LTC2326-18
18-Bit, 250ksps, ±10.24V
True Bipolar, Pseudo-Differential
Input ADC with 95dB SNR
Features
Description
250ksps Throughput Rate
nn ±5LSB INL (Max)
nn Guaranteed 18-Bit No Missing Codes
nn Pseudo-Differential Inputs
nn True Bipolar Input Ranges ±6.25V, ±10.24V, ±12.5V
nn 95dB SNR (Typ) at f = 2kHz
IN
nn –111dB THD (Typ) at f = 2kHz
IN
nn Guaranteed Operation to 125°C
nn Single 5V Supply
nn Low Drift (20ppm/°C Max) 2.048V Internal Reference
nn Onboard Single-Shot Capable Reference Buffer
nn No Pipeline Delay, No Cycle Latency
nn 1.8V to 5V I/O Voltages
nn SPI-Compatible Serial I/O with Daisy-Chain Mode
nn Internal Conversion Clock
nn Power Dissipation 28mW (Typ)
nn 16-Lead MSOP Package
The LTC®2326-18 is a low noise, high speed 18-bit successive approximation register (SAR) ADC with pseudodifferential inputs. Operating from a single 5V supply,
the LTC2326-18 has a ±10.24V true bipolar input range,
making it ideal for high voltage applications which require
a wide dynamic range. The LTC2326-18 achieves ±5LSB
INL maximum, no missing codes at 18 bits with 95dB SNR.
nn
Applications
Programmable Logic Controllers
Industrial Process Control
nn High Speed Data Acquisition
nn Portable or Compact Instrumentation
nn ATE
nn
nn
The LTC2326-18 has an onboard single-shot capable
reference buffer and low drift (20ppm/°C max) 2.048V
temperature compensated reference. The LTC2326-18
also has a high speed SPI-compatible serial interface that
supports 1.8V, 2.5V, 3.3V and 5V logic while also featuring
a daisy-chain mode. The fast 250ksps throughput with
no cycle latency makes the LTC2326-18 ideally suited
for a wide variety of high speed applications. An internal
oscillator sets the conversion time, easing external timing
considerations. The LTC2326-18 dissipates only 28mW
and automatically naps between conversions, leading to
reduced power dissipation that scales with the sampling
rate. A sleep mode is also provided to reduce the power
consumption of the LTC2326-18 to 300μW for further
power savings during inactive periods.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
SoftSpan is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners. Protected by U.S. Patents, including 7705765, 7961132.
Typical Application
32k Point FFT fS = 250ksps,
fIN = 2kHz
10µF
+10.24V
–10.24V
+
LT®1468
2.2µF
IN+
LTC2326-18
IN–
REF
REFBUF
47µF
REFIN
100nF
GND
SNR = 95.3dB
THD = –113dB
SINAD = 95.2dB
SFDR = –117dB
–20
–40
0.1µF
VDDLBYP OVDD
VDD
–
0
1.8V TO 5V
CHAIN
RDL/SDI
SDO
SCK
BUSY
CNV
AMPLITUDE (dBFS)
5V
SAMPLE CLOCK
232618 TA01
–60
–80
–100
–120
–140
–160
–180
0
25
50
75
FREQUENCY (kHz)
100
125
232618 TA01b
232618fb
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1
LTC2326-18
Absolute Maximum Ratings
Pin Configuration
(Notes 1, 2)
Supply Voltage (VDD)...................................................6V
Supply Voltage (OVDD).................................................6V
Supply Bypass Voltage (VDDLBYP)............................3.2V
Analog Input Voltage
IN+, IN–...............................................–16.5V to 16.5V
REFBUF....................................................................6V
REFIN ...................................................................2.8V
Digital Input Voltage
(Note 3)............................ (GND –0.3V) to (OVDD + 0.3V)
Digital Output Voltage
(Note 3)............................ (GND –0.3V) to (OVDD + 0.3V)
Power Dissipation............................................... 500mW
Operating Temperature Range
LTC2326C................................................. 0°C to 70°C
LTC2326I..............................................–40°C to 85°C
LTC2326H........................................... –40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
Order Information
TOP VIEW
VDDLBYP
VDD
GND
IN+
IN–
GND
REFBUF
REFIN
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
GND
OVDD
SDO
SCK
RDL/SDI
BUSY
CHAIN
CNV
MS PACKAGE
16-LEAD PLASTIC MSOP
TJMAX = 150°C, θJA = 110°C/W
http://www.linear.com/product/LTC2326-18#orderinfo
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC2326CMS-18#PBF
LTC2326CMS-18#TRPBF
232618
16-Lead Plastic MSOP
0°C to 70°C
LTC2326IMS-18#PBF
LTC2326IMS-18#TRPBF
232618
16-Lead Plastic MSOP
–40°C to 85°C
LTC2326HMS-18#PBF
LTC2326HMS-18#TRPBF 232618
16-Lead Plastic MSOP
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
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/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
2
232618fb
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LTC2326-18
Electrical Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 4)
SYMBOL
PARAMETER
CONDITIONS
VIN+
Absolute Input Range (IN+)
(Note 5)
l
–2.5 • VREFBUF – 0.5
2.5 • VREFBUF + 0.5
V
–
Absolute Input Range (IN–)
(Note 5)
l
–0.5
0.5
V
VIN+ – VIN–
Input Differential Voltage Range
VIN = VIN+ – VIN–
l
–2.5 • VREFBUF
2.5 • VREFBUF
IIN
Analog Input Current
l
–7.8
4.8
CIN
Analog Input Capacitance
5
pF
RIN
Analog Input Resistance
2.083
kΩ
CMRR
Input Common Mode Rejection Ratio
66
dB
VIN
MIN
TYP
fIN = 125kHz
MAX
UNITS
V
mA
Converter Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 4)
SYMBOL
PARAMETER
CONDITIONS
MIN
Resolution
l
18
No Missing Codes
l
18
TYP
Integral Linearity Error
Bits
DNL
Differential Linearity Error
BZE
Bipolar Zero-Scale Error
1.6
(Note 6)
l
Bipolar Full-Scale Error
LSBRMS
–5
±1
5
LSB
l
–1
±0.1
1.25
LSB
(Note 7)
l
–30
0
30
LSB
