LINER LTC2378-20 20-bit, 1msps, low power sar adc with 0.5ppm inl Datasheet

LTC2378-20
20-Bit, 1Msps, Low Power
SAR ADC with 0.5ppm INL
Description
Features
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1Msps Throughput Rate
±0.5ppm INL (Typ)
Guaranteed 20-Bit No Missing Codes
Low Power: 21mW at 1Msps, 21µW at 1ksps
104dB SNR (Typ) at fIN = 2kHz
–125dB THD (Typ) at fIN = 2kHz
Digital Gain Compression (DGC)
Guaranteed Operation to 85°C
2.5V Supply
Fully Differential Input Range ±VREF
VREF Input Range from 2.5V to 5.1V
No Pipeline Delay, No Cycle Latency
1.8V to 5V I/O Voltages
SPI-Compatible Serial I/O with Daisy-Chain Mode
Internal Conversion Clock
16-Lead MSOP and 4mm × 3mm DFN Packages
The LTC®2378-20 is a low noise, low power, high speed
20-bit successive approximation register (SAR) ADC.
Operating from a 2.5V supply, the LTC2378-20 has a
±VREF fully differential input range with VREF ranging from
2.5V to 5.1V. The LTC2378-20 consumes only 21mW and
achieves ±2ppm INL maximum, no missing codes at 20
bits with 104dB SNR.
The LTC2378-20 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 1Msps
throughput with no cycle latency makes the LTC2378-20
ideally suited for a wide variety of high speed applications.
An internal oscillator sets the conversion time, easing external timing considerations. The LTC2378-20 automatically
powers down between conversions, leading to reduced
power dissipation that scales with the sampling rate.
Applications
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The LTC2378-20 features a unique digital gain compression (DGC) function, which eliminates the driver amplifier’s
negative supply while preserving the full resolution of the
ADC. When enabled, the ADC performs a digital scaling
function that maps zero-scale code from 0V to 0.1 • VREF
and full-scale code from VREF to 0.9 • VREF. For a typical
reference voltage of 5V, the full-scale input range is now
0.5V to 4.5V, which provides adequate headroom for
powering the driving amplifier from a single 5.5V supply.
Medical Imaging
High Speed Data Acquisition
Portable or Compact Instrumentation
Industrial Process Control
Low Power Battery-Operated Instrumentation
ATE
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. Patents Pending. Protected by U.S. Patents, including
7705765, 7961132, 8319673.
Typical Application
Integral Nonlinearity vs Output Code
2.5V
2.0
1.8V TO 5V
10µF
1.5
0.1µF
0V
VREF
0V
+
10Ω
6800pF
IN+
LTC2378-20
3300pF
–
10Ω
OVDD
VDD
IN–
6800pF
REF
GND
CHAIN
RDL/SDI
SDO
SCK
BUSY
CNV
REF/DGC
2.5V TO 5.1V
SAMPLE CLOCK
VREF
237820 TA01
47µF
(X7R, 1210 SIZE)
INL ERROR (ppm)
1.0
VREF
0.5
0
–0.5
–1.0
–1.5
–2.0
0
262144
524288
786432
OUTPUT CODE
1048576
237820 TA02
237820f
For more information www.linear.com/LTC2378-20
1
LTC2378-20
Absolute Maximum Ratings
(Notes 1, 2)
Supply Voltage (VDD)................................................2.8V
Supply Voltage (OVDD).................................................6V
Reference Input (REF)..................................................6V
Analog Input Voltage (Note 3)
IN+, IN–.......................... (GND –0.3V) to (REF + 0.3V)
REF/DGC Input (Note 3)..... (GND –0.3V) to (REF + 0.3V)
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
LTC2378C................................................. 0°C to 70°C
LTC2378I..............................................–40°C to 85°C
Storage Temperature Range................... –65°C to 150°C
Pin Configuration
TOP VIEW
CHAIN
1
VDD
2
GND
3
+
4
IN–
5
GND
6
REF
7
REF/DGC
8
IN
16 GND
15 OVDD
17
GND
TOP VIEW
CHAIN
VDD
GND
IN+
IN–
GND
REF
REF/DGC
14 SDO
13 SCK
12 RDL/SDI
11 BUSY
10 GND
9 CNV
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
GND
OVDD
SDO
SCK
RDL/SDI
BUSY
GND
CNV
MS PACKAGE
16-LEAD PLASTIC MSOP
TJMAX = 150°C, θJA = 110°C/W
DE PACKAGE
16-LEAD (4mm × 3mm) PLASTIC DFN
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/
237820f
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 TA = 25°C. (Note 4)
SYMBOL
PARAMETER
CONDITIONS
VIN+
Absolute Input Range (IN+)
MIN
(Note 5)
l
VIN –
Absolute Input Range (IN–)
(Note 5)
VIN+ – VIN–
Input Differential Voltage Range
VIN = VIN+ – VIN–
VCM
Common-Mode Input Range
IIN
Analog Input Leakage Current
CIN
Analog Input Capacitance
CMRR
Input Common Mode Rejection Ratio
TYP
MAX
UNITS
–0.1
VREF + 0.1
V
l
–0.1
VREF + 0.1
V
l
–VREF
+VREF
V
l
VREF/2–
0.1
VREF/2+
0.1
V
VREF/2
0.01
µA
Sample Mode
Hold Mode
45
5
pF
pF
fIN = 500kHz
86
dB
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
TYP
MAX
UNITS
Resolution
l
20
Bits
No Missing Codes
l
20
Bits
Transition Noise
2.3
ppmRMS
INL
Integral Linearity Error
(Note 6)
REF/DGC = GND, (Note 6)
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l
–2
–2
±0.5
±0.5
2
2
ppm
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
FSE
Bipolar Full-Scale Error
(Note 7)
l
–100
±10
Bipolar Zero-Scale Error Drift
±7
Bipolar Full-Scale Error Drift
ppb/°C
100
±0.05
ppm
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
MIN
TYP
SINAD
Signal-to-(Noise + Distortion) Ratio
fIN = 2kHz, VREF = 5V
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101
104
dB
SNR
Signal-to-Noise Ratio
fIN = 2kHz, VREF = 5V
fIN = 2kHz, VREF = 5V, REF/DGC = GND
fIN = 2kHz, VREF = 2.5V
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101
99
95.4
104
102
98
dB
dB
dB
THD
Total Harmonic Distortion
fIN = 2kHz, VREF = 5V
fIN = 2kHz, VREF = 5V, REF/DGC = GND
fIN = 2kHz, VREF = 2.5V
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l
l
SFDR
Spurious Free Dynamic Range
fIN = 2kHz, VREF = 5V
l
–125
–125
–123
–114
