low power sleep state.
On the falling edge of EOC, the conversion result is loaded
into an internal static shift register. The device remains in
the sleep state until the first rising edge of SCK. Data is
16
shifted out the SDO pin on each falling edge of SCK
enabling external circuitry to latch data on the rising edge
of SCK. EOC can be latched on the first rising edge of SCK.
On the 32nd falling edge of SCK, SDO goes HIGH (EOC =
1) indicating a new conversion has begun.
Internal Serial Clock, Single Cycle Operation
This timing mode uses an internal serial clock to shift out
the conversion result and a CS signal to monitor and
control the state of the conversion cycle, see Figure 8.
In order to select the internal serial clock timing mode, the
serial clock pin (SCK) must be floating (HI-Z) or pulled
HIGH prior to the falling edge of CS. The device will not
enter the internal serial clock mode if SCK is driven LOW
on the falling edge of CS. An internal weak pull-up resistor
is active on the SCK pin during the falling edge of CS;
therefore, the internal serial clock timing mode is automatically selected if SCK is not externally driven.
The serial data output pin (SDO) is HI-Z as long as CS is
HIGH. At any time during the conversion cycle, CS may be
pulled LOW in order to monitor the state of the converter.
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2.7V TO 5.5V
VCC
1µF
VCC
= 50Hz REJECTION
= EXTERNAL OSCILLATOR
= 60Hz REJECTION
FO
LTC2400
VREF
0.1V TO VCC
VIN
–0.12VREF TO 1.12VREF
VREF
SCK
VIN
SDO
GND
CS
CS
BIT 31
SDO
BIT 30
EOC
BIT 29
BIT 28
BIT 27
SIG
EXR
MSB
BIT 26
BIT 0
BIT 4
LSB24
SCK
(EXTERNAL)
CONVERSION
SLEEP
DATA OUTPUT
CONVERSION
2400 F07
Figure 7. External Serial Clock, CS = 0 Operation
VCC
2.7V TO 5.5V
VCC
1µF
VCC
= 50Hz REJECTION
= EXTERNAL OSCILLATOR
= 60Hz REJECTION
FO
10k
LTC2400
VREF
0.1V TO VCC
VIN
–0.12VREF TO 1.12VREF
VREF
SCK
VIN
SDO
GND
CS
tEOCtest
CS
(VCC – 0.4V)].
Severe ground pin current disturbances can also occur
due to the undershoot of fast digital input signals. Undershoot and overshoot can occur because of the impedance
mismatch at the converter pin when the transition time of
an external control signal is less than twice the propagation delay from the driver to LTC2400. For reference, on
a regular FR-4 board, signal propagation velocity is approximately 183ps/inch for internal traces and 170ps/inch
for surface traces. Thus, a driver generating a control
signal with a minimum transition time of 1ns must be
connected to the converter pin through a trace shorter
than 2.5 inches. This problem becomes particularly difficult when shared control lines are used and multiple
reflections may occur. The solution is to carefully terminate all transmission lines close to their characteristic
impedance.
22
Parallel termination near the LTC2400 pin will eliminate
this problem but will increase the driver power dissipation.
A series resistor between 27Ω and 56Ω placed near the
driver or near the LTC2400 pin will also eliminate this
problem without additional power dissipation. The actual
resistor value depends upon the trace impedance and
connection topology.
Driving the Input and Reference
The analog input and reference of the typical delta-sigma
analog-to-digital converter are applied to a switched capacitor network. This network consists of capacitors
switching between the analog input (VIN), ground (Pin 4)
and the reference (VREF). The result is small current spikes
seen at both VIN and VREF. A simplified input equivalent
circuit is shown in Figure 15.