VREFBUF = 4.096V (REFBUF Overdriven)
(Notes 7, 9)
l
–125
REFIN = 2.048V (Note 7)
l
–150
Bipolar Zero-Scale Error Drift
FSE
UNITS
Bits
Transition Noise
INL
MAX
0.01
LSB/°C
125
LSB
150
Bipolar Full-Scale Error Drift
±0.5
LSB
ppm/°C
Dynamic Accuracy
The l denotes the specifications which apply over the full operating temperature range,
otherwise specifications are at TA = 25°C and AIN = –1dBFS. (Notes 4, 8)
SYMBOL
PARAMETER
CONDITIONS
SINAD
Signal-to-(Noise + Distortion) Ratio
±6.25V Range, fIN = 2kHz, REFIN = 1.25V
l
SNR
THD
SFDR
Signal-to-Noise Ratio
Total Harmonic Distortion
Spurious Free Dynamic Range
TYP
91
MAX
UNITS
dB
±10.24V Range, fIN = 2kHz, REFIN = 2.048V
l
91
95
dB
±12.5V Range, fIN = 2kHz, REFBUF = 5V
l
92
96.5
dB
±6.25V Range, fIN = 2kHz, REFIN = 1.25V
l
88
91.5
dB
±10.24V Range, fIN = 2kHz, REFIN = 2.048V
l
92
95
dB
±12.5V Range, fIN = 2kHz, REFBUF = 5V
l
94
97
dB
±6.25V Range, fIN = 2kHz, REFIN = 1.25V
l
–108
–98
dB
±10.24V Range, fIN = 2kHz, REFIN = 2.048V
l
–111
–98
dB
±12.5V Range, fIN = 2kHz, REFBUF = 5V
l
–106
–96
dB
±6.25V Range, fIN = 2kHz, REFIN = 1.25V
l
98
110
dB
±10.24V Range, fIN = 2kHz, REFIN = 2.048V
l
98
113
dB
±12.5V Range, fIN = 2kHz, REFBUF = 5V
l
96
108
dB
–3dB Input Linear Bandwidth
7
Aperture Delay
500
Aperture Jitter
Transient Response
MIN
87.6
Full-Scale Step
MHz
ps
4
psRMS
1
µs
232618fb
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3
LTC2326-18
Internal Reference Characteristics
The l denotes the specifications which apply over the
full operating temperature range, otherwise specifications are at TA = 25°C. (Note 4)
SYMBOL
PARAMETER
VREFIN
Internal Reference Output Voltage
CONDITIONS
VREFIN Temperature Coefficient
(Note 14)
MIN
TYP
MAX
UNITS
2.043
2.048
2.053
V
2
20
l
REFIN Output Impedance
ppm/°C
15
VREFIN Line Regulation
VDD = 4.75V to 5.25V
REFIN Input Voltage Range
(REFIN Overdriven) (Note 5)
kΩ
0.08
mV/V
1.25
2.4
V
Reference Buffer Characteristics
The l denotes the specifications which apply over the full
operating temperature range, otherwise specifications are at TA = 25°C. (Note 4)
SYMBOL
VREFBUF
IREFBUF
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
4.096
4.101
V
5
V
Reference Buffer Output Voltage
VREFIN = 2.048V
l
4.091
REFBUF Input Voltage Range
(REFBUF Overdriven) (Notes 5, 9)
l
2.5
REFBUF Output Impedance
VREFIN = 0V
REFBUF Load Current
VREFBUF = 5V (REFBUF Overdriven) (Notes 9, 10)
VREFBUF = 5V, Nap Mode (REFBUF Overdriven) (Note 9)
13
kΩ
0.56
0.39
l
0.6
mA
mA
Digital Inputs and Digital Outputs
The l denotes the specifications which apply over the
full operating temperature range, otherwise specifications are at TA = 25°C. (Note 4)
SYMBOL
PARAMETER
CONDITIONS
MIN
VIH
High Level Input Voltage
l
VIL
Low Level Input Voltage
l
IIN
Digital Input Current
CIN
Digital Input Capacitance
VOH
TYP
MAX
0.8 • OVDD
VIN = 0V to OVDD
l
–10
High Level Output Voltage
IO = –500µA
l
OVDD – 0.2
VOL
Low Level Output Voltage
IO = 500µA
l
l
UNITS
V
0.2 • OVDD
V
10
μA
5
pF
V
0.2
IOZ
Hi-Z Output Leakage Current
VOUT = 0V to OVDD
ISOURCE
Output Source Current
VOUT = 0V
–10
mA
ISINK
Output Sink Current
VOUT = OVDD
10
mA
4
–10
10
V
µA
232618fb
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LTC2326-18
Power Requirements
The l denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. (Note 4)
SYMBOL
PARAMETER
VDD
Supply Voltage
OVDD
Supply Voltage
IVDD
Supply Current
IOVDD
INAP
ISLEEP
Supply Current
Nap Mode Current
Sleep Mode Current
PD
Power Dissipation
Nap Mode
Sleep Mode
CONDITIONS
250ksps Sample Rate (IN+ = –10.24V, IN– = 0V)
250ksps Sample Rate (IN+ = IN– = 0V)
250ksps Sample Rate (CL = 20pF)
Conversion Done (IVDD + IOVDD, IN+ = –10.24V, IN– = 0V)
Sleep Mode (IVDD + IOVDD)
250ksps Sample Rate (IN+ = –10.24V, IN– = 0V)
250ksps Sample Rate (IN+ = IN– = 0V)
Conversion Done (IVDD + IOVDD, IN+ = –10.24V, IN– = 0V)
Sleep Mode (IVDD + IOVDD)
MIN
TYP
MAX
UNITS
l
4.75
5
5.25
V
l
1.71
5.25
V
9.9
5.6
0.1
8.4
60
11.5
mA
mA
mA
mA
μA
50
28
42
0.3
57.5
l
l
l
l
l
l
10
225
mW
mW
mW
mW
50
1.1
ADC Timing Characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Note 4)
SYMBOL
fSMPL
tCONV
tACQ
tHOLD
tCYC
tCNVH
tBUSYLH
tCNVL
tQUIET
tSCK
tSCKH
tSCKL
tSSDISCK
tHSDISCK
tSCKCH
tDSDO
PARAMETER
Maximum Sampling Frequency
Conversion Time
Acquisition Time
Maximum Time between Acquisitions
Time Between Conversions
CNV High Time
CNV↑ to BUSY Delay
Minimum Low Time for CNV
SCK Quiet Time from CNV↑
SCK Period
SCK High Time
SCK Low Time
SDI Setup Time From SCK↑
SDI Hold Time From SCK↑
SCK Period in Chain Mode
SDO Data Valid Delay from SCK↑
CONDITIONS
tHSDO
tDSDOBUSYL
tEN
tDIS
tWAKE
SDO Data Remains Valid Delay from SCK↑
SDO Data Valid Delay from BUSY↓
Bus Enable Time After RDL↓
Bus Relinquish Time After RDL↑
REFBUF Wakeup Time
MIN
TYP
l
l
tACQ = tCYC – tHOLD (Note 11)
l
1.9
3.460
540
l
l
l
CL = 20pF
(Note 12)
(Note 11)
(Notes 12, 13)
l
l
l
l
(Note 12)
(Note 12)
tSCKCH = tSSDISCK + tDSDO (Note 12)
CL = 20pF, OVDD = 5.25V
CL = 20pF, OVDD = 2.5V
CL = 20pF, OVDD = 1.71V
CL = 20pF (Note 11)
CL = 20pF (Note 11)
(Note 12)
(Note 12)
CREFBUF = 47μF, CREFIN = 100nF
4
20
13
l
l
l
l
l
20
20
10
4
4
4
1
13.5
7.5
8
9.5
l
l
l
l
MAX
250
3
1
5
16
13
l
l
l
200
UNITS
ksps
µs
µs
ns
µs
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ms
232618fb
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5
LTC2326-18
Electrical Characteristics
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 ground.