–114
–113
UNITS
dB
dB
dB
128
dB
–3dB Input Bandwidth
34
MHz
Aperture Delay
500
ps
4
ps
312
ns
Aperture Jitter
Transient Response
Full-Scale Step
115
MAX
237820f
For more information www.linear.com/LTC2378-20
3
LTC2378-20
Reference Input
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
VREF
Reference Voltage
(Note 5)
l
MIN
IREF
Reference Input Current
(Note 9)
l
VIHDGC
High Level Input Voltage REF/DGC Pin
l
VILDGC
Low Level Input Voltage REF/DGC Pin
l
TYP
2.5
0.94
MAX
UNITS
5.1
V
1.1
mA
0.8VREF
V
0.2VREF
V
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
TYP
MAX
UNITS
VIH
High Level Input Voltage
l
VIL
Low Level Input Voltage
l
IIN
Digital Input Current
CIN
Digital Input Capacitance
VOH
High Level Output Voltage
IO = –500µA
l
VOL
Low Level Output Voltage
IO = 500µA
l
IOZ
Hi-Z Output Leakage Current
VOUT = 0V to OVDD
l
ISOURCE
Output Source Current
VOUT = 0V
–10
mA
ISINK
Output Sink Current
VOUT = OVDD
10
mA
VIN = 0V to OVDD
0.8 • OVDD
V
–10
l
0.2 • OVDD
V
10
µA
5
pF
OVDD – 0.2
V
–10
0.2
V
10
µA
Power Requirements
The l denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. (Note 4)
SYMBOL
PARAMETER
MIN
TYP
MAX
UNITS
VDD
Supply Voltage
CONDITIONS
l
2.375
2.5
2.625
V
OVDD
Supply Voltage
l
1.71
5.25
V
IVDD
IOVDD
IPD
Supply Current
Supply Current
Power Down Mode
1Msps Sample Rate
1Msps Sample Rate (CL = 20pF)
Conversion Done (IVDD + IOVDD + IREF)
PD
Power Dissipation
Power Down Mode
1Msps Sample Rate
Conversion Done (IVDD + IOVDD + IREF)
l
l
8.4
0.2
1
10
90
mA
mA
µA
21
2.5
25
225
mW
µW
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
PARAMETER
CONDITIONS
fSMPL
Maximum Sampling Frequency
l
tCONV
Conversion Time
l
615
tACQ
Acquisition Time
l
312
ns
tACQ = tCYC – tCONV – tBUSYLH (Note 10)
MIN
TYP
MAX
UNITS
1
Msps
675
ns
tCYC
Time Between Conversions
l
1
µs
tCNVH
CNV High Time
l
20
ns
tBUSYLH
CNV↑ to BUSY Delay
CL = 20pF
l
13
ns
tCNVL
Minimum Low Time for CNV
(Note 11)
l
20
ns
tQUIET
SCK Quiet Time from CNV↑
(Note 10)
l
20
ns
tSCK
SCK Period
(Notes 11, 12)
l
10
ns
237820f
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 TA = 25°C. (Note 4)
SYMBOL
PARAMETER
tSCKH
SCK High Time
CONDITIONS
l
4
ns
tSCKL
SCK Low Time
l
4
ns
tSSDISCK
SDI Setup Time From SCK↑
(Note 11)
l
4
ns
tHSDISCK
SDI Hold Time From SCK↑
(Note 11)
l
1
ns
tSCKCH
SCK Period in Chain Mode
tSCKCH = tSSDISCK + tDSDO (Note 11)
l
13.5
ns
tDSDO
SDO Data Valid Delay from SCK↑
CL = 20pF, OVDD = 5.25V
CL = 20pF, OVDD = 2.5V
CL = 20pF, OVDD = 1.71V
l
l
l
tHSDO
SDO Data Remains Valid Delay from SCK↑
CL = 20pF (Note 10)
l
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
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may effect 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 REF or
OVDD, they will be clamped by internal diodes. This product can handle
input currents up to 100mA below ground or above REF or OVDD without
latch-up.
Note 4: VDD = 2.5V, OVDD = 2.5V, REF = 5V, VCM = 2.5V, fSMPL = 1MHz,
REF/DGC = VREF.
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.
MIN
TYP
MAX
7.5
8
9.5
1
UNITS
ns
ns
ns
ns
Note 7: Bipolar zero-scale error is the offset voltage measured from
–0.5LSB when the output code flickers between 0000 0000 0000 0000 0000
and 1111 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 ±5V input with a
5V reference voltage.
Note 9: fSMPL = 1MHz, IREF varies proportionately with sample rate.
Note 10: Guaranteed by design, not subject to test.
Note 11: Parameter tested and guaranteed at OVDD = 1.71V, OVDD = 2.5V
and OVDD = 5.25V.
Note 12: tSCK of 10ns maximum allows a shift clock frequency up to
100MHz for rising capture.
0.8*OVDD
tWIDTH
0.2*OVDD
tDELAY
tDELAY
0.8*OVDD
0.8*OVDD
0.2*OVDD
0.2*OVDD
50%
50%
237820 F01
Figure 1. Voltage Levels for Timing Specifications
237820f
For more information www.linear.com/LTC2378-20
5
LTC2378-20
Typical Performance Characteristics
REF = 5V, fSMPL = 1Msps, unless otherwise noted.
Integral Nonlinearity
vs Output Code
Differential Nonlinearity
vs Output Code
50000
1.5
0.4
45000
0.3
40000
0.2
35000
0.1
30000
0.5
0
–0.5
COUNTS
0.5
DNL ERROR (ppm)
0.0
–0.1
20000
–0.2
15000
–0.3
10000
–1.5
–0.4
5000
–2.0
–0.5
–1.0
0
262144
524288
786432
OUTPUT CODE
1048576
0
262144
524288
786432
OUTPUT CODE
237820 G01
0
SNR, SINAD (dBFS)
–80
–100
–120
–90
106
–95
98
–160
94
–180
92
200
300
FREQUENCY (kHz)
400
500
SINAD
100
96
100
SNR
102
–140
0
524295
THD
2ND
3RD
–100
–105
–110
–115
–120
–125
–130
–135
0
25
50
–140
75 100 125 150 175 200
FREQUENCY (kHz)
0
25
50
75 100 125 150 175 200
FREQUENCY (kHz)
237820 G06
237820 G05
237820 G04
105.0
524287 524291
OUTPUT CODE
THD, Harmonics
vs Input Frequency
108
104
–60
524283
237820 G03
SNR, SINAD vs Input Frequency
SNR = 104dB
THD = –128dB
SINAD = 104dB
SFDR = 132dB
–40
0
524279
1048576
237820 G02
128k Point FFT fS = 1Msps,
fIN = 2kHz
–20
σ = 2.3
25000
HARMONICS, THD (dBFS)
INL ERROR (ppm)
DC Histogram
2.0
1.0
AMPLITUDE (dBFS)
TA = 25°C, VDD = 2.5V, OVDD = 2.5V, VCM = 2.5V,
SNR, SINAD vs Input level,
fIN = 2kHz
105
SNR, SINAD vs Reference
Voltage, fIN = 2kHz
–110
THD, Harmonics vs Reference
Voltage, fIN = 2kHz
104
SINAD
103.5
102
HARMONICS, THD (dBFS)
SNR, SINAD (dBFS)
SNR, SINAD (dBFS)
SNR
104.0
–115
103
104.5
SNR
SINAD
101
100
99
98
97
THD
–120
3RD
–125
–130
–135
96
103.0
–40
–30
–20
–10
INPUT LEVEL (dB)
0
237820 G07
95
2ND
2.5
3.0
3.5
4.0
4.5
REFERENCE VOLTAGE (V)
5.0
237820 G08
–140
2.5
3.0
3.5
4.0
4.5
REFERENCE VOLTAGE (V)
5.0
237820 G09
237820f
6
For more information www.linear.com/LTC2378-20
LTC2378-20
Typical Performance Characteristics
REF = 5V, fSMPL = 1Msps, unless otherwise noted.