VCC
IREF(LEAK)
RSW
5k
VREF
IREF(LEAK)
IIN
VCC
IIN(LEAK) RSW
5k
AVERAGE INPUT CURRENT:
IIN = 0.25(VIN – 0.5 • VREF)fCEQ
VIN
CEQ
10pF (TYP)
IIN(LEAK)
RSW
5k
GND
2400 F15
SWITCHING FREQUENCY
f = 153.6kHz FOR INTERNAL OSCILLATOR (fO = LOGIC LOW OR HIGH)
f = fEOSC FOR EXTERNAL OSCILLATORS
Figure 15. LTC2400 Equivalent Analog Input Circuit
The key to understanding the effects of this dynamic input
current is based on a simple first order RC time constant
model. Using the internal oscillator, the LTC2400’s internal switched capacitor network is clocked at 153,600Hz
corresponding to a 6.5µs sampling period. Fourteen time
constants are required each time a capacitor is switched in
order to achieve 1ppm settling accuracy.
Therefore, the equivalent time constant at VIN and VREF
should be less than 6.5µs/14 = 460ns in order to achieve
1ppm accuracy.
LTC2400
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APPLICATIONS INFORMATION
Input Current (VIN)
If complete settling occurs on the input, conversion results will be uneffected by the dynamic input current. If the
settling is incomplete, it does not degrade the linearity
performance of the device. It simply results in an offset/
full-scale shift, see Figure 16. To simplify the analysis of
input dynamic current, two separate cases are assumed:
large capacitance at VIN (CIN > 0.01µF) and small capacitance at VIN (CIN < 0.01µF).
If the total capacitance at VIN (see Figure 17) is small
(< 0.01µF), relatively large external source resistances (up
to 20k for 20pF parasitic capacitance) can be tolerated
without any offset/full-scale error. Figures 18 and 19 show
a family of offset and full-scale error curves for various
small valued input capacitors (CIN < 0.01µF) as a function
of input source resistance.
For large input capacitor values (CIN > 0.01µF), the input
spikes are averaged by the capacitor into a DC current. The
gain shift becomes a linear function of input source
resistance independent of input capacitance, see Figures
20 and 21. The equivalent input impedance is 1.66MΩ.
This results in ±1.5µA of input dynamic current at the
extreme values of VIN (VIN = 0V and VIN = VREF, when
TUE
0
VREF/2
VREF
VIN
2400 F16
Figure 16. Offset/Full-Scale Shift
FULL-SCALE ERROR (ppm)
0
VCC = 5V
VREF = 5V
VIN = 5V
TA = 25°C
–10
–20
CIN = 0pF
CIN = 100pF
CIN = 1000pF
–30
CIN = 0.01µF
–40
RSOURCE
VIN
INTPUT
SIGNAL
SOURCE
CIN
CPAR
≅ 20pF
–50
1
10
LTC2400
1k
100
RSOURCE (Ω)
10k
100k
2400 F19
2400 F17
Figure 19. Full-Scale Error vs RSOURCE (Small C)
Figure 17. An RC Network at VIN
300
50
OFFSET ERROR (ppm)
40
30
OFFSET ERROR (ppm)
VCC = 5V
VREF = 5V
VIN = 0V
TA = 25°C
CIN = 0pF
CIN = 100pF
CIN = 1000pF
20
CIN = 0.01µF
10
0
VCC = 5V
VREF = 5V
250 VIN = 0V
TA = 25°C
CIN = 1µF
CIN = 10µF
200
CIN = 0.1µF
150
100
CIN = 0.01µF
50
0
1
10
1k
100
RSOURCE (Ω)
10k
100k
2400 F18
Figure 18. Offset vs RSOURCE (Small C)
0 100 200 300 400 500 600 700 800 900 1000
RSOURCE (Ω)
2400 F20
Figure 20. Offset vs RSOURCE (Large C)
23
LTC2400
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APPLICATIONS INFORMATION
600
CIN = 0.01µF
FULL-SCALE ERROR (ppm)
–50
VCC = 5V
VREF = 5V
VIN = 5V
TA = 25°C
–100
CIN = 0.1µF
–150
CIN = 1µF
CIN = 10µF
–200
VCC = 5V
VREF = 5V
VIN = 5V
TA = 25°C
500
FULL-SCALE ERROR (ppm)
0
400
CVREF = 10µF
300
CVREF = 1µF
200
CVREF = 0.1µF
100
–250
CVREF = 0.01µF
0
–300
0
200
400
600
RSOURCE (Ω)
800
0
1000
2400 F22
2400 F21
Figure 21. Full-Scale Error vs RSOURCE (Large C)
Figure 22. Full-Scale Error vs RVREF (Large C)
VREF = 5V). This corresponds to a 0.3ppm shift in offset
and full-scale readings for every 1Ω of input source
resistance.