Note 3: When these pin voltages are taken below ground or above VDD or
OVDD, they will be clamped by internal diodes. This product can handle
input currents up to 100mA below ground or above VDD or OVDD without
latch-up.
Note 4: VDD = 5V, OVDD = 2.5V, ±10.24V Range, REFIN = 2.048V,
fSMPL = 250kHz.
Note 5: Recommended operating conditions.
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: Bipolar zero error is the offset voltage measured from –0.5LSB
when the output code flickers between 00 0000 0000 0000 0000 and 11
1111 1111 1111 1111. Full-scale bipolar error is the worst-case of –FS
or +FS untrimmed deviation from ideal first and last code transitions and
includes the effect of offset error.
Note 8: All specifications in dB are referred to a full-scale ±10.24V input
with REFIN = 2.048V.
Note 9: When REFBUF is overdriven, the internal reference buffer must be
turned off by setting REFIN = 0V.
Note 10: fSMPL = 250kHz, IREFBUF varies proportionally with sample rate.
Note 11: Guaranteed by design, not subject to test.
Note 12: Parameter tested and guaranteed at OVDD = 1.71V, OVDD = 2.5V
and OVDD = 5.25V.
Note 13: tSCK of 10ns maximum allows a shift clock frequency up to
100MHz for rising edge capture.
Note 14: Temperature coefficient is calculated by dividing the maximum
change in output voltage by the specified temperature range.
0.8 • OVDD
tWIDTH
0.2 • OVDD
tDELAY
tDELAY
0.8 • OVDD
0.8 • OVDD
0.2 • OVDD
0.2 • OVDD
50%
50%
232618 F01
Figure 1. Voltage Levels for Timing Specifications
6
232618fb
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LTC2326-18
T
Typical
Performance Characteristics A = 25°C, VDD = 5V, OVDD = 2.5V, REFIN = 2.048V,
fSMPL = 250ksps, unless otherwise noted.
Integral Nonlinearity
vs Output Code
Differential Nonlinearity
vs Output Code
3.0
0.5
2.5
0.4
2.0
0.5
0
–0.5
–1.0
–1.5
0.2
2000
0.1
COUNTS
DNL ERROR (LSB)
1.0
0.0
–0.1
500
–0.4
–2.5
–3.0
–131072
–65536
0
65536
OUTPUT CODE
–0.5
–131072
131072
–65536
0
65536
OUTPUT CODE
CODE
THD, Harmonics
vs Input Frequency
–70
100
SNR
–60
–80
–100
–120
–80
THD, HARMONICS (dBFS)
90
SNR, SINAD (dBFS)
AMPLITUDE (dBFS)
232618 G03
SNR, SINAD vs Input Frequency
SNR = 95.3dB
THD = –113dB
SINAD = 95.2dB
SFDR = –117dB
–40
SINAD
80
70
–140
0
25
50
75
FREQUENCY (kHz)
100
60
125
0
25
50
75
FREQUENCY (kHz)
–100
–110
–120
THD
2ND
3RD
100
–140
125
0
25
50
75
FREQUENCY (kHz)
100
125
232618 G06
232618 G04
96.0
–90
–130
–160
–180
–6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7
232618 G02
32k Point FFT fS = 250ksps,
fIN = 2kHz
–20
0
131072
232618 G01
0
1500
1000
–0.2
–0.3
–2.0
σ = 1.6
2500
0.3
1.5
INL ERROR (LSB)
DC Histogram
3000
232618 G05V
SNR, SINAD vs Input Level,
fIN = 2kHz
98
SNR, SINAD vs Temperature,
fIN = 2kHz
–105
THD, Harmonics vs Temperature,
fIN = 2kHz
95.5
SNR
SINAD
95.0
THD, HARMONICS (dBFS)
SNR, SINAD (dBFS)
MAGNITUDE (dBFS)
97
96
95
SNR
SINAD
94
–110
THD
–115
3RD
2ND
–120
93
94.5
–40
–30
–10
–20
INPUT LEVEL (dB)
0
232618 G07
92
–40 –25 –10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
–125
–40 –25 –10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
232618 G09
232618 G08
232618fb
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7
LTC2326-18
T
Typical
Performance Characteristics A = 25°C, VDD = 5V, OVDD = 2.5V, REFIN = 2.048V,
fSMPL = 250ksps, unless otherwise noted.
Full-Scale Error vs Temperature
Offset Error vs Temperature
20
5
1.5
15
4
0.5
0
–0.5
–1.0
MAX INL
MAX DNL
MIN DNL
MIN INL
3
10
OFFSET ERROR (LSB)
1.0
FULL-SCALE ERROR (LSB)
INL, DNL ERROR (LSB)
INL/DNL vs Temperature
2.0
5
0
–5
–10
2
1
0
–1
–2
–3
–1.5
–15
–4
–2.0
–40 –25 –10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
–20
–40 –25 –10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
–5
–40 –25 –10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
232618 G11
232618 G10
Supply Current vs Temperature
120
2.0484
INTERNAL REFERENCE OUTPUT (V)
VDD
100
4
CURRENT (µA)
CURRENT (mA)
Internal Reference Output vs
Temperature
Sleep Current vs Temperature
6
5
IIN+ = IIN– = 0V)
3
2
80
60
40
20
1
OVDD
0
–40 –25 –10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
0
–40 –25 –10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
Internal Reference Output
Temperature Coefficient
Distribution
2.0482
2.0481
2.0480
2.0479
2.0478
2.0477
2.0476
–40 –25 –10 5 20 35 50 65 80 95 110 125
TEMPERATURE (°C)
232618 G15
CMRR vs Input Frequency
Supply Current vs Sampling Rate
80
12
30
75
10
25
70
20
15
65
60
10
–10 –8 –6 –4 –2 0 2 4
DRIFT (ppm/°C)
6
8 10
232618 G16
50
VDD (IN+ = –10.24V)
8
6
VDD (IN+ = 0V)
4
VDD (IN+ = 10.24V)
2
55
5
0
SUPPLY CURRENT (mA)
35
CMRR (dB)
NUMBER OF PARTS
2.0483
232618 G14
232618 G13
8
232618 G12
0
25
50
75
FREQUENCY (kHz)
100
125
232618 G17
0
OVDD
0
50
100
150
200
SAMPLING FREQUENCY (kHz)
250
232618 G18
232618fb
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LTC2326-18
Pin Functions
VDDLBYP (Pin 1): 2.5V Supply Bypass Pin. The voltage
on this pin is generated via an onboard regulator off of
VDD. This pin must be bypassed with a 2.2μF ceramic
capacitor to GND.
VDD (Pin 2): 5V Power Supply. The range of VDD is 4.75V to
5.25V. Bypass VDD to GND with a 10µF ceramic capacitor.
GND (Pins 3, 6 and 16): Ground.
IN+ (Pin 4): Analog Input. IN+ operates differential with
respect to IN– with an IN+-IN– range of –2.5 • VREFBUF to
2.5 • VREFBUF.