106
SNR, SINAD vs Temperature,
fIN = 2kHz
–120
TA = 25°C, VDD = 2.5V, OVDD = 2.5V, VCM = 2.5V,
THD, Harmonics vs Temperature,
fIN = 2kHz
INL vs Temperature
2.0
1.5
105
SINAD
103
102
–125
1.0
INL ERROR (ppm)
HARMONICS, THD (dBFS)
SNR, SINAD (dBFS)
SNR
104
THD
–130
3RD
–135
5 25 45 65 85 105 125
TEMPERATURE (°C)
–140
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
–2.0
–55 –35 –15
Full-Scale Error vs Temperature
Supply Current vs Temperature
Offset Error vs Temperature
15
3
9
10
2
–FS
0
+FS
–5
–10
POWER SUPPLY CURRENT (mA)
10
OFFSET ERROR (ppm)
4
5
1
0
–1
–2
–3
–15
–20
–55 –35 –15
–4
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
IVDD
8
7
6
5
4
3
2
IREF
1
0
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
IOVDD
5 25 45 65 85 105 125
TEMPERATURE (°C)
237820 G14
237820 G13
Shutdown Current vs Temperature
237820 G15
Reference Current
vs Reference Voltage
CMRR vs Input Frequency
100
IVDD + IOVDD + IREF
1.0
0.9
REFERENCE CURRENT (mA)
95
35
90
CMRR (dB)
30
25
20
85
80
15
10
75
5
0
–50
5 25 45 65 85 105 125
TEMPERATURE (°C)
237820 G12
20
40
MIN INL
–0.5
237820 G11
237820 G10
FULL-SCALE ERROR (ppm)
0
–1.5
100
–55 –35 –15
POWER-DOWN CURRENT (µA)
0.5
–1.0
2ND
101
45
MAX INL
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
–25
0
25
50
75
TEMPERATURE (°C)
100
125
237820 G16
70
0
100
200
300
FREQUENCY (kHz)
400
500
237820 G17
0
2.5
3.0
3.5
4.0
4.5
REFERENCE VOLTAGE (V)
5.0
237820 G18
237820f
For more information www.linear.com/LTC2378-20
7
LTC2378-20
Pin Functions
CHAIN (Pin 1): Chain Mode Selector Pin. When low, the
LTC2378-20 operates in normal mode and the RDL/SDI
input pin functions to enable or disable SDO. When high,
the LTC2378-20 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.
VDD (Pin 2): 2.5V Power Supply. The range of VDD is
2.375V to 2.625V. Bypass VDD to GND with a 10µF ceramic
capacitor.
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.
GND (Pins 3, 6, 10 and 16): Ground.
IN+, IN– (Pins 4, 5): Positive and Negative Differential
Analog Inputs.
REF (Pin 7): Reference Input. The range of REF is 2.5V
to 5.1V. This pin is referred to the GND pin and should be
decoupled closely to the pin with a 47µF ceramic capacitor
(X7R, 1210 size, 10V rating).
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.
REF/DGC (Pin 8): When tied to REF, digital gain compression is disabled and the LTC2378-20 defines full-scale according to the ±VREF analog input range. When tied to GND,
digital gain compression is enabled and the LTC2378‑20
defines full-scale with inputs that swing between 10% and
90% of the ±VREF analog input range.
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.
GND (Exposed Pad Pin 17 – DFN Package Only): Ground.
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 OVDD.
Functional Block Diagram
VDD = 2.5V
REF = 5V
IN+
+
20-BIT SAMPLING ADC
IN–
OVDD = 1.8V to 5V
SPI
PORT
–
CONTROL LOGIC
CHAIN
SDO
RDL/SDI
SCK
CNV
BUSY
REF/DGC
GND
237820 BD01
237820f
8
For more information www.linear.com/LTC2378-20
LTC2378-20
Timing Diagram
Conversion Timing Using the Serial Interface
CHAIN, RDL/SDI = 0
CNV
BUSY
POWER-DOWN AND ACQUIRE
CONVERT
SCK
SDO
D19 D18 D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
237820 TD01
Applications Information
OVERVIEW
CONVERTER OPERATION
The LTC2378-20 is a low noise, low power, high speed
20-bit successive approximation register (SAR) ADC.
Operating from a single 2.5V supply, the LTC2378-20
supports a large and flexible ±VREF fully differential input
range with VREF ranging from 2.5V to 5.1V, making it ideal
for high performance applications which require a wide
dynamic range. The LTC2378-20 achieves ±2ppm INL
maximum, no missing codes at 20 bits and 104dB SNR.
The LTC2378-20 operates in two phases. During the acquisition phase, the charge redistribution capacitor D/A
converter (CDAC) is connected to the IN+ and IN– pins
to sample the differential analog input voltage. A rising
edge on the CNV pin initiates a conversion. During the
conversion phase, the 20-bit CDAC is sequenced through a
successive approximation algorithm, effectively comparing
the sampled input with binary-weighted fractions of the
reference voltage (e.g. VREF/2, VREF/4 … VREF/1048576)
using the differential comparator. At the end of conversion,
the CDAC output approximates the sampled analog input.
The ADC control logic then prepares the 20-bit digital
output code for serial transfer.
Fast 1Msps throughput with no cycle latency makes the
LTC2378-20 ideally suited for a wide variety of high speed
applications. An internal oscillator sets the conversion time,
easing external timing considerations. The LTC2378‑20
dissipates only 21mW at 1Msps, while an auto power-down
feature is provided to further reduce power dissipation
during inactive periods.
The LTC2378-20 features a unique digital gain compression (DGC) function, which eliminates the driver amplifier’s
negative supply while preserving the full resolution of the
ADC. When enabled, the ADC performs a digital scaling
function that maps zero-scale code from 0V to 0.1 • VREF
and full-scale code from VREF to 0.9 • VREF. For a typical
reference voltage of 5V, the full-scale input range is now
0.5V to 4.5V, which provides adequate headroom for
powering the driving amplifier from a single 5.5V supply.
TRANSFER FUNCTION
The LTC2378-20 digitizes the full-scale voltage of 2 × REF
into 220 levels, resulting in an LSB size of 9.5µV with
REF = 5V. Note that 1 LSB at 20 bits is approximately
1ppm. The ideal transfer function is shown in Figure 2.
The output data is in 2’s complement format.