50
FULL-SCALE ERROR (ppm)
In addition to the input current spikes, the input ESD
protection diodes have a temperature dependent leakage
current. This leakage current, nominally 1nA (±10nA
max), results in a fixed offset shift of 10µV for a 10k source
resistance.
VCC = 5V
VREF = 5V
40 V = 5V
IN
TA = 25°C
30
CVREF = 100pF
CVREF = 1000pF
20
CVREF = 0.01µF
10
0
Reference Current (VREF)
–10
Similar to the analog input, the reference input has a
dynamic input current. This current has negligible effect
on the offset. However, the reference current at VIN = VREF
is similar to the input current at full-scale. For large values
of reference capacitance (CVREF > 0.01µF), the full-scale
error shift is 0.3ppm/Ω of external reference resistance
independent of the capacitance at VREF, see Figure 22. If
the capacitance tied to VREF is small (CVREF < 0.01µF), an
input resistance of up to 20k (20pF parasitic capacitance
at VREF) may be tolerated, see Figure 23.
–20
24
CVREF = 0pF
1
10
100
10k
1k
RESISTANCE AT VREF(Ω)
100k
2400 F23
Figure 23. Full-Scale Error vs RVREF (Small C)
50
VCC = 5V
VREF = 5V
TA = 25°C
40
INL ERROR (ppm)
Unlike the analog input, the integral nonlinearity of the
device can be degraded with excessive external RC time
constants tied to the reference input. If the capacitance at
node VREF is small (CVREF < 0.01µF), the reference input
can tolerate large external resistances without reduction
in INL, see Figure 24. If the external capacitance is large
(CVREF > 0.01µF), the linearity will be degraded by
0.15ppm/Ω independent of capacitance at VREF, see
Figure 25.
1000
200
400
600
800
RESISTANCE AT VREF (Ω)
CVREF = 0pF
CVREF = 100pF
CVREF = 1000pF
30
20
CVREF = 0.01µF
10
0
–10
1
10
100
1k
10k
RESISTANCE AT VREF (Ω)
100k
2400 F24
Figure 24. INL Error vs RVREF (Small C)
LTC2400
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160
–20
CVREF = 0.1µF
CVREF = 1µF
CVREF = 10µF
100
80
60
–40
CVREF = 0.01µF
40
REJECTION (dB)
120
INL ERROR (ppm)
0
VCC = 5V
VREF = 5V
TA = 25°C
140
–60
–80
–100
20
–120
0
–140
–20
0
800
200
600
400
RESISTANCE AT VREF (Ω)
1000
0
fS/2
fS
INPUT FREQUENCY
2400 F25
2400 F26
Figure 25. INL Error vs RVREF (Large C)
Figure 26. Sinc4 Filter Rejection
In addition to the dynamic reference current, the VREF ESD
protection diodes have a temperature dependent leakage
current. This leakage current, nominally 1nA (±10nA max),
results in a fixed full-scale shift of 10µV for a 10k source
resistance.
digital filter is narrow (≈ 0.2%) compared to the bandwidth
of the frequencies rejected.
ANTIALIASING
One of the advantages delta-sigma ADCs offer over conventional ADCs is on-chip digital filtering. Combined with
a large oversampling ratio, the LTC2400 significantly
simplifies antialiasing filter requirements.
The digital filter provides very high rejection except at
integer multiples of the modulator sampling frequency
(fS), see Figure 26. The modulator sampling frequency is
256 • FO, where FO is the notch frequency (typically 50Hz
or 60Hz). The bandwidth of signals not rejected by the
As a result of the oversampling ratio (256) and the digital
filter, minimal (if any) antialias filtering is required in front
of the LTC2400. If passive RC components are placed in
front of the LTC2400 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
input dynamic current.