IN– (Pin 5): Analog Ground Sense. IN– has an input range
of ±500mV with respect to GND and must be tied to the
ground plane or a remote sense.
REFBUF (Pin 7): Reference Buffer Output. An onboard
buffer nominally outputs 4.096V to this pin. This pin is
referred to GND and should be decoupled closely to the pin
with a 47μF ceramic capacitor. The internal buffer driving
this pin may be disabled by grounding its input at REFIN.
Once the buffer is disabled, an external reference may
overdrive this pin in the range of 2.5V to 5V. A resistive
load greater than 500kΩ can be placed on the reference
buffer output.
REFIN (Pin 8): Reference Output/Reference Buffer Input.
An onboard bandgap reference nominally outputs 2.048V
at this pin. Bypass this pin with a 100nF ceramic capacitor
to GND to limit the reference output noise. If more accuracy is desired, this pin may be overdriven by an external
reference in the range of 1.25V to 2.4V.
CHAIN (Pin 10): Chain Mode Selector Pin. When low,
the LTC2326-18 operates in normal mode and the
RDL/SDI input pin functions to enable or disable SDO.
When high, the LTC2326-18 operates in chain mode and the
RDL/SDI pin functions as SDI, the daisy-chain serial data
input. Logic levels are determined by OVDD.
BUSY (Pin 11): BUSY Indicator. Goes high at the start of
a new conversion and returns low when the conversion
has finished. Logic levels are determined by OVDD.
RDL/SDI (Pin 12): When CHAIN is low, the part is in normal mode and the pin is treated as a bus enabling input.
When CHAIN is high, the part is in chain mode and the
pin is treated as a serial data input pin where data from
another ADC in the daisy chain is input. Logic levels are
determined by OVDD.
SCK (Pin 13): Serial Data Clock Input. When SDO is enabled,
the conversion result or daisy-chain data from another
ADC is shifted out on the rising edges of this clock MSB
first. Logic levels are determined by OVDD.
SDO (Pin 14): Serial Data Output. The conversion result or
daisy-chain data is output on this pin on each rising edge
of SCK MSB first. The output data is in 2’s complement
format. Logic levels are determined by OVDD.
OVDD (Pin 15): I/O Interface Digital Power. The range of
OVDD is 1.71V to 5.25V. This supply is nominally set to
the same supply as the host interface (1.8V, 2.5V, 3.3V,
or 5V). Bypass OVDD to GND with a 0.1μF capacitor.
CNV (Pin 9): Convert Input. A rising edge on this input
powers up the part and initiates a new conversion. Logic
levels are determined by OVDD.
232618fb
For more information www.linear.com/LTC2326-18
9
LTC2326-18
Functional Block Diagram
VDD = 5V
VDDLBYP = 2.5V
REFIN = 1.25V
TO 2.4V
OVDD = 1.8V
TO 5V
REFBUF = 2.5V
TO 5V
LDO
15k
2.048V
REFERENCE
2× REFERENCE
BUFFER
IN+
IN–
4R
4R
R
0.63× BUFFER
+
R
18-BIT SAMPLING ADC
–
CHAIN
SDO
RDL/SDI
SCK
SPI
PORT
CNV
BUSY
CONTROL LOGIC
GND
232618 BD01
Timing Diagram
Conversion Timing Using the Serial Interface
CHAIN, RDL/SDI = 0
CNV
BUSY
CONVERT
HOLD
NAP
ACQUIRE
SCK
SDO
10
D17 D16 D15
D2 D1 D0
232618 TD01
232618fb
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LTC2326-18
Applications Information
Overview
Transfer Function
The LTC2326-18 is a low noise, high speed 18-bit successive approximation register (SAR) ADC with pseudodifferential inputs. Operating from a single 5V supply,
the LTC2326-18 has a ±10.24V true bipolar input range,
making it ideal for high voltage applications which require
a wide dynamic range. The LTC2326-18 achieves ±5LSB
INL maximum, no missing codes at 18-bits and 95dB SNR.
The LTC2326-18 digitizes the full-scale voltage of ±2.5 •
REFBUF into 218 levels, resulting in an LSB size of 78µV
with REFBUF = 4.096V. The ideal transfer function is shown
in Figure 2. The output data is in 2’s complement format.
The analog inputs of the LTC2326-18 are pseudo-differential in order to reduce any unwanted signal that is common
to both inputs. The analog inputs can be modeled by the
equivalent circuit shown in Figure 3. The back-to-back
diodes at the inputs form clamps that provide ESD protection. Each input drives a resistor divider network that has
011...111
OUTPUT CODE (TWO’S COMPLEMENT)
The LTC2326-18 has an onboard single-shot capable
reference buffer and low drift (20ppm/°C max) 2.048V
temperature-compensated reference. The LTC2326-18
also has a high speed SPI-compatible serial interface that
supports 1.8V, 2.5V, 3.3V and 5V logic while also featuring
a daisy-chain mode. The fast 250ksps throughput with
no cycle latency makes the LTC2326-18 ideally suited
for a wide variety of high speed applications. An internal
oscillator sets the conversion time, easing external timing
considerations. The LTC2326-18 dissipates only 28mW
and automatically naps between conversions, leading to
reduced power dissipation that scales with the sampling
rate. A sleep mode is also provided to reduce the power
consumption of the LTC2326-18 to 300μW for further
power savings during inactive periods.
Analog Input
BIPOLAR
ZERO
011...110
000...001
000...000
111...111
111...110
100...001
FSR = +FS – –FS
1LSB = FSR/262144
100...000
–FSR/2
Converter Operation
The LTC2326-18 operates in two phases. During the acquisition phase, the charge redistribution capacitor D/A
converter (CDAC) is connected to the outputs of the resistor divider networks that pins IN+ and IN– drive to sample
an attenuated and level-shifted version of the pseudodifferential analog input voltage as shown in Figure 3. A
rising edge on the CNV pin initiates a conversion. During
the conversion phase, the 18-bit CDAC is sequenced
through a successive approximation algorithm, effectively
comparing the sampled input with binary-weighted fractions of the reference voltage (e.g. VREFBUF/2, VREFBUF/4
… VREFBUF/262144) using the differential comparator. At
the end of conversion, the CDAC output approximates the
sampled analog input. The ADC control logic then prepares
the 18-bit digital output code for serial transfer.
–1 0V 1
FSR/2 – 1LSB
LSB
LSB
INPUT VOLTAGE (V)
232618 F02
Figure 2. LTC2326-18 Transfer Function
0.63 • VREFBUF
IN+
1.6k
400Ω
RON
50Ω
CIN
45pF
0.63 • VREFBUF
IN–
1.6k
400Ω
RON
50Ω
CIN
45pF
BIAS
VOLTAGE
232618 F03
Figure 3. The Equivalent Circuit for the Differential
Analog Input of the LTC2326-18
232618fb
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11
LTC2326-18
Applications Information
a total impedance of 2kΩ. The resistor divider network
attenuates and level shifts the ±2.5 • REFBUF true bipolar
signal swing of each input to the 0-REFBUF input signal
swing of the ADC core. In the acquisition phase, 45pF (CIN)
from the sampling CDAC in series with approximately 50Ω
(RON) from the on-resistance of the sampling switch is
connected to the output of the resistor divider network.
