TI1 ADS8887 18-bit, 100-ksps, serial interface, micropower, miniature true-differential input, sar analog-to-digital converter Datasheet

ADS8887
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SBAS613A – MAY 2013 – REVISED DECEMBER 2013
18-Bit, 100-kSPS, Serial Interface, microPower, Miniature,
True-Differential Input, SAR Analog-to-Digital Converter
Check for Samples: ADS8887
FEATURES
APPLICATIONS
• Sample Rate: 100 kHz
• No Latency Output
• Unipolar, True-Differential Input Range:
–VREF to +VREF
• Wide Common-Mode Voltage Range:
0 V to VREF with 90-dB CMRR (min)
• SPI™-Compatible Serial Interface with
Daisy-Chain Option
• Excellent AC and DC Performance:
– SNR: 100 dB, THD: –115 dB
– INL: ±1.5 LSB (typ), ±3.0 LSB (max)
– DNL: +1.5 and –1 LSB (max), 18-Bit NMC
• Wide Operating Range:
– AVDD: 2.7 V to 3.6 V
– DVDD: 1.65 V to 3.6 V
(Independent of AVDD)
– REF: 2.5 V to 5 V (Independent of AVDD)
– Operating Temperature: –40°C to +85°C
• Low-Power Dissipation:
– 0.7 mW at 100 kSPS
– 70 µW at 10 kSPS
• Power-Down Current (AVDD): 50 nA
• Full-Scale Step Settling to 18 Bits: 1200 ns
• Packages: MSOP-10 and SON-10
•
•
•
•
1
234
Traditional Input Range
ADS8887 Input Range
VDIFF
Automatic Test Equipment (ATE)
Instrumentation and Process Controls
Precision Medical Equipment
Low-Power, Battery-Operated Instruments
DESCRIPTION
The ADS8887 is an 18-bit, 100-kSPS, true-differential
input, analog-to-digital converter (ADC). The device
operates with a 2.5-V to 5-V external reference,
offering a wide selection of signal ranges without
additional input signal scaling. The reference voltage
setting is independent of, and can exceed, the analog
supply voltage (AVDD).
The device offers an SPI-compatible serial interface
that also supports daisy-chain operation for
cascading multiple devices.
The device supports unipolar, true-differential analog
input signals with a differential input swing of –VREF to
+VREF. This true-differential analog input structure
allows for a common-mode voltage of any value in
the range of 0 V to +VREF (when both inputs are
within the operating input range of –0.1 V to VREF
+ 0.1 V).
Device operation is optimized for very low-power
operation. Power consumption directly scales with
speed. This feature makes the ADS8887 excellent for
lower-speed applications.
True-Differential Input Range
+VREF
2.5 V to 5 V
2.7 V to 3.6 V
REF
AVDD
1.65 V to 3.6 V
VREF
VCM
0V
0 V - VREF
DVDD
AINP
VCM
DIN
ADS8887
VREF
SCLK
DOUT
0V
AINM
CONVST
0 V - VREF
VREF/2
GND
-VREF
1
2
3
4
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
TINA is a trademark of Texas Instruments Inc..
SPI is a trademark of Motorola Inc.
All other trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2013, Texas Instruments Incorporated
ADS8887
SBAS613A – MAY 2013 – REVISED DECEMBER 2013
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This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling and installation procedures can cause damage.
ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more
susceptible to damage because very small parametric changes could cause the device not to meet its published specifications.
FAMILY INFORMATION (1)
(1)
(2)
THROUGHPUT
18-BIT, TRUE-DIFFERENTIAL
16-BIT, SINGLE-ENDED
16-BIT, TRUE-DIFFERENTIAL
100 kSPS
ADS8887
ADS8866
ADS8867
250 kSPS
—
—
—
400 kSPS
ADS8885
ADS8864
ADS8865
500 kSPS
—
ADS8319 (2)
ADS8318 (2)
680 kSPS
ADS8883
ADS8862
ADS8863
1 MSPS
ADS8881
ADS8860
ADS8861
For the most current package and ordering information, see the Package Option Addendum at the end of this document, or visit the
device product folder at www.ti.com.
Pin-to-pin compatible device with AVDD = 5 V.
ABSOLUTE MAXIMUM RATINGS (1)
Over operating free-air temperature range, unless otherwise noted.