The modulator contained within the LTC2400 can handle
large-signal level perturbations without saturating. Signal
levels up to 40% of VREF do not saturate the analog modulator. These signals are limited by the input ESD protection
to 300mV below ground and 300mV above VCC.
25
LTC2400
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TYPICAL APPLICATIONS
SYNCHRONIZATION OF MULTIPLE LTC2400s
Increasing the Output Rate Using Multiple LTC2400s
Since the LTC2400’s absolute accuracy (total unadjusted
error) is 10ppm, applications utilizing multiple matched
ADCs are possible.
A second application uses multiple LTC2400s to increase
the effective output rate by 4×, see Figure 28. In this case,
four LTC2400s are interleaved under the control of separate CS signals. This increases the effective output rate
from 7.5Hz to 30Hz (up to a maximum of 60Hz). Additionally, the one-shot output spectrum is unfolded allowing
further digital signal processing of the conversion results.
SCK and SDO may be common to all four LTC2400s. The
four CS rising edges equally divide one LTC2400 conversion cycle (7.5Hz for 60Hz notch frequency). In order to
synchronize the start of conversion to CS, 31 or less SCK
clock pulses must be applied to each ADC.
Simultaneous Sampling with Two LTC2400s
One such application is synchronizing multiple LTC2400s,
see Figure 27. The start of conversion is synchronized to
the rising edge of CS. In order to synchronize multiple
LTC2400s, CS is a common input to all the ADCs.
To prevent the converters from autostarting a new conversion at the end of data output read, 31 or fewer SCK
clock signals are applied to the LTC2400 instead of 32 (the
32nd falling edge would start a conversion). The exact
timing and frequency for the SCK signal is not critical
since it is only shifting out the data. In this case, two
LTC2400’s simultaneously start and end their conversion
cycles under the external control of CS.
Both the synchronous and 4× output rate applications use
the external serial clock and single cycle operation with
reduced data output length (see Serial Interface Timing
Modes section and Figure 6). An external oscillator clock
is applied commonly to the FO pin of each LTC2400 in
order to synchronize the sampling times. Both circuits
may be extended to include more LTC2400s.
SCK2
SCK1
LTC2400
#1
VCC
µCONTROLLER
EXTERNAL OSCILLATOR
(153,600HZ)
LTC2400
#2
FO
VCC
FO
VREF
SCK
VREF
SCK
VIN
SDO
VIN
SDO
GND
CS
GND
CS
CS
SDO1
SDO2
VREF
(0.1V TO VCC)
CS
SCK1
31 OR LESS CLOCK CYCLES
SCK2
31 OR LESS CLOCK CYCLES
SDO1
SDO2
2400 F27
Figure 27. Synchronous Conversion—Extendable
26
LTC2400
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TYPICAL APPLICATIONS
LTC2400
#2
LTC2400
#1
VCC
µCONTROLLER
FO
VCC
LTC2400
#3
FO
VCC
VREF
(0.1V TO VCC)
EXTERNAL OSCILLATOR
(153,600HZ)
LTC2400
#4
FO
VCC
FO
VREF
SCK
VREF
SCK
VREF
SCK
VREF
SCK
VIN
SDO
VIN
SDO
VIN
SDO
VIN
SDO
GND
CS
GND
CS
GND
CS
GND
CS
SCK
SDO
CS1
CS2
CS3
CS4
CS1
CS2
CS3
CS4
SCK
31 OR LESS
CLOCK PULSES
SDO
2400 F28
Figure 28. 4 × Output Rate LTC2400 System
Differential to Single-Ended
Analog Conditioning
Simple Differential Front-End
for the LTC2400
The circuits in Figures 29 and 30 use the LTC1043 dual
precision, switched capacitor building block. Each circuit
uses one-half of an LTC1043 to perform a differential to
single-ended conversion over an input common mode
range that includes the power supplies. The LTC1043
samples a differential input voltage, holds it on CS and
transfers it to a ground-referenced capacitor CH. The
voltage on CH is applied to the LTC2400’s input and
converted to a digital value.
The circuit in Figure 29 is ideal for wide dynamic range
differential signals in applications where absolute accuracy is secondary to high resolution, have large signal
swings, source impedances under 500Ω and use a 5V or
±5V supply.