Any unwanted signal that is common to both inputs will
be reduced by the common mode rejection of the ADC
core and resistor divider network. The IN+ input of the
ADC core draws a current spike while charging the CIN
capacitor during acquisition.
Input Drive Circuits
A low impedance source can directly drive the high impedance input of the LTC2326-18 without gain error. A
high impedance source should be buffered to minimize
settling time during acquisition and to optimize the distortion performance of the ADC. Minimizing settling time
is important even for DC inputs, because the ADC input
draws a current spike when entering acquisition.
For best performance, a buffer amplifier should be used
to drive the analog input of the LTC2326-18. The amplifier
provides low output impedance to minimize gain error
and allows for fast settling of the analog signal during
the acquisition phase. It also provides isolation between
the signal source and the ADC input which draws a small
current spike during acquisition.
Input Filtering
The noise and distortion of the buffer amplifier and signal
source must be considered since they add to the ADC noise
and distortion. Noisy input signals should be filtered prior
to the buffer amplifier input with a low bandwidth filter to
minimize noise. The simple 1-pole RC lowpass filter shown
in Figure 4 is sufficient for many applications.
The input resistor divider network, sampling switch onresistance (RON) and the sample capacitor (CIN) form a
second lowpass filter that limits the input bandwidth to
the ADC core to 7MHz. A buffer amplifier with a low noise
density must be selected to minimize the degradation of
the SNR over this bandwidth.
12
50Ω
±10.24V
+
66nF
LT1468
BW = 48kHz
IN+
–
LTC2326-18
IN–
232618 F04
Figure 4. Input Signal Chain
High quality capacitors and resistors should be used in the
RC filters since these components can add distortion. NPO
and silver mica type dielectric capacitors have excellent
linearity. Carbon surface mount resistors can generate
distortion from self heating and from damage that may
occur during soldering. Metal film surface mount resistors
are much less susceptible to both problems.
Pseudo-Differential Bipolar Inputs
For most applications, we recommend the low power
LT1468 ADC driver to drive the LTC2326-18. With a low
noise density of 5nV/√Hz and a low supply current of 3mA,
the LT1468 is flexible and may be configured to convert
signals of various amplitudes to the ±10.24V input range
of the LTC2326-18.
To achieve the full distortion performance of the
LTC2326‑18, a low distortion single-ended signal source
driven through the LT1468 configured as a unity-gain
buffer as shown in Figure 4 can be used to get the full
data sheet THD specification of –111dB.
ADC Reference
There are three ways of providing the ADC reference. The
first is to use both the internal reference and reference
buffer. The second is to externally overdrive the internal
reference and use the internal reference buffer. The third
is to disable the internal reference buffer and overdrive
the REFBUF pin from an external source. The following
tables give examples of these cases and the resulting
bipolar input ranges.
232618fb
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LTC2326-18
Applications Information
External Reference with Internal Buffer
Table 1. Internal Reference with Internal Buffer
REFIN
REFBUF
BIPOLAR INPUT RANGE
2.048V
4.096V
±10.24V
Table 2. External Reference with Internal Buffer
REFIN
(OVERDRIVE)
REFBUF
BIPOLAR INPUT RANGE
1.25V (Min)
2.5V
±6.25V
2.048V
4.096V
±10.24V
2.4V (Max)
4.8V
±12V
Table 3. External Reference Unbuffered
REFIN
REFBUF
(OVERDRIVE)
BIPOLAR INPUT RANGE
0V
2.5V (Min)
±6.25V
0V
5V (Max)
±12.5V
Internal Reference with Internal Buffer
The LTC2326-18 has an on-chip, low noise, low drift
(20ppm/°C max), temperature compensated bandgap
reference that is factory trimmed to 2.048V. It is internally
connected to a reference buffer as shown in Figure 5a and
is available at REFIN (Pin 8). REFIN should be bypassed to
GND with a 100nF ceramic capacitor to minimize noise. The
reference buffer gains the REFIN voltage by 2 to 4.096V at
REFBUF (Pin 7). So the input range is ±10.24V, as shown
in Table 1. Bypass REFBUF to GND with at least a 47μF
ceramic capacitor (X7R, 10V, 1210 size) to compensate
the reference buffer and minimize noise.
15k
REFIN
BANDGAP
REFERENCE
100nF
REFBUF
REFERENCE
BUFFER
6.5k
47µF
6.5k
GND
LTC2326-18
232618 F05a
Figure 5a. LTC2326-18 Internal Reference Circuit
If more accuracy and/or lower drift is desired, REFIN
can be easily overdriven by an external reference since
a 15k resistor is in series with the reference as shown
in Figure 5b. REFIN can be overdriven in the range from
1.25V to 2.4V. The resulting voltage at REFBUF will be
2 • REFIN. So the input range is ±5 • REFIN, as shown
in Table 2. Linear Technology offers a portfolio of high
performance references designed to meet the needs of
many applications. With its small size, low power, and high
accuracy, the LTC6655-2.048 is well suited for use with
the LTC2326-18 when overdriving the internal reference.
The LTC6655-2.048 offers 0.025% (max) initial accuracy
and 2ppm/°C (max) temperature coefficient for high precision applications. The LTC6655-2.048 is fully specified
over the H-grade temperature range and complements
the extended temperature range of the LTC2326-18 up to
125°C. Bypassing the LTC6655-2.048 with a 2.7μF to 100µF
ceramic capacitor close to the REFIN pin is recommended.
External Reference Unbuffered
The internal reference buffer can also be overdriven from
2.5V to 5V with an external reference at REFBUF as shown
in Figure 5c. So the input ranges are ±6.25V to ±12.5V,
respectively, as shown in Table 3. To do so, REFIN must
be grounded to disable the reference buffer. A 13k resistor loads the REFBUF pin when the reference buffer is
disabled. To maximize the input signal swing and corresponding SNR, the LTC6655-5 is recommended when
overdriving REFBUF. The LTC6655-5 offers the same small
size, accuracy, drift and extended temperature range as
the LTC6655-2.048. By using this 5V reference, an SNR
of 97dB can be achieved. Bypassing the LTC6655-5 with
a 47μF ceramic capacitor (X5R, 0805 size) close to the
REFBUF pin is recommended.
The REFBUF pin of the LTC2326-18 draws a charge (QCONV)
from the external bypass capacitor during each conversion
cycle. If the internal reference buffer is overdriven, the
external reference must provide all of this charge with a
DC current equivalent to IREFBUF = QCONV/tCYC. Thus, the
DC current draw of REFBUF depends on the sampling
232618fb
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13
LTC2326-18
Applications Information
15k
REFIN
BANDGAP
REFERENCE
2.7µF
REFBUF
REFERENCE
BUFFER
Internal Reference Buffer Transient Response
47µF
LTC6655-2.048
6.5k
6.5k
LTC2326-18
GND
232618 F05b
Figure 5b. Using the LTC6655-2.048 as an External Reference
15k
REFIN
REFBUF
of the output code. If an external reference is used to
overdrive REFBUF, the fast settling LTC6655-5 reference
is recommended.
