LTC2378-20 20-Bit, 1Msps, Low Power SAR ADC with 0.5ppm INL

28 Pages·2015·0.95 MB·English

by  Linear Technology Corporation

LTC2378-20
20-Bit, 1Msps, Low Power
SAR ADC with 0.5ppm INL
FEATURES DESCRIPTION
n 1Msps Throughput Rate The LTC®2378-20 is a low noise, low power, high speed
n ±0.5ppm INL (Typ) 20-bit successive approximation register (SAR) ADC.
n Guaranteed 20-Bit No Missing Codes Operating from a 2.5V supply, the LTC2378-20 has a
n Low Power: 21mW at 1Msps, 21µW at 1ksps ±V fully differential input range with V ranging from
REF REF
n 104dB SNR (Typ) at f = 2kHz 2.5V to 5.1V. The LTC2378-20 consumes only 21mW and
IN
n –125dB THD (Typ) at f = 2kHz achieves ±2ppm INL maximum, no missing codes at 20
IN
n Digital Gain Compression (DGC) bits with 104dB SNR.
n Guaranteed Operation to 85°C
The LTC2378-20 has a high speed SPI-compatible serial
n 2.5V Supply
interface that supports 1.8V, 2.5V, 3.3V and 5V logic
n Fully Differential Input Range ±V
REF while also featuring a daisy-chain mode. The fast 1Msps
n V Input Range from 2.5V to 5.1V
REF throughput with no cycle latency makes the LTC2378-20
n No Pipeline Delay, No Cycle Latency
ideally suited for a wide variety of high speed applications.
n 1.8V to 5V I/O Voltages
An internal oscillator sets the conversion time, easing exter-
n SPI-Compatible Serial I/O with Daisy-Chain Mode
nal timing considerations. The LTC2378-20 automatically
n Internal Conversion Clock
powers down between conversions, leading to reduced
n 16-Lead MSOP and 4mm × 3mm DFN Packages
power dissipation that scales with the sampling rate.
APPLICATIONS
The LTC2378-20 features a unique digital gain compres-
sion (DGC) function, which eliminates the driver amplifier’s
n Medical Imaging
negative supply while preserving the full resolution of the
n High Speed Data Acquisition
ADC. When enabled, the ADC performs a digital scaling
n Portable or Compact Instrumentation
function that maps zero-scale code from 0V to 0.1 • V
REF
n Industrial Process Control
and full-scale code from V to 0.9 • V . For a typical
REF REF
n Low Power Battery-Operated Instrumentation
reference voltage of 5V, the full-scale input range is now
n ATE
0.5V to 4.5V, which provides adequate headroom for
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and powering the driving amplifier from a single 5.5V supply.
SoftSpan is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners. Patents Pending. Protected by U.S. Patents, including
7705765, 7961132, 8319673.
TYPICAL APPLICATION
Integral Nonlinearity vs Output Code
2.5V 1.8V TO 5V 2.0
1.5
10µF 0.1µF
1.0
VREF 10Ω 6800pF VDD OVDD CHAIN pm) 0.5
0V + IN+ RDSLD/SODI OR (p 0
VREF – 3300pF IN– LTC2378-20 BCSUNCSKVY SAMPLE CLOCK NL ERR –0.5
0V 10Ω 6800pF RREEFF GND REF/DGC VREF I –1.0
2.5V TO 5.1V 237820 TA01
–1.5
47µF
(X7R, 1210 SIZE) –2.0
0 262144 524288 786432 1048576
OUTPUT CODE
237820 TA02
237820fa
1
For more information www.linear.com/LTC2378-20
LTC2378-20
ABSOLUTE MAXIMUM RATINGS
(Notes 1, 2)
Supply Voltage (V ) ...............................................2.8V Digital Output Voltage
DD
Supply Voltage (OV ) ................................................6V (Note 3) ...........................(GND –0.3V) to (OV + 0.3V)
DD DD
Reference Input (REF) .................................................6V Power Dissipation ..............................................500mW
Analog Input Voltage (Note 3) Operating Temperature Range