The LTC1043 achieves its best differential to single-ended
conversion when its internal switching frequency operates at a nominal 300Hz, as set by the 0.01µF capacitor C1,
and when 1µF capacitors are used for CS and CH. CS and
CH should be a film-type capacitor such as mylar or
polypropylene.
The circuit achieves a nonlinearity of ±35ppm (a linearity
accuracy of 14.5 bits), noise of 1.5µVRMS and 21-bit
resolution. The circuit exhibits a typical 2.75mV zero
offset. However, this is not an offset that simply shifts the
output code by a constant value. It is a gain error that alters
the transfer function’s slope. The gain error revolves
around midscale (VREF/2). This gain error can be corrected
in software by measuring the error at 0V input and using
the result to create a correction factor.
27
LTC2400
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TYPICAL APPLICATIONS
5V 0.1µF
VREFIN
5V
0.1µF
1
VCC
2
4
7
8
3
VIN
LTC2400
SDO
SCK
11
LARGE
MAGNITUDE
DIFFERENTIAL
INPUT
CS
VREF
GND
CS
1µF
EXT
CH
1µF
4
5
6
7
CHIP SELECT
SERIAL
SERIAL
FO
8
12
13
14
16
C1
0.01µF
1/2
LTC1043
17
0.1µF
– 5V
2400 F29
Figure 29. Simple Rail-to-Rail Circuit Converts Differential Signals to Single-Ended Signals
LTC2400 High Accuracy Differential to Single-Ended
Converter for ±5V Supplies
The circuit in Figure 30 is ideal for low level differential
signals in applications that have a ±5V supply and need
high accuracy without calibration. The circuit combines an
LTC1043 and LTC1050 as a differential to single-ended
amplifier that has an input common mode range that
includes the power supplies. Resistors R1 and R2 set the
LTC1050’s gain at 101.
The circuit schematic shows an optional resistor RS. This
resistor can be placed in series with the LTC2400’s input
to limit current if the input goes below – 300mV. The
resistor does not degrade the converter’s performance as
long as any capacitance, stray or otherwise, connected
between the LTC2400’s input and ground is less than
100pF. Higher capacitance will increase offset and fullscale errors (see Input Current section).
The circuit achieves a nonlinearity of ±1ppm, input referred noise of 0.05µVRMS (averaging 64 samples), 19.6
bits resolution for a full-scale input of 40mV, and an overall
accuracy of 20 bits when using an LTC1236-5 precision
5V reference.
28
Multiple Inputs
The simple circuit shown in Figure 31 takes advantage of
the LTC2400’s single conversion settling. The LTC1391
serially programmed multiplexer allows accurate conversions on each of its eight channels without introducing any
offset, gain or linearity errors with its input signal between
0V and VREF, as long as the total capacitance connected to
the LTC2400’s input is less than 1000pF. A small 2ppm
(typ) error occurs when an active input channel’s signal
voltage reaches –300mV (typ). If the excursion below
ground is above – 200mV (typ), the error is less than the
LTC2400’s 0.3ppmRMS noise. On the topside, the selected
input signal’s magnitude can go above the 5V supply with
no linearity degradation or increased noise. Figure 31’s
circuit can tolerate overdrive on the unselected channel
without conversion degradation as long as the overdrive is
less than 250mV above the supply voltage or 250mV
below ground. The linearity performance is similar to that
shown in the Typical Performance Characteristics section.
Errors caused by channel-to-channel crosstalk are less
than the LTC2400’s typical input noise. This remains the
case for a frequency range of 1Hz to 153.6kHz (the
LTC2400’s internal clock frequency or 10fS). When the
frequency reaches 1.536MHz (4VP-P), the RMS noise typically doubles and the linearity is degraded by 30ppm (typ).