BANDGAP
REFERENCE
REFERENCE
BUFFER
6.5k
For optimum transient performance, the internal reference
buffer should be used. The internal reference buffer uses a
proprietary design that results in an output voltage change
at REFBUF of less than 1LSB when responding to a sudden
burst of conversions. This makes the internal reference
buffer of the LTC2326-18 truly single-shot capable since the
first sample taken after idling will yield the same result as
a sample taken after the transient response of the internal
reference buffer has settled. Figure 7 shows the transient
responses of the LTC2326-18 with the internal reference
buffer and with the internal reference buffer overdriven by
the LTC6655-5, both with a bypass capacitance of 47μF.
47µF
LTC6655-5
6.5k
2
LTC2326-18
232618 F05c
Figure 5c. Overdriving REFBUF Using the LTC6655-5
rate and output code. In applications where a burst of
samples is taken after idling for long periods, as shown in
Figure 6, IREFBUF quickly goes from approximately 390µA
to a maximum of 0.6mA for REFBUF = 5V at 250ksps. This
step in DC current draw triggers a transient response in
the external reference that must be considered since any
deviation in the voltage at REFBUF will affect the accuracy
DEVIATION FROM FINAL VALUE (LSB)
GND
INTERNAL REFERENCE BUFFER
0
–2
EXTERNAL SOURCE ON REFBUF
–4
–6
–8
0 100 200 300 400 500 600 700 800 900 1000
TIME (µs)
232618 F07
Figure 7. Transient Response of the LTC2326-18
CNV
IDLE
PERIOD
IDLE
PERIOD
232618 F06
Figure 6. CNV Waveform Showing Burst Sampling
14
232618fb
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LTC2326-18
Applications Information
Dynamic Performance
Total Harmonic Distortion (THD)
Fast Fourier Transform (FFT) techniques are used to test
the ADC’s frequency response, distortion and noise at the
rated throughput. By applying a low distortion sine wave
and analyzing the digital output using an FFT algorithm,
the ADC’s spectral content can be examined for frequencies outside the fundamental. The LTC2326-18 provides
guaranteed tested limits for both AC distortion and noise
measurements.
Total Harmonic Distortion (THD) is the ratio of the RMS sum
of all harmonics of the input signal to the fundamental itself.
The out-of-band harmonics alias into the frequency band
between DC and half the sampling frequency (fSMPL/2).
THD is expressed as:
Signal-to-Noise and Distortion Ratio (SINAD)
where V1 is the RMS amplitude of the fundamental
frequency and V2 through VN are the amplitudes of the
second through Nth harmonics.
The signal-to-noise and distortion ratio (SINAD) is the
ratio between the RMS amplitude of the fundamental input
frequency and the RMS amplitude of all other frequency
components at the A/D output. The output is band limited
to frequencies from above DC and below half the sampling
frequency. Figure 8 shows that the LTC2326-18 achieves
a typical SINAD of 95dB at a 250kHz sampling rate with
a 2kHz input.
Signal-to-Noise Ratio (SNR)
The signal-to-noise ratio (SNR) is the ratio between the
RMS amplitude of the fundamental input frequency and
the RMS amplitude of all other frequency components
except the first five harmonics and DC. Figure 8 shows
that the LTC2326-18 achieves a typical SNR of 95dB at a
250kHz sampling rate with a 2kHz input.
0
SNR = 95.3dB
THD = –113dB
SINAD = 95.2dB
SFDR = –117dB
–20
AMPLITUDE (dBFS)
–40
–60
–80
–100
–120
V22 + V32 + V42 +…+ VN
THD= 20log
V1
2
Power Considerations
The LTC2326-18 provides two power supply pins: the 5V
power supply (VDD), and the digital input/output interface
power supply (OVDD). The flexible OVDD supply allows
the LTC2326-18 to communicate with any digital logic
operating between 1.8V and 5V, including 2.5V and 3.3V
systems.
Power Supply Sequencing
The LTC2326-18 does not have any specific power supply
sequencing requirements. Care should be taken to adhere
to the maximum voltage relationships described in the
Absolute Maximum Ratings section. The LTC2326‑18
has a power-on reset (POR) circuit that will reset the
LTC2326-18 at initial power-up or whenever the power
supply voltage drops below 2V. Once the supply voltage
reenters the nominal supply voltage range, the POR will
re-initialize the ADC. No conversions should be initiated
until 200μs after a POR event to ensure the re-initialization
period has ended. Any conversions initiated before this
time will produce invalid results.
–140
Timing and Control
–160
–180
0
25
50
75
FREQUENCY (kHz)
100
125
232618 F08
Figure 8. 32k Point FFT of the LTC2326-18
CNV Timing
The LTC2326-18 conversion is controlled by CNV. A rising edge on CNV will start a conversion and power up
the LTC2326-18. Once a conversion has been initiated,
232618fb
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15
LTC2326-18
Applications Information
A proprietary sampling architecture allows the LTC2326-18
to begin acquiring the input signal for the next conversion 527ns after the start of the current conversion. This
extends the acquisition time to 3.460µs, easing settling
requirements and allowing the use of extremely low power
ADC drivers. (Refer to the Timing Diagram.)
Internal Conversion Clock
The LTC2326-18 has an internal clock that is trimmed to
achieve a maximum conversion time of 3µs.
Auto Nap Mode
The LTC2326-18 automatically enters nap mode after a
conversion has been completed and completely powers
up once a new conversion is initiated on the rising edge of
CNV. During nap mode, only the ADC core powers down
and all other circuits remain active. During nap, data from
the last conversion can be clocked out. The auto nap mode
feature will reduce the power dissipation of the LTC2326-18
as the sampling frequency is reduced. Since full power is
consumed only during a conversion, the ADC core of the
LTC2326-18 remains powered down for a larger fraction of
the conversion cycle (tCYC) at lower sample rates, thereby
reducing the average power dissipation which scales with
the sampling rate as shown in Figure 9.
Sleep Mode
The auto nap mode feature provides limited power savings
since only the ADC core powers down. To obtain greater
power savings, the LTC2326-18 provides a sleep mode.
During sleep mode, the entire part is powered down
except for a small standby current resulting in a power
dissipation of 300μW. To enter sleep mode, toggle CNV
VDD (IN+ = –10.24V)
10
8
6
VDD (IN+ = 0V)
4
VDD (IN+ = 10.24V)
2
0
Acquisition
16
12
SUPPLY CURRENT (mA)
it cannot be restarted until the conversion is complete.
For optimum performance, CNV should be driven by a
clean low jitter signal. Converter status is indicated by the
BUSY output which remains high while the conversion is
in progress. To ensure that no errors occur in the digitized
results, any additional transitions on CNV should occur
within 40ns from the start of the conversion or after the
conversion has been completed. Once the conversion has
completed, the LTC2326-18 powers down.