IN+, IN– .........................(GND –0.3V) to (REF + 0.3V) LTC2378C ................................................0°C to 70°C
REF/DGC Input (Note 3) ....(GND –0.3V) to (REF + 0.3V) LTC2378I .............................................–40°C to 85°C
Digital Input Voltage Storage Temperature Range ..................–65°C to 150°C
(Note 3) ...........................(GND –0.3V) to (OV + 0.3V)
DD
PIN CONFIGURATION
TOP VIEW
CHAIN 1 16 GND
TOP VIEW
VDD 2 15 OVDD CHAIN 1 16 GND
GND 3 14 SDO VDD 2 15 OVDD
IN+ 4 17 13 SCK GND 3 14 SDO
IN– 5 GND 12 RDL/SDI IINN+– 45 1132 SRCDKL/SDI
GND 6 11 BUSY GND 6 11 BUSY
REF 7 10 GND REF 7 10 GND
REF/DGC 8 9 CNV
REF/DGC 8 9 CNV
MS PACKAGE
DE PACKAGE 16-LEAD PLASTIC MSOP
16-LEAD (4mm × 3mm) PLASTIC DFN TJMAX = 150°C, θJA = 110°C/W
TJMAX = 150°C, θJA = 40°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC2378CMS-20#PBF LTC2378CMS-20#TRPBF 237820 16-Lead Plastic MSOP 0°C to 70°C
LTC2378IMS-20#PBF LTC2378IMS-20#TRPBF 237820 16-Lead Plastic MSOP –40°C to 85°C
LTC2378CDE-20#PBF LTC2378CDE-20#TRPBF 23780 16-Lead (4mm × 3mm) Plastic DFN 0°C to 70°C
LTC2378IDE-20#PBF LTC2378IDE-20#TRPBF 23780 16-Lead (4mm × 3mm) Plastic DFN –40°C to 85°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 non-standard 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/
237820fa
2
For more information www.linear.com/LTC2378-20
LTC2378-20
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T = 25°C. (Note 4)
A
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V + Absolute Input Range (IN+) (Note 5) l –0.1 V + 0.1 V
IN REF
V – Absolute Input Range (IN–) (Note 5) l –0.1 V + 0.1 V
IN REF
V + – V – Input Differential Voltage Range V = V + – V – l –V +V V
IN IN IN IN IN REF REF
V Common-Mode Input Range l V /2– V /2 V /2+ V
CM REF REF REF
0.1 0.1
I Analog Input Leakage Current 0.01 µA
IN
C Analog Input Capacitance Sample Mode 45 pF
IN
Hold Mode 5 pF
CMRR Input Common Mode Rejection Ratio f = 500kHz 86 dB
IN
CONVERTER CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T = 25°C. (Note 4)
A
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Resolution l 20 Bits
No Missing Codes l 20 Bits
Transition Noise 2.3 ppm
RMS
INL Integral Linearity Error (Note 6) l –2 ±0.5 2 ppm
REF/DGC = GND, (Note 6) l –2 ±0.5 2 ppm
DNL Differential Linearity Error (Note 10) l –0.5 ±0.2 0.5 ppm
BZE Bipolar Zero-Scale Error (Note 7) l –13 0 13 ppm
Bipolar Zero-Scale Error Drift ±7 ppb/°C
FSE Bipolar Full-Scale Error (Note 7) l –100 ±10 100 ppm
Bipolar Full-Scale Error Drift ±0.05 ppm/°C
DYNAMIC ACCURACY
The l denotes the specifications which apply over the full operating temperature range,
otherwise specifications are at T = 25°C and A = –1dBFS. (Notes 4, 8)
A IN
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
SINAD Signal-to-(Noise + Distortion) Ratio f = 2kHz, V = 5V l 101 104 dB
IN REF
SNR Signal-to-Noise Ratio f = 2kHz, V = 5V l 101 104 dB
IN REF
f = 2kHz, V = 5V, REF/DGC = GND l 99 102 dB
IN REF
f = 2kHz, V = 2.5V l 95.4 98 dB
IN REF
THD Total Harmonic Distortion f = 2kHz, V = 5V l –125 –114 dB
IN REF
f = 2kHz, V = 5V, REF/DGC = GND l –125 –114 dB
IN REF
f = 2kHz, V = 2.5V l –123 –113 dB
IN REF
SFDR Spurious Free Dynamic Range f = 2kHz, V = 5V l 115 128 dB
IN REF
–3dB Input Bandwidth 34 MHz
Aperture Delay 500 ps
Aperture Jitter 4 ps
Transient Response Full-Scale Step 312 ns
237820fa
3
For more information www.linear.com/LTC2378-20
LTC2378-20
REFERENCE INPUT
The l denotes the specifications which apply over the full operating temperature range, otherwise
specifications are at T = 25°C. (Note 4)