ANALOG INPUT
The analog inputs of the LTC2378-20 are fully differential
in order to maximize the signal swing that can be digitized.
The analog inputs can be modeled by the equivalent circuit
237820f
For more information www.linear.com/LTC2378-20
9
LTC2378-20
OUTPUT CODE (TWO’S COMPLEMENT)
Applications Information
tling of the analog signal during the acquisition phase. It
also provides isolation between the signal source and the
ADC input currents.
011...111
BIPOLAR
ZERO
011...110
000...001
Noise and Distortion
000...000
111...111
111...110
100...001
FSR = +FS – –FS
1LSB = FSR/1048576 ≈ 1ppm
100...000
–1 0V 1
FSR/2 – 1LSB
LSB
LSB
INPUT VOLTAGE (V)
–FSR/2
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 an appropriate filter to
minimize noise. The simple 1-pole RC lowpass filter (LPF1)
shown in Figure 4 is sufficient for many applications.
237820 F02
Figure 2. LTC2378-20 Transfer Function
shown in Figure 3. The diodes at the input provide ESD
protection. In the acquisition phase, each input sees approximately 45pF (CIN) from the sampling CDAC in series
with 40Ω (RON) from the on-resistance of the sampling
switch. Any unwanted signal that is common to both
inputs will be reduced by the common mode rejection of
the ADC. The inputs draw a current spike while charging
the CIN capacitors during acquisition. During conversion,
the analog inputs draw only a small leakage current.
INPUT DRIVE CIRCUITS
A low impedance source can directly drive the high impedance inputs of the LTC2378-20 without gain error. A high
impedance source should be buffered to minimize settling
time during acquisition and to optimize ADC linearity. For
best performance, a buffer amplifier should be used to
drive the analog inputs of the LTC2378-20. The amplifier
provides low output impedance, which produces fast setREF
RON
40Ω
IN+
REF
IN–
RON
40Ω
CIN
45pF
CIN
45pF
BIAS
VOLTAGE
LPF2
SINGLE-ENDEDINPUT SIGNAL LPF1
500Ω
6600pF
6800pF
10Ω
IN+
3300pF
10Ω
SINGLE-ENDED- 6800pF
BW = 48kHz TO-DIFFERENTIAL
DRIVER
BW = 1.2MHz
LTC2378-20
IN–
237820 F04
Figure 4. Input Signal Chain
A coupling filter network (LPF2) should be used between
the buffer and ADC input to minimize disturbances reflected
into the buffer from sampling transients. Long RC time
constants at the analog inputs will slow down the settling
of the analog inputs. Therefore, LPF2 typically requires a
wider bandwidth than LPF1. This filter also helps minimize
the noise contribution from the buffer. A buffer amplifier
with a low noise density must be selected to minimize
degradation of the SNR.
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.
Input Currents
237820 F03
Figure 3. The Equivalent Circuit for the
Differential Analog Input of the LTC2378-20
One of the biggest challenges in coupling an amplifier to
the LTC2378-20 is in dealing with current spikes drawn
by the ADC inputs at the start of each acquisition phase.
237820f
10
For more information www.linear.com/LTC2378-20
LTC2378-20
Applications Information
Fully Settled – This case is characterized by filter time
constants and an overall settling time that is considerably shorter than the sample period. When acquisition
begins, the coupling filter is disturbed. For a typical first
order RC filter, the disturbance will look like an initial step
with an exponential decay. The amplifier will have its own
response to the disturbance, which may include ringing. If
the input settles completely (to within the accuracy of the
LTC2378-20), the disturbance will not contribute any error.
Partially Settled – In this case, the beginning of acquisition
causes a disturbance of the coupling filter, which then
begins to settle out towards the nominal input voltage.
However, acquisition ends (and the conversion begins)
before the input settles to its final value. This generally
produces a gain error, but as long as the settling is linear,
no distortion is produced. The coupling filter’s response
is affected by the amplifier’s output impedance and other
parameters. A linear settling response to fast switchedcapacitor current spikes can NOT always be assumed for
precision, low bandwidth amplifiers. The coupling filter
serves to attenuate the current spikes’ high-frequency
energy before it reaches the amplifier.
Fully Averaged – If the coupling filter capacitors (CFILT) at the
ADC inputs are much larger than the ADC’s sample capacitors
(45pF), then the sampling glitch is greatly attenuated. The
driving amplifier effectively only sees the average sampling
current, which is quite small. At 1Msps, the equivalent input
resistance is approximately 22k (as shown in Figure 5), a
benign resistive load for most precision amplifiers. However,
resistive voltage division will occur between the coupling
filter’s DC resistance and the ADC’s equivalent (switchedcapacitor) input resistance, thus producing a gain error.
The input leakage currents of the LTC2378-20 should
also be considered when designing the input drive circuit,
because source impedances will convert input leakage
currents to an added input voltage error. The input leakage
currents, both common mode and differential, are typically
extremely small over the entire operating temperature
range. Figure 6 shows input leakage currents over temperature for a typical part.
IN+
REQ
LTC2378-20
CFILT >> 45pF
BIAS
VOLTAGE
IN–
CFILT >> 45pF
REQ
237820 F05
1
REQ =
fSMPL • 45pF
Figure 5. Equivalent Circuit for the Differential Analog
Input of the LTC2378-20 at 1Msps
30
INPUT LEAKAGE (nA)
The ADC inputs may be modeled as a switched capacitor
load of the drive circuit. A drive circuit may rely partially
on attenuating switched-capacitor current spikes with
small filter capacitors CFILT placed directly at the ADC
inputs, and partially on the driver amplifier having sufficient bandwidth to recover from the residual disturbance.
Amplifiers optimized for DC performance may not have
sufficient bandwidth to fully recover at the ADC’s maximum
conversion rate, which can produce nonlinearity and other
errors. Coupling filter circuits may be classified in three
broad categories:
20
10
DIFFERENTIAL
0
–10
–55
COMMON
–35
–15
5
25
45
TEMPERATURE (°C)
65
85
237820 F06
Figure 6. Common Mode and Differential Input Leakage Current
over Temperature
Let RS1 and RS2 be the source impedances of the differential input drive circuit shown in Figure 7, and let IL1
and IL2 be the leakage currents flowing out of the ADC’s
analog inputs. The voltage error, VE, due to the leakage
currents can be expressed as:
VE =
RS1 +RS2
I +I
• (IL1 –IL2 ) + (RS1 –RS2 ) • L1 L2
2
2
The common mode input leakage current, (IL1 + IL2)/2, is
typically extremely small (Figure 6) over the entire operat237820f
For more information www.linear.com/LTC2378-20
11
LTC2378-20
Applications Information
Single-Ended-to-Differential Conversion
IL1
RS1
IN+
+
VE
–
RS2
LTC2378-20
IN–
IL2
237820 F07
Figure 7. Source Impedances of a Driver and Input Leakage
Currents of the LTC2378-20
ing temperature range and common mode input voltage
range. Thus, any reasonable mismatch (below 5%) of the
source impedances RS1 and RS2 will cause only a negligible
error. The differential input leakage current, (IL1 – IL2),
depends on temperature and is maximum when VIN = VREF,
as shown in Figure 6. The differential leakage current is
also typically very small, and its nonlinear component is
even smaller. Only the nonlinear component will impact
the ADC’s linearity.