OVDD
0
50
100
150
200
SAMPLING FREQUENCY (kHz)
250
232618 G18
Figure 9. Power Supply Current of the LTC2326-18
Versus Sampling Rate.
twice with no intervening rising edge on SCK. The part
will enter sleep mode on the falling edge of BUSY from
the last conversion initiated. Once in sleep mode, a rising
edge on SCK will wake the part up. Upon emerging from
sleep mode, wait tWAKE ms before initiating a conversion
to allow the reference and reference buffer to wake up
and charge the bypass capacitors at REFIN and REFBUF.
(Refer to the Timing Diagrams section for more detailed
timing information about sleep mode.)
Digital Interface
The LTC2326-18 has a serial digital interface. The flexible
OVDD supply allows the LTC2326-18 to communicate with
any digital logic operating between 1.8V and 5V, including
2.5V and 3.3V systems.
The serial output data is clocked out on the SDO pin when
an external clock is applied to the SCK pin if SDO is enabled.
Clocking out the data after the conversion will yield the
best performance. With a shift clock frequency of at least
20MHz, a 250ksps throughput is still achieved. The serial
output data changes state on the rising edge of SCK and
can be captured on the falling edge or next rising edge of
SCK. D17 remains valid till the first rising edge of SCK.
The serial interface on the LTC2326-18 is simple and
straightforward to use. The following sections describe the
operation of the LTC2326-18. Several modes are provided
depending on whether a single or multiple ADCs share the
SPI bus or are daisy-chained.
For more information www.linear.com/LTC2326-18
232618fb
LTC2326-18
Applications Information
Normal Mode, Single Device
When CHAIN = 0, the LTC2326-18 operates in normal
mode. In normal mode, RDL/SDI enables or disables the
serial data output pin SDO. If RDL/SDI is high, SDO is in
high impedance. If RDL/SDI is low, SDO is driven. Figure 10
shows a single LTC2326-18 operated in normal mode
with CHAIN and RDL/SDI tied to ground. With RDL/SDI
grounded, SDO is enabled and the MSB(D17) of the new
conversion data is available at the falling edge of BUSY.
This is the simplest way to operate the LTC2326-18.
CONVERT
DIGITAL HOST
CNV
CHAIN
LTC2326-18
RDL/SDI
BUSY
IRQ
SDO
DATA IN
SCK
CLK
NAP
ACQUIRE
CONVERT
NAP
CONVERT
ACQUIRE
CHAIN = 0
RDL/SDI = 0
tCYC
tCNVH
tCNVL
CNV
tHOLD
tACQ
tACQ = tCYC – tHOLD
tCONV
BUSY
tSCK
tBUSYLH
tSCKH
1
SCK
2
3
tHSDO
tDSDOBUSYL
SDO
tQUIET
16
17
18
tSCKL
tDSDO
D17
D16
D15
D1
D0
232618 F12
Figure 10. Using a Single LTC2326-18 in Normal Mode
232618fb
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17
LTC2326-18
Applications Information
Normal Mode, Multiple Devices
be used to allow only one LTC2326-18 to drive SDO at a
time in order to avoid bus conflicts. As shown in Figure 11,
the RDL/SDI inputs idle high and are individually brought
low to read data out of each device between conversions.
When RDL/SDI is brought low, the MSB of the selected
device is output onto SDO.
Figure 11 shows multiple LTC2326-18 devices operating
in normal mode (CHAIN = 0) sharing CNV, SCK and SDO.
By sharing CNV, SCK and SDO, the number of required
signals to operate multiple ADCs in parallel is reduced.
Since SDO is shared, the RDL/SDI input of each ADC must
RDLB
RDLA
CONVERT
CNV
CHAIN
CNV
CHAIN
LTC2326-18
SDO
B
BUSY
LTC2326-18
A
IRQ
DIGITAL HOST
SDO
RDL/SDI
RDL/SDI
SCK
SCK
DATA IN
CLK
NAP
CONVERT
NAP
ACQUIRE
CONVERT
ACQUIRE
CHAIN = 0
tCNVL
CNV
tHOLD
BUSY
tCONV
tBUSYLH
RDL/SDIA
RDL/SDIB
tSCK
1
SCK
2
tSCKH
3
16
17
18
tHSDO
SDO
Hi-Z
D17A
D16A
D15A
19
20
21
34
35
36
tSCKL
tDSDO
tEN
tQUIET
tDIS
D1A
D0A
Hi-Z
D17B
D16B
D15B
D1B
D0B
Hi-Z
232618 F11
Figure 11. Normal Mode with Multiple Devices Sharing CNV, SCK, and SDO
18
232618fb
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LTC2326-18
Applications Information
Chain Mode, Multiple Devices
may limit the number of lines needed to interface to a large
number of converters. Figure 12 shows an example with
two daisy-chained devices. The MSB of converter A will
appear at SDO of converter B after 18 SCK cycles. The
MSB of converter A is clocked in at the SDI/RDL pin of
converter B on the rising edge of the first SCK.
When CHAIN = OVDD, the LTC2326-18 operates in
chain mode. In chain mode, SDO is always enabled and
RDL/SDI serves as the serial data input pin (SDI) where
daisy-chain data output from another ADC can be input.
This is useful for applications where hardware constraints
CONVERT
OVDD
OVDD
CNV
CHAIN
CNV
CHAIN
LTC2326-18
RDL/SDI
RDL/SDI
SDO
A
DIGITAL HOST
LTC2326-18
IRQ
BUSY
B
DATA IN
SDO
SCK
SCK
CLK
NAP
ACQUIRE
CONVERT
NAP
ACQUIRE
CONVERT
CHAIN = OVDD
RDL/SDIA = 0
tCYC
tCNVL
CNV
tHOLD
BUSY
tCONV
tBUSYLH
tSCKCH
SCK
1
2
3
16
17
tSSDISCK
18
19
20
34
35
36
tSCKL
tHSDO
tHSDISCK
SDOA = RDL/SDIB
tQUIET
tSCKH
tDSDO
D17A
D16A
D15A
D1A
D0A
D17B
D16B
D15B
D1B
D0B
tDSDOBUSYL
SDOB
D17A
D16A
D1A
D0A
232618 F12
Figure 12. Chain Mode Timing Diagram
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19
LTC2326-18
Applications Information
Sleep Mode
To enter sleep mode, toggle CNV twice with no intervening rising edge on SCK as shown in Figure 13. The part
will enter sleep mode on the falling edge of BUSY from
CHAIN = DON’T CARE
RDL/SDI = DON’T CARE
CONVERT
NAP
the last conversion initiated. Once in sleep mode, a rising
edge on SCK will wake the part up. Upon emerging from
sleep mode, wait tWAKE ms before initiating a conversion
to allow the reference and reference buffer to wake up
and charge the bypass capacitors at REFIN and REFBUF.