A
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V Reference Voltage (Note 5) l 2.5 5.1 V
REF
I Reference Input Current (Note 9) l 0.94 1.1 mA
REF
V High Level Input Voltage REF/DGC Pin l 0.8V V
IHDGC REF
V Low Level Input Voltage REF/DGC Pin l 0.2V V
ILDGC REF
DIGITAL INPUTS AND DIGITAL OUTPUTS
The l denotes the specifications which apply over the
full operating temperature range, otherwise specifications are at T = 25°C. (Note 4)
A
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V High Level Input Voltage l 0.8 • OV V
IH DD
V Low Level Input Voltage l 0.2 • OV V
IL DD
I Digital Input Current V = 0V to OV l –10 10 µA
IN IN DD
C Digital Input Capacitance 5 pF
IN
V High Level Output Voltage I = –500µA l OV –0.2 V
OH O DD
V Low Level Output Voltage I = 500µA l 0.2 V
OL O
I Hi-Z Output Leakage Current V = 0V to OV l –10 10 µA
OZ OUT DD
I Output Source Current V = 0V –10 mA
SOURCE OUT
I Output Sink Current V = OV 10 mA
SINK OUT DD
POWER REQUIREMENTS
The l denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at T = 25°C. (Note 4)
A
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
V Supply Voltage l 2.375 2.5 2.625 V
DD
OV Supply Voltage l 1.71 5.25 V
DD
I Supply Current 1Msps Sample Rate l 8.4 10 mA
VDD
I Supply Current 1Msps Sample Rate (C = 20pF) 0.2 mA
OVDD L
I Power Down Mode Conversion Done (I + I + I ) l 1 90 µA
PD VDD OVDD REF
P Power Dissipation 1Msps Sample Rate 21 25 mW
D
Power Down Mode Conversion Done (I + I + I ) 2.5 225 µW
VDD OVDD REF
ADC TIMING CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T = 25°C. (Note 4)
A
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
f Maximum Sampling Frequency l 1 Msps
SMPL
t Conversion Time l 615 675 ns
CONV
t Acquisition Time t = t – t – t (Note 10) l 312 ns
ACQ ACQ CYC CONV BUSYLH
t Time Between Conversions l 1 µs
CYC
t CNV High Time l 20 ns
CNVH
tBUSYLH CNV↑ to BUSY Delay CL = 20pF l 13 ns
t Minimum Low Time for CNV (Note 11) l 20 ns
CNVL
tQUIET SCK Quiet Time from CNV↑ (Note 10) l 20 ns
t SCK Period (Notes 11, 12) l 10 ns
SCK
237820fa
4
For more information www.linear.com/LTC2378-20
LTC2378-20
ADC TIMING CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T = 25°C. (Note 4)
A
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
t SCK High Time l 4 ns
SCKH
t SCK Low Time l 4 ns
SCKL
tSSDISCK SDI Setup Time From SCK↑ (Note 11) l 4 ns
tHSDISCK SDI Hold Time From SCK↑ (Note 11) l 1 ns
t SCK Period in Chain Mode t = t + t (Note 11) l 13.5 ns
SCKCH SCKCH SSDISCK DSDO
tDSDO SDO Data Valid Delay from SCK↑ CL = 20pF, OVDD = 5.25V l 7.5 ns
C = 20pF, OV = 2.5V l 8 ns
L DD
C = 20pF, OV = 1.71V l 9.5 ns
L DD
tHSDO SDO Data Remains Valid Delay from SCK↑ CL = 20pF (Note 10) l 1 ns
tDSDOBUSYL SDO Data Valid Delay from BUSY↓ CL = 20pF (Note 10) l 5 ns
tEN Bus Enable Time After RDL↓ (Note 11) l 16 ns
tDIS Bus Relinquish Time After RDL↑ (Note 11) l 13 ns
Note 1: Stresses beyond those listed under Absolute Maximum Ratings Note 7: Bipolar zero-scale error is the offset voltage measured from
may cause permanent damage to the device. Exposure to any Absolute –0.5LSB when the output code flickers between 0000 0000 0000 0000 0000
Maximum Rating condition for extended periods may effect device and 1111 1111 1111 1111 1111. Full-scale bipolar error is the worst-case
reliability and lifetime. of –FS or +FS untrimmed deviation from ideal first and last code transitions
Note 2: All voltage values are with respect to ground. and includes the effect of offset error.