For optimal performance, it is recommended that the
source impedances, RS1 and RS2, be between 10Ω and
50Ω and with 1% tolerance. For source impedances in
this range, the voltage and temperature coefficients of
RS1 and RS2 are usually not critical. The guaranteed AC
and DC specifications are tested with 10Ω source impedances, and the specifications will gradually degrade with
increased source impedances due to incomplete settling
of the inputs.
Fully Differential Inputs
For single-ended input signals, a single-ended-todifferential conversion circuit must be used to produce
a differential signal at the inputs of the LTC2378-20. The
LT6203 ADC driver is recommended for performing singleended-to-differential conversions. The LT6203 is flexible
and may be configured to convert single-ended signals
of various amplitudes to the ±5V differential input range
of the LTC2378-20.
Figure 9a shows the LT6203 being used to convert a 0V to
5V single-ended input signal. In this case, the first amplifier
is configured as a unity gain buffer and the single-ended
input signal directly drives the high-impedance input of the
amplifier. As shown in the FFT of Figure 9b, the LT6203
drives the LTC2378-20 to near full data sheet performance.
499Ω
LT6203
5V
6
5
+
–
3
0V
2
2
5V
5
0V
6
OUT2
0V
5V
OUT1
0V
249Ω
10µF
+
–
VCM = REF/2
Figure 9a. LT6203 Converting a 0V to 5V Single-Ended
Signal to a ±5V Differential Input Signal
0
SNR = 104dB
THD = –112.7dB
SINAD = 103.6dB
SFDR = 115dB
–20
+
–
1
+
–
7
5V
0V
5V
0V
237820 F08
Figure 8. LT6203 Buffering a Fully Differential Signal Source
AMPLITUDE (dBFS)
0V
3
7
1
–40
LT6203
5V
–
+
237820 F09a
A low distortion fully differential signal source driven
through the LT6203 configured as two unity gain buffers
as shown in Figure 8 can be used to get the full data sheet
distortion performance of –125dB.
5V
499Ω
–60
–80
–100
–120
–140
–160
–180
0
100
200
300
FREQUENCY (kHz)
400
500
237820 F09b
Figure 9b. 128k Point FFT Plot with fIN = 2kHz
for Circuit Shown in Figure 9a
237820f
12
For more information www.linear.com/LTC2378-20
LTC2378-20
Applications Information
Digital Gain Compression
The LTC2378-20 offers a digital gain compression (DGC)
feature which defines the full-scale input swing to be between 10% and 90% of the ±VREF analog input range. To
enable digital gain compression, bring the REF/DGC pin
low. This feature allows the SAR ADC driver to be powered
off of a single positive supply since each input swings
between 0.5V and 4.5V as shown in Figure 10. Needing
only one positive supply to power the SAR ADC driver
results in additional power savings for the entire system.
With DGC enabled, the LTC2378-20 can be driven by the
low power LTC6362 differential driver which is powered
from a single 5V supply. Figure 11a shows how to configure
the LTC6362 to accept a ±3.28V true bipolar single-ended
input signal and level shift the signal to the reduced input
range of the LTC2378-20 when digital gain compression
is enabled. When paired with the LTC6655-4.096 for the
reference, the entire signal chain solution can be powered
from a single 5V supply, minimizing power consump-
DC Accuracy
Many driver circuits presented in this data sheet emphasize
AC performance (Distortion and Signal to Noise Ratio), and
the amplifiers are chosen accordingly. The very low level
of distortion is a direct consequence of the excellent INL of
the LTC2378-20, and this property can be exploited in DC
applications as well. Note that while the LT6362 and LT6203
are characterized by excellent AC specifications, their DC
specifications do not match those of the LTC2378-20. The
offset of these amplifiers, for example, is more than 500μV
under certain conditions. In contrast, the LTC2378-20 has
a guaranteed maximum offset error of 130µV (typical drift
±0.007ppm/°C), and a guaranteed maximum full-scale
error of 100ppm (typical drift ±0.05ppm/°C). Low drift
is important to maintain accuracy over wide temperature
ranges in a calibrated system.
Amplifiers have to be selected very carefully to provide a
20-bit accurate DC signal chain. A large-signal open-loop
gain of at least 126dB may be required to ensure 1ppm
linearity for amplifiers configured for a gain of negative
1. However, less gain is sufficient if the amplifier’s gain
characteristic is known to be (mostly) linear. An amplifier’s offset versus signal level must be considered for
amplifiers configured as unity gain buffers. For example,
5V
4.5V
0.5V
0V
tion and reducing complexity. As shown in the FFT of
Figure 11b, the single 5V supply solution can achieve up
to 100dB of SNR.
237820 F10
Figure 10. Input Swing of the LTC2378 with Gain
Compression Enabled
VIN LTC6655-4.096
5V
0
VOUT_F
4.096V
VOUT_S
–40
1k
VCM
10µF
1k
3
V+
1k
3.28V
0V
–3.28V
8
LTC6362
1k
VCM
4
+
1
5
V–
1k
2.5V
6800pF
0.41V
IN+
35.7Ω
6
IN–
35.7Ω
3.69V
REF
VDD
LTC2378-20
3300pF
–
2
3.69V
6800pF
REF/DGC
237820 F11a
0.41V
AMPLITUDE (dBFS)
1k
47µF
SNR = 100dB
THD = –110dB
SINAD = 99.7dB
SFDR = 113dB
–20
–60
–80
–100
–120
–140
–160
–180
0
100
200
300
FREQUENCY (kHz)
400
500
237820 F11b
Figure 11a. LTC6362 Configured to Accept a ±3.28V Input Signal While Running from
a Single 5V Supply When Digital Gain Compression Is Enabled in the LTC2378-20
Figure 11b. 64k Point FFT Plot
with fIN = 2kHz for Circuit Shown
in Figure 11a
237820f
For more information www.linear.com/LTC2378-20
13
LTC2378-20
Applications Information
1ppm linearity may require that the offset is known to
vary less than 5μV for a 5V swing. However, greater offset
variations may be acceptable if the relationship is known
to be (mostly) linear. Unity-gain buffer amplifiers typically
require substantial headroom to the power supply rails for
best performance. Inverting amplifier circuits configured
to minimize swing at the amplifier input terminals may
perform better with only little headroom than unity-gain
buffer amplifiers. The linearity and thermal properties
of an inverting amplifier’s feedback network should be
considered carefully to ensure DC accuracy.
ADC Reference
The LTC2378-20 requires an external reference to define
its input range. A low noise, low temperature drift reference is critical to achieving the full data sheet performance
of the ADC. 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 LTC6655-5 is particularly well suited for
use with the LTC2378-20. The LTC6655-5 offers 0.025%
(max) initial accuracy and 2ppm/°C (max) temperature
coefficient for high precision applications.