VALUE
MIN
MAX
AINP to GND or AINN to GND
–0.3
REF + 0.3
V
AVDD to GND or DVDD to GND
–0.3
4
V
REF to GND
–0.3
5.7
V
Digital input voltage to GND
–0.3
DVDD + 0.3
V
Digital output to GND
–0.3
DVDD + 0.3
V
Operating temperature range, TA
–40
+85
°C
Storage temperature range, Tstg
–65
+150
°C
(1)
UNIT
Stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. These are stress ratings
only, and functional operation of the device at these or any other conditions beyond those indicated under electrical characteristics is not
implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
THERMAL INFORMATION
ADS8887
THERMAL METRIC
(1)
DGS
DRC
10 PINS
10 PINS
θJA
Junction-to-ambient thermal resistance
151.9
111.1
θJCtop
Junction-to-case (top) thermal resistance
45.4
46.4
θJB
Junction-to-board thermal resistance
72.2
45.9
ψJT
Junction-to-top characterization parameter
3.3
3.5
ψJB
Junction-to-board characterization parameter
70.9
45.5
θJCbot
Junction-to-case (bottom) thermal resistance
N/A
N/A
(1)
2
UNITS
°C/W
For more information about traditional and new thermal metrics, see the IC Package Thermal Metrics application report, SPRA953.
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ELECTRICAL CHARACTERISTICS
All minimum and maximum specifications are at TA = –40°C to +85°C, AVDD = 3 V, DVDD = 3 V, VREF = 5 V,
VCM = VREF / 2 V, and fSAMPLE = 100 kSPS, unless otherwise noted.
Typical specifications are at TA = +25°C, AVDD = 3 V, and DVDD = 3 V.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
ANALOG INPUT
Full-scale input span (1) (2)
Operating input range
(1) (2)
AINP – AINN
–VREF
VREF
V
AINP
–0.1
VREF + 0.1
V
AINN
–0.1
VREF + 0.1
V
VCM
Input common-mode range
0
CI
Input capacitance
AINP and AINN terminal to GND
Input leakage current
During acquisition for dc input
VREF / 2
VREF
V
59
pF
5
nA
18
Bits
SYSTEM PERFORMANCE
Resolution
NMC
No missing codes
DNL
Differential linearity
INL
Integral linearity (4)
EO
Offset error
18
(5)
±0.7
1.5
LSB (3)
–3
±1.5
3
LSB (3)
±1
4
–4
Offset error drift with temperature
EG
Bits
–0.99
mV
±1.5
Gain error
–0.01
Gain error drift with temperature
±0.005
µV/°C
0.01
±0.15
CMRR
Common-mode rejection ratio
With common-mode input signal = 5 VPP at dc
PSRR
Power-supply rejection ratio
At mid-code
90
Transition noise
%FSR
ppm/°C
100
dB
80
dB
0.7
LSB
SAMPLING DYNAMICS
tconv
Conversion time
500
tACQ
Acquisition time
1200
8800
ns
Maximum throughput rate
with or without latency
(1)
(2)
(3)
(4)
(5)
ns
100
kHz
Aperture delay
4
ns
Aperture jitter, RMS
5
ps
Step response
Settling to 18-bit accuracy
1200
ns
Overvoltage recovery
Settling to 18-bit accuracy
1200
ns
Ideal input span, does not include gain or offset error.
Specified for VCM = VREF / 2. Refer to the Analog Input section for the effect of VCM on the full-scale input range.
LSB = least significant bit.
This parameter is the endpoint INL, not best-fit.
Measured relative to actual measured reference.
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ELECTRICAL CHARACTERISTICS (continued)
All minimum and maximum specifications are at TA = –40°C to +85°C, AVDD = 3 V, DVDD = 3 V, VREF = 5 V,
VCM = VREF / 2 V, and fSAMPLE = 100 kSPS, unless otherwise noted.
Typical specifications are at TA = +25°C, AVDD = 3 V, and DVDD = 3 V.