Note 3: When these pin voltages are taken below ground or above REF or Note 8: All specifications in dB are referred to a full-scale ±5V input with a
OV , they will be clamped by internal diodes. This product can handle 5V reference voltage.
DD
input currents up to 100mA below ground or above REF or OV without Note 9: f = 1MHz, I varies proportionately with sample rate.
DD SMPL REF
latch-up. Note 10: Guaranteed by design, not subject to test.
Note 4: VDD = 2.5V, OVDD = 2.5V, REF = 5V, VCM = 2.5V, fSMPL = 1MHz, Note 11: Parameter tested and guaranteed at OVDD = 1.71V, OVDD = 2.5V
REF/DGC = VREF. and OVDD = 5.25V.
Note 5: Recommended operating conditions. Note 12: t of 10ns maximum allows a shift clock frequency up to
SCK
Note 6: Integral nonlinearity is defined as the deviation of a code from a 100MHz for rising capture.
straight line passing through the actual endpoints of the transfer curve.
The deviation is measured from the center of the quantization band.
0.8*OVDD tWIDTH
0.2*OVDD
tDELAY tDELAY 50% 50%
0.8*OVDD 0.8*OVDD 237820 F01
0.2*OVDD 0.2*OVDD
Figure 1. Voltage Levels for Timing Specifications
237820fa
5
For more information www.linear.com/LTC2378-20
LTC2378-20
TYPICAL PERFORMANCE CHARACTERISTICS
T = 25°C, V = 2.5V, OV = 2.5V, V = 2.5V,
A DD DD CM
REF = 5V, f = 1Msps, unless otherwise noted.
SMPL
Integral Nonlinearity Differential Nonlinearity
vs Output Code vs Output Code DC Histogram
2.0 0.5 50000
σ = 2.3
0.4 45000
1.5
0.3 40000
1.0
m) m) 0.2 35000
NL ERROR (pp –00..505 NL ERROR (pp–000...101 COUNTS 232005000000000
I D–0.2 15000
–1.0
–0.3 10000
–1.5
–0.4 5000
–2.0 –0.5 0
0 262144 524288 786432 1048576 0 262144 524288 786432 1048576 524279 524283 524287 524291 524295
OUTPUT CODE OUTPUT CODE OUTPUT CODE
237820 G01 237820 G02 237820 G03
128k Point FFT f = 1Msps, THD, Harmonics
S
f = 2kHz SNR, SINAD vs Input Frequency vs Input Frequency
IN
0 108 –90
SNR = 104dB THD
–20 THD = –128dB 106 –95 2ND
SINAD = 104dB 3RD
–40 SFDR = 132dB 104 SNR S)–100
dBFS) –60 dBFS) 102 D (dBF––111005
E ( –80 D ( SINAD TH
MPLITUD––110200 NR, SINA 19080 MONICS, ––112105
A S R–125
–140 96 HA
–130
–160 94 –135
–180 92 –140
0 100 200 300 400 500 0 25 50 75 100 125 150 175 200 0 25 50 75 100 125 150 175 200
FREQUENCY (kHz) FREQUENCY (kHz) FREQUENCY (kHz)
237820 G04 237820 G05 237820 G06
SNR, SINAD vs Input level, SNR, SINAD vs Reference THD, Harmonics vs Reference
f = 2kHz Voltage, f = 2kHz Voltage, f = 2kHz
IN IN IN
105.0 105 –110
104
–115
103
104.5 S)
dBFS) SNR dBFS)110021 SNR SINAD D (dBF –120 3TRHDD
D ( SINAD D ( TH
R, SINA104.0 R, SINA19090 ONICS, –125
N N M –130
S S 98 R
103.5 HA
97
–135
96 2ND
103.0 95 –140
–40 –30 –20 –10 0 2.5 3.0 3.5 4.0 4.5 5.0 2.5 3.0 3.5 4.0 4.5 5.0
INPUT LEVEL (dB) REFERENCE VOLTAGE (V) REFERENCE VOLTAGE (V)
237820 G07 237820 G08 237820 G09
237820fa
6
For more information www.linear.com/LTC2378-20
LTC2378-20
TYPICAL PERFORMANCE CHARACTERISTICS
T = 25°C, V = 2.5V, OV = 2.5V, V = 2.5V,
A DD DD CM
REF = 5V, f = 1Msps, unless otherwise noted.