When choosing a bypass capacitor for the LTC6655-5, the
capacitor’s voltage rating, temperature rating, and package size should be carefully considered. Physically larger
capacitors with higher voltage and temperature ratings tend
to provide a larger effective capacitance, better filtering
the noise of the LTC6655-5, and consequently producing
a higher SNR. Therefore, we recommend bypassing the
LTC6655-5 with a 47μF ceramic capacitor (X7R, 1210
size, 10V rating) close to the REF pin.
The REF pin of the LTC2378-20 draws charge (QCONV) from
the 47µF bypass capacitor during each conversion cycle.
The reference replenishes this charge with a DC current,
IREF = QCONV/tCYC. The DC current draw of the REF pin,
IREF, depends on the sampling rate and output code. If
the LTC2378-20 is used to continuously sample a signal
at a constant rate, the LTC6655-5 will keep the deviation
of the reference voltage over the entire code span to less
than 0.5LSBs.
When idling, the REF pin on the LTC2378-20 draws only
a small leakage current (< 1µA). In applications where a
burst of samples is taken after idling for long periods as
shown in Figure 12, IREF quickly goes from approximately
0µA to a maximum of 1.1mA at 1Msps. This step in DC
current draw triggers a transient response in the reference
that must be considered since any deviation in the reference output voltage will affect the accuracy of the output
code. In applications where the transient response of the
reference is important, the fast settling LTC6655-5 reference is also recommended.
DYNAMIC PERFORMANCE
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 LTC2378-20 provides guaranteed tested
limits for both AC distortion and noise measurements.
Signal-to-Noise and Distortion Ratio (SINAD)
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 13 shows that the LTC2378-20 achieves
a typical SINAD of 104dB at a 1MHz sampling rate with a
2kHz input.
CNV
IDLE
PERIOD
IDLE
PERIOD
237820 F12
Figure 12. CNV Waveform Showing Burst Sampling
237820f
14
For more information www.linear.com/LTC2378-20
LTC2378-20
Applications Information
Signal-to-Noise Ratio (SNR)
Power Supply Sequencing
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 13 shows
that the LTC2378-20 achieves a typical SNR of 104dB at
a 1MHz sampling rate with a 2kHz input.
The LTC2378-20 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 LTC2378‑20
has a power-on-reset (POR) circuit that will reset the
LTC2378-20 at initial power-up or whenever the power
supply voltage drops below 1V. Once the supply voltage
re-enters the nominal supply voltage range, the POR will
reinitialize the ADC. No conversions should be initiated
until 200µs after a POR event to ensure the reinitialization
period has ended. Any conversions initiated before this
time will produce invalid results.
0
SNR = 104dB
THD = –128dB
SINAD = 104dB
SFDR = 132dB
–20
AMPLITUDE (dBFS)
–40
–60
–80
–100
TIMING AND CONTROL
–120
–140
CNV Timing
–160
–180
0
100
200
300
FREQUENCY (kHz)
400
500
237820 F13
Figure 13. 128k Point FFT Plot with fIN = 2kHz of the LTC2378-20
Total Harmonic Distortion (THD)
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:
THD= 20log
V22 + V32 + V42 +…+ VN2
V1
where V1 is the RMS amplitude of the fundamental fre­
quency and V2 through VN are the amplitudes of the second
through Nth harmonics.
POWER CONSIDERATIONS
The LTC2378-20 provides two power supply pins: the
2.5V power supply (VDD), and the digital input/output
interface power supply (OVDD). The flexible OVDD supply
allows the LTC2378-20 to communicate with any digital
logic operating between 1.8V and 5V, including 2.5V and
3.3V systems.
The LTC2378-20 conversion is controlled by CNV. A rising edge on CNV will start a conversion and power up
the LTC2378-20. Once a conversion has been initiated,
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 LTC2378-20 powers down and begins
acquiring the input signal.
Internal Conversion Clock
The LTC2378-20 has an internal clock that is trimmed to
achieve a maximum conversion time of 675ns. With a min­
imum acquisition time of 312ns, throughput performance
of 1Msps is guaranteed without any external adjustments.
Auto Power-Down
The LTC2378-20 automatically powers down after a
conversion has been completed and powers up once a
new conversion is initiated on the rising edge of CNV.
During power down, data from the last conversion can
be clocked out. To minimize power dissipation during
237820f
For more information www.linear.com/LTC2378-20
15
LTC2378-20
Applications Information
power down, disable SDO and turn off SCK. The auto
power-down feature will reduce the power dissipation of
the LTC2378-20 as the sampling frequency is reduced.
Since power is consumed only during a conversion, the
LTC2378-20 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 14.
10
POWER SUPPLY CURRENT (mA)
9
8
7
6
5
IVDD
4
3
IOVDD
2
1
0
IREF
0 100 200 300 400 500 600 700 800 900 1000
SAMPLING RATE (kHz)
DIGITAL INTERFACE
The LTC2378-20 has a serial digital interface. The flexible
OVDD supply allows the LTC2378-20 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
64MHz, a 1Msps 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. D19 remains valid until the first rising edge of SCK.
The serial interface on the LTC2378-20 is simple and
straightforward to use. The following sections describe the
operation of the LTC2378-20. Several modes are provided
depending on whether a single or multiple ADCs share the
SPI bus or are daisy chained.
237820 F14
Figure 14. Power Supply Current of the LTC2378-20
Versus Sampling Rate
237820f
16
For more information www.linear.com/LTC2378-20
LTC2378-20
Timing Diagrams
Normal Mode, Single Device
Figure 15 shows a single LTC2378-20 operated in normal
mode with CHAIN and RDL/SDI tied to ground. With RDL/
SDI grounded, SDO is enabled and the MSB(D19) of the
new conversion data is available at the falling edge of
BUSY. This is the simplest way to operate the LTC2378-20.
When CHAIN = 0, the LTC2378-20 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.
CONVERT
DIGITAL HOST
CNV
CHAIN
BUSY
IRQ
LTC2378-20
RDL/SDI
SDO
DATA IN
SCK
CLK
237820 F15a
POWER-DOWN
AND ACQUIRE
CONVERT
POWER-DOWN AND ACQUIRE
CHAIN = 0
RDL/SDI = 0
CONVERT
tCYC
tCNVH
tCNVL
CNV
tACQ = tCYC – tCONV – tBUSYLH
tCONV
BUSY
tACQ
tSCK
tBUSYLH
tSCKH
1
SCK
2
3
tHSDO
tDSDOBUSYL
SDO
tQUIET
18
19
20
tSCKL
tDSDO
D19
D18
D17
D1
D0
237820 F15
Figure 15. Using a Single LTC2378-20 in Normal Mode
237820f
For more information www.linear.com/LTC2378-20
17
LTC2378-20
timing diagrams
Normal Mode, Multiple Devices
Since SDO is shared, the RDL/SDI input of each ADC must
be used to allow only one LTC2378-20 to drive SDO at a
time in order to avoid bus conflicts. As shown in Figure 16,
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 16 shows multiple LTC2378-20 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.