LTC2400
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TYPICAL APPLICATIONS
5V
5V 0.1µF
VREFIN
0.1µF
5V 0.1µF
BRIDGETYPICAL
INPUT
350Ω 350Ω
1
4
VFS = 40mV
7
3
8
2
CS
1µF
EXT
CH
1µF
–
VIN
LTC2400
0.1µF
4
5
6
7
CHIP SELECT
SERIAL
SERIAL
FO
8
R1
9.09k
– 5V
14
SDO
SCK
R2
90.9k
16
C1
0.01µF
CS
VREF
GND
350Ω 350Ω
AGND OR
– VEXT
3
4
12
13
2
RS*
5.1k
6
LTC1050
11
DIFFERENTIAL
INPUT
7
+
VCC
1/2
LTC1043
*OPTIONAL: LIMITS INPUT CURRENT IF THE
INPUT VOLTAGE GOES BELOW – 300mV
17
0.1µF
– 5V
2400 F30
Figure 30. Differential to Single-Ended Converter for Low Level Inputs, Such as Bridges, Maintains the LTC2400’s High Accuracy
VREFIN
5V
CH1
CH2
CH3
CH4
CH5
CH6
CH7
1
2
3
4
5
6
7
8
S0
V+
S1
D
S2
V–
S3
DATA 2
S4
DATA 1
S5
CS
S6
CLK
S7
GND
1
0.1µF
LTC1391
CH0
5V 0.1µF
2
16
15
3
VCC
CS
VREF
VIN
LTC2400
SDO
14
SCK
13
GND
12
4
5
6
7
CS
SDO
SCK
FO
8
11
10
9
2400 F31
Figure 31. Multiplex 8-Signal Sources with the LTC1391 and Maintain the LTC2400’s Conversion Accuracy
29
LTC2400
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TYPICAL APPLICATIONS
Sample Driver for LTC2400 SPI Interface
The LTC2400 has a very simple serial interface that makes
interfacing to microprocessors and microcontrollers very
easy. Shown in Figures 32 and 34 are listings of sample
source codes that can be used to initiate conversions and
retrieve data from the LTC2400.
The listing in Figure 32 was created by Parallax, Inc. (916624-8333), for the BASIC Stamp. This code uses individual port lines to control the LTC2400’s conversion and
'LTC2400
'03/17/99
'
'
'
'
retrieve the 32-bit result. A fourth port line is used to
power the LTC2400, a vivid example of the converter’s
micropower operation. The program’s main sequence
activates the LTC2400’s serial interface, uses a loop to
retrieve the 32 conversion bits, and then places the
converter’s interface in a high impedance state and starting the next conversion. All bits are retained in variables
ADlo and ADhi. The code can be found on their web site,
www.parallaxinc.com.
Sample Driver
This program is an example showing how to access the
LTC2400 using the Basic Stamp2 from Parallax. Since
the BS2 is based on a 16-bit architecture, only the
upper 16 bits of the 24-bit result are displayed,
although all 24 bits are retrieved.
ADlo
ADhi
Ctr
Temp
var
var
var
var
word
word
byte
bit
'A/D result - lower 16 bits
'A/D result - upper 8 bits
'loop counter
'temporary bit used for shift
SDO
SCK
CS
Pwr
con
con
con
con
0
1
2
3
'Serial data connected to P0
'Serial clock connected to P1
'Chip Select connected to P2
'Stamp supplies power connected to P3
'(Uses only 0.3mA!)
Init
dira = $E
outa = $0
pause 100
high Pwr
pause 1
high CS
Start
pause 125
low CS
for Ctr = 0 to 31
high SCK
gosub ShiftL
30
'Set up data direction
'Pwr, CS, and SCK are outputs
'SDO is an input
'Initialize outputs
'Pwr, CS, and SCK are low
'Wait 100mS for I/O to settle
'Power up the LTC2400
'Wait 1mS for power-on sequence
'Disable the device until we
'wish to read it.
'Eight times second
'Enable the LTC2400
'Cycle clock 32 times
LTC2400
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TYPICAL APPLICATIONS
ADlo.bit0 = in0
'and sample data line
low SCK
next
high CS
'Disable the LTC2400
ADhi = (ADhi12)
debug ?ADhi
'Discard the lower eight bits
goto Start
'and display (debug command).
ShiftL
Temp = ADlo.bit15
ADlo = ADlo
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