SMPL
SNR, SINAD vs Temperature, THD, Harmonics vs Temperature,
f = 2kHz f = 2kHz INL vs Temperature
IN IN
106 –120 2.0
1.5
105
SNR S) –125 1.0 MAX INL
F
NR, SINAD (dBFS) 111000243 SINAD MONICS, THD (dB –130 3RD THD NL ERROR (ppm)–00..505 MIN INL
S R I
HA –135 2ND –1.0
101
–1.5
100 –140 –2.0
–55 –35 –15 5 25 45 65 85 105 125 –55 –35 –15 5 25 45 65 85 105 125 –55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C) TEMPERATURE (°C) TEMPERATURE (°C)
237820 G10 237820 G11 237820 G12
Full-Scale Error vs Temperature Offset Error vs Temperature Supply Current vs Temperature
20 4 10
9
15 3 IVDD
ULL-SCALE ERROR (ppm)–11–00505 +–FFSS OFFSET ERROR (ppm) ––20121 WER SUPPLY CURRENT (mA) 876345
F O 2
–15 –3 P IREF
1
IOVDD
–20 –4 0
–55 –35 –15 5 25 45 65 85 105 125 –55 –35 –15 5 25 45 65 85 105 125 –55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C) TEMPERATURE (°C) TEMPERATURE (°C)
237820 G13 237820 G14 237820 G15
Reference Current
Shutdown Current vs Temperature CMRR vs Input Frequency vs Reference Voltage
45 100 1.0
IVDD + IOVDD + IREF
40 0.9
95
µA) 35 A) 0.8
RRENT ( 30 B) 90 RENT (m 00..76
POWER-DOWN CU 11220505 CMRR (d 887505 REFERENCE CUR 0000....2543
5 0.1
0 70 0
–50 –25 0 25 50 75 100 125 0 100 200 300 400 500 2.5 3.0 3.5 4.0 4.5 5.0
TEMPERATURE (°C) FREQUENCY (kHz) REFERENCE VOLTAGE (V)
237820 G16 237820 G17 237820 G18
237820fa
7
For more information www.linear.com/LTC2378-20
LTC2378-20
PIN FUNCTIONS
CHAIN (Pin 1): Chain Mode Selector Pin. When low, the BUSY (Pin 11): BUSY Indicator. Goes high at the start of
LTC2378-20 operates in normal mode and the RDL/SDI a new conversion and returns low when the conversion
input pin functions to enable or disable SDO. When high, has finished. Logic levels are determined by OV .
DD
the LTC2378-20 operates in chain mode and the RDL/SDI
RDL/SDI (Pin 12): When CHAIN is low, the part is in Nor-
pin functions as SDI, the daisy-chain serial data input.
mal Mode and the pin is treated as a bus enabling input.
Logic levels are determined by OV .
DD When CHAIN is high, the part is in chain mode and the
V (Pin 2): 2.5V Power Supply. The range of V is pin is treated as a serial data input pin where data from
DD DD
2.375V to 2.625V. Bypass V to GND with a 10µF ceramic another ADC in the daisy chain is input. Logic levels are
DD
capacitor. determined by OV .