RDLB
RDLA
CONVERT
CNV
CHAIN
CNV
CHAIN
LTC2378-20
B
BUSY
LTC2378-20
SDO
A
IRQ
DIGITAL HOST
SDO
RDL/SDI
RDL/SDI
SCK
SCK
DATA IN
CLK
237820 F16a
POWER-DOWN
AND ACQUIRE
CONVERT
CONVERT
POWER-DOWN AND ACQUIRE
CHAIN = 0
tCNVL
CNV
tCONV
BUSY
tBUSYLH
RDL/SDIA
RDL/SDIB
tSCK
SCK
1
tSCKH
2
3
18
19
20
tHSDO
SDO
Hi-Z
D19A
D18A
D17A
21
22
23
38
39
40
tSCKL
tDSDO
tEN
tQUIET
tDIS
D1A
D0A
Hi-Z
D19B
D18B
D17B
D1B
D0B
Hi-Z
237820 F16
Figure 16. Normal Mode With Multiple Devices Sharing CNV, SCK and SDO
237820f
18
For more information www.linear.com/LTC2378-20
LTC2378-20
timing diagrams
Chain Mode, Multiple Devices
This is useful for applications where hardware constraints
may limit the number of lines needed to interface to a large
number of converters. Figure 17 shows an example with
two daisy-chained devices. The MSB of converter A will
appear at SDO of converter B after 20 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 LTC2378-20 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.
CONVERT
OVDD
OVDD
CNV
CHAIN
RDL/SDI
DIGITAL HOST
LTC2378-20
RDL/SDI
SDO
A
CNV
CHAIN
LTC2378-20
IRQ
BUSY
B
DATA IN
SDO
SCK
SCK
CLK
237820 F17a
POWER-DOWN
AND ACQUIRE
CONVERT
CONVERT
POWER-DOWN AND ACQUIRE
CHAIN = OVDD
RDL/SDIA = 0
tCYC
tCNVL
CNV
BUSY
tCONV
tBUSYLH
tSCKCH
SCK
1
2
3
18
19
tSSDISCK
20
21
22
38
39
40
tSCKL
tHSDO
tHSDISCK
SDOA = RDL/SDIB
tQUIET
tSCKH
tDSDO
D19A
D18A
D17A
D1A
D0A
D19B
D18B
D17B
D1B
D0B
tDSDOBUSYL
SDOB
D19A
D18A
D1A
D0A
237820 F17
Figure 17. Chain Mode Timing Diagram
237820f
For more information www.linear.com/LTC2378-20
19
LTC2378-20
Board Layout
To obtain the best performance from the LTC2378-20
a printed circuit board is recommended. Layout for the
printed circuit board (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 DC1925A, the
evaluation kit for the LTC2378-20.
Top Silkscreen
237820f
20
For more information www.linear.com/LTC2378-20
LTC2378-20
board layout
Layer 1 Component Side
237820f
For more information www.linear.com/LTC2378-20
21
LTC2378-20
board layout
Layer 2 Ground Plane
237820f
22
For more information www.linear.com/LTC2378-20
LTC2378-20
board layout
Layer 3 PWR Plane
237820f
For more information www.linear.com/LTC2378-20
23
LTC2378-20
board layout
Layer 4 Bottom Layer
237820f
24
For more information www.linear.com/LTC2378-20
C78
4.7µF
6.3V
VREF
R87
499k
R86
499k
2
3
V–
4
5
V+
C45
0.1µF
25V
C67
OPT
C18
10µF
6.3V
1
R55
0Ω
R54
OPT
AC
EXT_CM
R88
1Ω
JP8
COUPLING
C77
0.1µF
25V
U5
LT6202CS8
R53
0Ω
J2
BNC
AIN –
0 – VREF
–
+
AIN+
0 – VREF
J4
BNC
AC
1
DC
E5
C8
1µF
DC
EXT
C60
10µF
6.3V
R56
OPT
C59
10µF
6.3V
C74
1µF
25V
JP4
CM
R18
249Ω
C68
15pF
COG
VREF/2
C70
OPT C69
10µF
6.3V
1
3
JP7
COUPLING
2
2
3
2
1
For more information www.linear.com/LTC2378-20
2
4
C48
0.1µF
25V
U18B
LT6203MS8
7
C19
0.1µF
25V
R49
OPT
R58
OPT
R52
0Ω
C75
OPT
C76
OPT
6
5
2
3
R45
OPT
V–
R46
0Ω
C39
OPT
R50
0Ω
7
C44
1µF
25V
C49
1µF
25V
C57
C55 0.1µF
1µF 25V
25V
V+
C17
1µF
25V
C40
1µF
25V
C42
C58 0.1µF
1µF 25V
25V
V+
C47
0.1µF
25V
U10B
LT6203MS8
R47
OPT
C65
OPT
4
BYPASS CAPACITORS FOR U10
C62
10µF
6.3V
5
6
C64
OPT
R34
499Ω
R57
OPT
R44
OPT
C11
0.1µF
9V TO 10V
V+ U10A
LT6203MS8
5
1
R3
CLK
33Ω
TO CPLD
–V
R40
OPT
R39
0Ω
C61
OPT
R41
0Ω
4
5
BYPASS CAPACITORS FOR U18
C63
OPT
R33
499Ω
V–
4
2
5
U2
R6 3 U8
3
NC7SZ04P5X NC7SVU04P5X
1k
V+ U18A
LT6203MS8
5
1
R5
49.9Ω
1206
C5
0.1µF
+3.3V
C2
0.1µF
–V
R36
OPT
2
3
R38
249Ω
R30
0Ω
C43
0.1µF
25V
C66
OPT
R35
OPT
J1
CLKIN
+
–
–
+
3
C9
10µF
6.3V
C10
0.1µF
C6
10µF
6.3V
C7
0.1µF
C20
47µF
10V
1206
X7R
3
2
1
JP6
FS
C56
0.1µF
R1
33Ω
0.8VREF
VREF
6
4
3
SDO
1
3
5
7
9
11
13
9V TO
10V
J3
DC590
2
4
6
8
10
12
14
R17 R13
2k
1k
4
VSS
6
5
7
3
2
1
237820 BL
U7
C14
0.1µF 8 24LC025-I/ST
VCC
SCL
SCK
SDA
WP
CNV
ARRAY
A2
EEPROM
A1
A0
3
7
8
R10
4.99k
C16
0.1µF
R11
4.99k
R12
4.99k
R15
33Ω 4
+3.3V
C3
0.1µF
DC590 DETECT
TO CPLD
R8
33Ω
PR\
Q
5
D
VCC
2
CLR\
Q\
1
CP
GND
+3.3V
C13
0.1µF
U3
NL17SZ74
+3.3V
HD1X3-100
U6
OPT NC7SZ66P5X 5
C71
CNV
VCC
6800pF R9
9
A 1
2 B
0Ω 4
NPO
CNV
13 SCK
IN+
SCK
OE 4
C72
14 SDO
SDO
3300pF
R4
LTC2378-20
GND
11 BUSY
BUSY
1206 NPO
0Ω
3
IN–
12 RD
C73
5
RDL/SDI
6800pF
NPO
R7
+3.3V
R51
1k
10Ω
U9
C15
NC7SZ04P5X
5
0.1µF
2
4
R48
10Ω
+2.5V
+3.3V
U15
LTC6655AHMS8-5
1
8
SHDN
GND
2
7
VIN
OUT_F
3
6
GND OUT_S
4
5
GND
GND
VDD 2
OVDD 15
REF 7
8
REF/DGC
GND
GND
GND
GND
3
6
10
16
1
+3.3V
C1
0.1µF
R2
1k
–
+
–
+
+3.3V
3
5
DB1
DB0
DB3
DB2
DB5
DB4
DB7
DB6
DB9
DB8
DB11
DB10
DB13
DB12
DB15
DB14
CLKOUT
CLK2
DB17
DB16
DB19
DB18
2 CNVST_33
FROM CPLD
U4
NC7SVU04P5X
+3.3V
C4
0.1µF
P1