CONVERT
SLEEP
ACQUIRE
NAP AND
ACQUIRE
CONVERT
tWAKE
tCNVH
CNV
tHOLD
BUSY
tACQ
tCONV
tCONV
tBUSYLH
SCK
CHAIN = DON’T CARE
RDL/SDI = DON’T CARE
CONVERT
SLEEP
NAP AND
ACQUIRE
CONVERT
tWAKE
tCNVH
CNV
tCONV
BUSY
tBUSYLH
SCK
232618 F13
Figure 13. Sleep Mode Timing Diagram
20
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LTC2326-18
Board Layout
To obtain the best performance from the LTC2326-18 a
printed circuit board (PCB) is recommended. Layout for the
PCB should ensure the digital and analog signal lines are
separated as much as possible. In particular, care should
be taken not to run any digital clocks or signals alongside
analog signals or underneath the ADC.
Recommended Layout
The following is an example of a recommended PCB layout.
A single solid ground plane is used. Bypass capacitors to
the supplies are placed as close as possible to the supply
pins. Low impedance common returns for these bypass
capacitors are essential to the low noise operation of the
ADC. The analog input traces are screened by ground.
For more details and information refer to DC1908, the
evaluation kit for the LTC2326-18.
Partial Top Silkscreen
Partial Layer 1 Component Side
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21
LTC2326-18
Board Layout
Partial Layer 2 Ground Plane
Partial Layer 3 Power Plane
Partial Layer 4 Bottom Layer
22
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LTC2326-18
Board Layout
Partial Schematic of Demo Board
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23
LTC2326-18
Package Description
Please refer to http://www.linear.com/product/LTC2326-18#packaging for the most recent package drawings.
MS Package
16-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1669 Rev A)
0.889 ±0.127
(.035 ±.005)
5.10
(.201)
MIN
3.20 – 3.45
(.126 – .136)
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
0.50
(.0197)
BSC
0.305 ±0.038
(.0120 ±.0015)
TYP
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
DETAIL “A”
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
4.90 ±0.152
(.193 ±.006)
0° – 6° TYP
0.280 ±0.076
(.011 ±.003)
REF
16151413121110 9
GAUGE PLANE
0.53 ±0.152
(.021 ±.006)
DETAIL “A”
0.18
(.007)
SEATING
PLANE
1.10
(.043)
MAX
0.17 – 0.27
(.007 – .011)
TYP
1234567 8
0.50
(.0197)
BSC
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
24
0.86
(.034)
REF
0.1016 ±0.0508
(.004 ±.002)
MSOP (MS16) 0213 REV A
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LTC2326-18
Revision History
REV
DATE
DESCRIPTION
A
10/14
Minimum SINAR/SNR Numbers Updated for ±6.25V Range
B
07/16
PAGE NUMBER
3
Added Conditions to G13
8
Replaced Figure 9
16
Updated graphs G01, G02 and G03
7
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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 itsinformation
circuits as described
herein will not infringe on existing patent rights.
For more
www.linear.com/LTC2326-18
25
LTC2326-18
Typical Application
LT1468 Configured to Buffer a ±10.24V Single-Ended Signal Into the LTC2326-18
15V
7
LT1468
5V
+10.24V
3
+
–10.24V
IN+
6
2
–
VDD
LTC2326-18
IN
–
REFBUF
4
REFIN
47µF
–15V
100nF
232618 TA02
Related Parts
PART NUMBER
DESCRIPTION
COMMENTS
ADCs
LTC2338-18/LTC2337-18/ 18-Bit, 1Msps/500ksps/250ksps Serial,
LTC2336-18
Low Power ADC
5V Supply, ±10.24V True Bipolar, Differential Input, 100dB SNR, Pin-Compatible
Family in MSOP-16 Package
LTC2378-20/LTC2377-20/ 20-Bit, 1Msps/500ksps/250ksps Serial,
LTC2376-20
Low Power ADC
2.5V Supply, Differential Input, 0.5ppm INL, ±5V Input Range, DGC, PinCompatible Family in MSOP-16 and 4mm × 3mm DFN-16 Packages
LTC2379-18/LTC2378-18/ 18-Bit, 1.6Msps/1Msps/500ksps/250ksps
LTC2377-18/LTC2376-18 Serial, Low Power ADC
2.5V Supply, Differential Input, 101.2dB SNR, ±5V Input Range, DGC,
Pin-Compatible Family in MSOP-16 and 4mm × 3mm DFN-16 Packages
LTC2380-16/LTC2378-16/ 16-Bit, 2Msps/1Msps/500ksps/250ksps
LTC2377-16/LTC2376-16 Serial, Low Power ADC
2.5V Supply, Differential Input, 96.2dB SNR, ±5V Input Range, DGC,
Pin-Compatible Family in MSOP-16 and 4mm × 3mm DFN-16 Packages
LTC2369-18/LTC2368-18/ 18-Bit, 1.6Msps/1Msps/500ksps/250ksps
LTC2367-18/LTC2364-18 Serial, Low Power ADC
2.5V Supply, Pseudo-Differential Unipolar Input, 96.5dB SNR, 0V to 5V Input
Range, Pin-Compatible Family in MSOP-16 and 4mm × 3mm DFN-16 Packages
LTC2370-16/LTC2368-16/ 16-Bit, 2Msps/1Msps/500ksps/250ksps
LTC2367-16/LTC2364-16 Serial, Low Power ADC
2.5V Supply, Pseudo-Differential Unipolar Input, 94dB SNR, 0V to 5V Input
Range, Pin-Compatible Family in MSOP-16 and 4mm × 3mm DFN-16 Packages
LTC2389-18/LTC2389-16 18-Bit/16-Bit, 2.5Msps Parallel/Serial ADC
5V Supply, Pin-Configurable Input Range, 99.8dB/96dB SNR, Parallel or Serial
I/O 7mm × 7mm LQFP-48 and QFN-48 Packages
LTC1609
±10V, Configurable Unipolar/Bipolar Input, Single 5V Supply, SSOP-28 and
SO-20 Packages
16-Bit, 200ksps Serial ADC
DACs
LTC2756/LTC2757
18-Bit, Single Serial/Parallel IOUT SoftSpan™ ±1LSB INL/DNL, Software-Selectable Ranges, SSOP-28/7mm × 7mm LQFP-48
DAC
Package
LTC2641
16-Bit/14-Bit/12-Bit Single Serial VOUT DAC
±1LSB INL /DNL, MSOP-8 Package, 0V to 5V Output
LTC2630
12-Bit/10-Bit/8-Bit Single VOUT DACs
±1LSB INL (12 Bits), Internal Reference, SC70 6-Pin Package
LTC6655
Precision Low Drift Low Noise Buffered
Reference
5V/2.5V/2.048V/1.2V, 2ppm/°C, 0.25ppm Peak-to-Peak Noise, MSOP-8 Package
LTC6652
Precision Low Drift Low Noise Buffered
Reference
5V/2.5V/2.048V/1.2V, 5ppm/°C, 2.1ppm Peak-to-Peak Noise, MSOP-8 Package
References
Amplifiers
LT1468/LT1469
Single/Dual 90MHz, 22V/μs, 16-Bit Accurate Low Input Offset: 75μV/125µV
Op Amp
26 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LTC2326-18
●
●
(408) 432-1900 FAX: (408) 434-0507
www.linear.com/LTC2326-18
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LT 0716 REV B • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2014
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