DD
GND (Pins 3, 6, 10 and 16): Ground. SCK (Pin 13): Serial Data Clock Input. When SDO is enabled,
the conversion result or daisy-chain data from another
IN+, IN– (Pins 4, 5): Positive and Negative Differential
ADC is shifted out on the rising edges of this clock MSB
Analog Inputs.
first. Logic levels are determined by OV .
DD
REF (Pin 7): Reference Input. The range of REF is 2.5V
SDO (Pin 14): Serial Data Output. The conversion result or
to 5.1V. This pin is referred to the GND pin and should be
daisy-chain data is output on this pin on each rising edge
decoupled closely to the pin with a 47µF ceramic capacitor
of SCK MSB first. The output data is in 2’s complement
(X7R, 1210 size, 10V rating).
format. Logic levels are determined by OV .
DD
REF/DGC (Pin 8): When tied to REF, digital gain compres-
OV (Pin 15): I/O Interface Digital Power. The range of
sion is disabled and the LTC2378-20 defines full-scale ac- DD
OV is 1.71V to 5.25V. This supply is nominally set to
cording to the ±V analog input range. When tied to GND, DD
REF
the same supply as the host interface (1.8V, 2.5V, 3.3V,
digital gain compression is enabled and the LTC2378-20
or 5V). Bypass OV to GND with a 0.1µF capacitor.
defines full-scale with inputs that swing between 10% and DD
90% of the ±V analog input range. GND (Exposed Pad Pin 17 – DFN Package Only): Ground.
REF
Exposed pad must be soldered directly to the ground plane.
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 OV .
DD
FUNCTIONAL BLOCK DIAGRAM
VDD = 2.5V
REF = 5V OVDD = 1.8V to 5V
CHAIN
IN+ + SDO
SPI RDL/SDI
20-BIT SAMPLING ADC PORT
SCK
IN– –
CNV
BUSY
CONTROL LOGIC
REF/DGC
GND
237820 BD01
237820fa
8
For more information www.linear.com/LTC2378-20
LTC2378-20
TIMING DIAGRAM
Conversion Timing Using the Serial Interface
CHAIN, RDL/SDI = 0
CNV
POWER-DOWN AND ACQUIRE
BUSY CONVERT
SCK
D19D18DD1177D16D15D14D13D12D11D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
SDO
237820 TD01
APPLICATIONS INFORMATION
OVERVIEW CONVERTER OPERATION
The LTC2378-20 is a low noise, low power, high speed The LTC2378-20 operates in two phases. During the ac-
20-bit successive approximation register (SAR) ADC. quisition phase, the charge redistribution capacitor D/A
Operating from a single 2.5V supply, the LTC2378-20 converter (CDAC) is connected to the IN+ and IN– pins
supports a large and flexible ±V fully differential input to sample the differential analog input voltage. A rising
REF
range with V ranging from 2.5V to 5.1V, making it ideal edge on the CNV pin initiates a conversion. During the
REF
for high performance applications which require a wide conversion phase, the 20-bit CDAC is sequenced through a
dynamic range. The LTC2378-20 achieves ±2ppm INL successive approximation algorithm, effectively comparing
maximum, no missing codes at 20 bits and 104dB SNR. the sampled input with binary-weighted fractions of the
reference voltage (e.g. V /2, V /4 … V /1048576)
Fast 1Msps throughput with no cycle latency makes the REF REF REF
using the differential comparator. At the end of conversion,
LTC2378-20 ideally suited for a wide variety of high speed
the CDAC output approximates the sampled analog input.
applications. An internal oscillator sets the conversion time,
The ADC control logic then prepares the 20-bit digital
easing external timing considerations. The LTC2378-20
output code for serial transfer.
dissipates only 21mW at 1Msps, while an auto power-down
feature is provided to further reduce power dissipation
during inactive periods. TRANSFER FUNCTION
The LTC2378-20 features a unique digital gain compres- The LTC2378-20 digitizes the full-scale voltage of 2 × REF
sion (DGC) function, which eliminates the driver amplifier’s into 220 levels, resulting in an LSB size of 9.5µV with
negative supply while preserving the full resolution of the REF = 5V. Note that 1 LSB at 20 bits is approximately
ADC. When enabled, the ADC performs a digital scaling 1ppm. The ideal transfer function is shown in Figure 2.
function that maps zero-scale code from 0V to 0.1 • V The output data is in 2’s complement format.