55
57
59
61
63
65
67
69
71
73
75
77
79
81
83
85
87
89
91
93
95
97
99
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
33
35
37
39
41
43
45
47
49
51
53
R80
2k
0603
EDGE-CON-100
56
58
60
62
64
66
68
70
72
74
76
78
80
82
84
86
88
90
92
94
96
98
100
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
52
54
R81
4.99k
3.3V
LTC2378-20
board layout
Partial Schematic of Demoboard
237820f
25
LTC2378-20
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
DE Package
16-Lead Plastic DFN (4mm × 3mm)
(Reference LTC DWG # 05-08-1732 Rev Ø)
4.00 ±0.10
(2 SIDES)
R = 0.05
TYP
0.70 ±0.05
3.60 ±0.05
2.20 ±0.05
3.30 ±0.05
1.70 ±0.05
PACKAGE
OUTLINE
PIN 1
TOP MARK
(SEE NOTE 6)
9
R = 0.115
TYP
3.30 ±0.10
3.00 ±0.10
(2 SIDES)
1.70 ±0.10
0.200 REF
PIN 1 NOTCH
R = 0.20 OR
0.35 × 45°
CHAMFER
(DE16) DFN 0806 REV Ø
8
0.25 ±0.05
0.45 BSC
0.40 ±0.10
16
1
0.23 ±0.05
0.45 BSC
0.75 ±0.05
3.15 REF
3.15 REF
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE MADE VARIATION OF VERSION (WGED-3) IN JEDEC
PACKAGE OUTLINE MO-229
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
237820f
26
For more information www.linear.com/LTC2378-20
LTC2378-20
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
MS Package
16-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1669 Rev Ø)
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
0.889 ±0.127
(.035 ±.005)
5.23
(.206)
MIN
16151413121110 9
3.20 – 3.45
(.126 – .136)
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
GAUGE PLANE
0.305 ±0.038
(.0120 ±.0015)
TYP
0.53 ±0.152
(.021 ±.006)
0.50
(.0197)
BSC
RECOMMENDED SOLDER PAD LAYOUT
DETAIL “A”
0.18
(.007)
SEATING
PLANE
1.10
(.043)
MAX
0.17 – 0.27
(.007 – .011)
TYP
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
1234567 8
0.50
(.0197)
BSC
0.86
(.034)
REF
0.1016 ±0.0508
(.004 ±.002)
MSOP (MS16) 1107 REV Ø
237820f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection
of its
circuits as described
herein will not infringe on existing patent rights.
For more
information
www.linear.com/LTC2378-20
27
LTC2378-20
Typical Application
LTC6362 Configured to Accept a ±3.28V Input Signal While Running from a Single
5V Supply with Digital Gain Compression Enabled in the LTC2378-20
VIN LTC6655-4.096
5V
VOUT_F
4.096V
VOUT_S
1k
10µF
1k
V+
1k
3.28V
0V
–3.28V
47µF
1k
VCM
8
4
+
LTC6362
1k
VCM
3
1
2.5V
6800pF
0.41V
IN+
35.7Ω
5
V–
6
1k
IN–
35.7Ω
3.69V
REF
VDD
LTC2378-20
3300pF
–
2
3.69V
6800pF
REF/DGC
237820 TA03
0.41V
Related Parts
PART NUMBER
DESCRIPTION
COMMENTS
LTC2379-18/LTC2378-18
LTC2377-18/LTC2376-18
18-Bit, 1.6Msps/1Msps/500ksps/250ksps 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
LTC2377-16/LTC2376-16
16-Bit, 2Msps/1Msps/500ksps/250ksps 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
ADCs
LTC2369-18/LTC2368-18/ 18-Bit, 1.6Msps/1Msps/500ksps/250ksps Serial,
LTC2367-18/LTC2364-18 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
LTC2367-16/LTC2364-16
16-Bit, 2Msps/1Msps/500ksps/250ksps 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-/16-Bit, 2.5Msps Parallel/Serial, ADC
5V Supply, Pin Configurable Input Range, 99.8dB SNR, Pin Compatible
Parts in 7mm × 7mm LQFP-48 and QFN-48 Packages
LTC2756/LTC2757
18-Bit, Single Serial/Parallel IOUT SoftSpan™ DAC
±1LSB INL/DNL, SSOP-28 and 7mm × 7mm LQFP-48 Packages
LTC2641
16-/14-/12-Bit Single Serial VOUT DAC
±1LSB INL /DNL, MSOP-8 Package, 0V to 5V Output
LTC2630
12-/10-/8-Bit Single VOUT DACs
SC70 6-Pin Package, Internal Reference, ±1LSB INL (12 Bits)
LTC6655
Precision Low Drift Low Noise Buffered Reference
5V/2.5V, 5ppm/°C, 0.25ppm Peak-to-Peak Noise, MSOP-8 Package
LTC6652
Precision Low Drift Low Noise Buffered Reference
5V/2.5V, 5ppm/°C, 2.1ppm Peak-to-Peak Noise, MSOP-8 Package
LTC6362
Low Power Rail-to-Rail Input/Output Differential
Output Amplifier/ADC Driver
Single 2.8V to 5.25V Supply, 1mA Supply Current, MSOP-8 and
3mm × 3mm DFN-8 Packages
LT6200/LT6200-5/
LT6200-10
165MHz/800MHz/1.6GHz Op Amp with
Unity Gain/AV = 5/AV = 10
Low Noise Voltage: 0.95nV/√Hz (100kHz), Low Distortion: –80dB at
1MHz, TSOT23-6 Package
LT6202/LT6203
Single/Dual 100MHz Rail-to-Rail Input/Output Noise 1.9nV√Hz, 3mA Maximum, 100MHz Gain Bandwidth, TSOT23-5, SO-8 ,
Low Power Amplifiers
MSOP-8 and 3mm × 3mm DFN-8 Packages
DACS
REFERENCES
AMPLIFIERS
237820f
28 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LTC2378-20
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LTC2378-20
LT 0413 • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2013