REF
and full-scale code from V to 0.9 • V . For a typical
REF REF
reference voltage of 5V, the full-scale input range is now ANALOG INPUT
0.5V to 4.5V, which provides adequate headroom for
The analog inputs of the LTC2378-20 are fully differential
powering the driving amplifier from a single 5.5V supply.
in order to maximize the signal swing that can be digitized.
The analog inputs can be modeled by the equivalent circuit
237820fa
9
For more information www.linear.com/LTC2378-20
LTC2378-20
APPLICATIONS INFORMATION
tling of the analog signal during the acquisition phase. It
T) 011...111 also provides isolation between the signal source and the
N
E 011...110 BIPOLAR
EM ZERO ADC input currents.
L
P
M
O 000...001
S C 000...000 Noise and Distortion
O’
W
T 111...111 The noise and distortion of the buffer amplifier and signal
DE ( 111...110
O source must be considered since they add to the ADC noise
C
T
U and distortion. Noisy input signals should be filtered prior
P 100...001
T FSR = +FS – –FS
OU 100...000 1LSB = FSR/1048576 ≈ 1ppm to the buffer amplifier input with an appropriate filter to
minimize noise. The simple 1-pole RC lowpass filter (LPF1)
–FSR/2 –1 0V 1 FSR/2 – 1LSB
LSB LSB shown in Figure 4 is sufficient for many applications.
INPUT VOLTAGE (V)
237820 F02
Figure 2. LTC2378-20 Transfer Function LPF2
6800pF
shown in Figure 3. The diodes at the input provide ESD SINGLE-ENDED-
INPUT SIGNAL LPF1 10Ω IN+
protection. In the acquisition phase, each input sees ap-
500Ω 3300pF
LTC2378-20
proximately 45pF (C ) from the sampling CDAC in series
IN
6600pF IN–
with 40Ω (R ) from the on-resistance of the sampling
ON 10Ω
switch. Any unwanted signal that is common to both SINGLE-ENDED- 6800pF 237820 F04
inputs will be reduced by the common mode rejection of BW = 48kHz TO-DIFFERENTIAL
DRIVER
BW = 1.2MHz
the ADC. The inputs draw a current spike while charging
Figure 4. Input Signal Chain
the C capacitors during acquisition. During conversion,
IN
the analog inputs draw only a small leakage current.
A coupling filter network (LPF2) should be used between
the buffer and ADC input to minimize disturbances reflected
INPUT DRIVE CIRCUITS
into the buffer from sampling transients. Long RC time
A low impedance source can directly drive the high imped- constants at the analog inputs will slow down the settling
ance inputs of the LTC2378-20 without gain error. A high of the analog inputs. Therefore, LPF2 typically requires a
impedance source should be buffered to minimize settling wider bandwidth than LPF1. This filter also helps minimize
time during acquisition and to optimize ADC linearity. For the noise contribution from the buffer. A buffer amplifier
best performance, a buffer amplifier should be used to with a low noise density must be selected to minimize
drive the analog inputs of the LTC2378-20. The amplifier degradation of the SNR.
provides low output impedance, which produces fast set-
High quality capacitors and resistors should be used in the
RC filters since these components can add distortion. NPO
REF
and silver mica type dielectric capacitors have excellent
CIN
RON 45pF linearity. Carbon surface mount resistors can generate
40Ω
IN+ distortion from self heating and from damage that may
occur during soldering. Metal film surface mount resistors
BIAS are much less susceptible to both problems.
REF VOLTAGE
CIN
RON 45pF
40Ω Input Currents
IN–
One of the biggest challenges in coupling an amplifier to
237820 F03 the LTC2378-20 is in dealing with current spikes drawn
Figure 3. The Equivalent Circuit for the by the ADC inputs at the start of each acquisition phase.
Differential Analog Input of the LTC2378-20
237820fa
10
For more information www.linear.com/LTC2378-20