PARAMETER
TEST CONDITIONS
MIN
TYP
98
MAX
UNIT
DYNAMIC CHARACTERISTICS
At 1 kHz, VREF = 5 V
SINAD
Signal-to-noise + distortion (6)
99.9
dB
At 10 kHz, VREF = 5 V
98.7
dB
At 49 kHz, VREF = 5 V
93.3
dB
100
dB
At 10 kHz, VREF = 5 V
99.5
dB
At 49 kHz, VREF = 5 V
93.5
dB
At 1 kHz, VREF = 5 V
–115
dB
At 10 kHz, VREF = 5 V
–112
dB
At 49 kHz, VREF = 5 V
–102
dB
At 1 kHz, VREF = 5 V
115
dB
At 10 kHz, VREF = 5 V
112
dB
At 49 kHz, VREF = 5 V
102
dB
30
MHz
At 1 kHz, VREF = 5 V
Signal-to-noise ratio (6)
SNR
Total harmonic distortion (6) (7)
THD
SFDR
BW–3dB
Spurious-free dynamic range (6)
98.5
–3-dB small-signal bandwidth
EXTERNAL REFERENCE INPUT
VREF
Input range
2.5
Reference input current
During conversion, 100-kHz sample rate, mid-code
Reference leakage current
CREF
Decoupling capacitor at the REF
input
10
5
V
35
μA
250
nA
22
µF
POWER-SUPPLY REQUIREMENTS
Power-supply
voltage
AVDD
Analog supply
DVDD
Digital supply range
Supply current
AVDD
PVA
Power dissipation
IAPD
Device power-down current (8)
2.7
3
3.6
1.65
1.8
3.6
V
V
100-kHz sample rate, AVDD = 3 V
0.23
0.4
mA
100-kHz sample rate, AVDD = 3 V
0.7
1.2
mW
10-kHz sample rate, AVDD = 3 V
70
μW
50
nA
DIGITAL INPUTS: LOGIC FAMILY (CMOS)
VIH
High-level input voltage
VIL
Low-level input voltage
ILK
Digital input leakage current
1.65 V < DVDD < 2.3 V
0.8 × DVDD
DVDD + 0.3
V
2.3 V < DVDD < 3.6 V
0.7 × DVDD
DVDD + 0.3
V
1.65 V < DVDD < 2.3 V
–0.3
0.2 × DVDD
V
2.3 V < DVDD < 3.6 V
–0.3
0.3 × DVDD
V
±100
nA
0.8 × DVDD
DVDD
V
0
0.2 × DVDD
V
–40
+85
°C
±10
DIGITAL OUTPUTS: LOGIC FAMILY (CMOS)
VOH
High-level output voltage
IO = 500-μA source, CLOAD = 20 pF
VOL
Low-level output voltage
IO = 500-μA sink, CLOAD = 20 pF
TEMPERATURE RANGE
TA
(6)
(7)
(8)
4
Operating free-air temperature
All specifications expressed in decibels (dB) refer to the full-scale input (FSR) and are tested with an input signal 0.5 dB below full-scale,
unless otherwise specified.
Calculated on the first nine harmonics of the input frequency.
The device automatically enters a power-down state at the end of every conversion, and remains in power-down during the acquisition
phase.
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TIMING CHARACTERISTICS
3-WIRE OPERATION
1/fsample
DIN = HIGH
tconv-max
tACQ
tclkh
th-CK-DO
CONVST
tclkl
tquiet
œœ
SCLK
1
2
td-CNV-DO
D17
DOUT
3
16
17
18
D1
D0
tSCLK
œœ
D16
D15
D2
œœ
twh-CNV-min
td-CK-DO
td-CK-DOhz
Figure 1. 3-Wire Operation: CONVST Functions as Chip Select
Table 1. TIMING REQUIREMENTS: 3-Wire Operation (1)
PARAMETER
MIN
TYP
MAX
UNIT
tACQ
Acquisition time
1200
tconv
Conversion time
500
1/fsample
Time between conversions
twh-CNV
Pulse duration: CONVST high
fSCLK
SCLK frequency
tSCLK
SCLK period
62.5
tclkl
SCLK low time
0.45
0.55
tSCLK
tclkh
SCLK high time
0.45
0.55
tSCLK
th-CK-DO
SCLK falling edge to current data invalid
td-CK-DO
SCLK falling edge to next data valid delay
13.4
ns
td-CNV-DO
Enable time: CONVST low to MSB valid
12.3
ns
td-CNV-DOhz
Disable time: CONVST high or last SCLK falling edge to DOUT 3-state (CS mode)
13.2
ns
tquiet
Quiet time
(1)
ns
8800
10000
ns
ns
10
ns
16
MHz
ns
3
ns
20
ns
All specifications are at TA = –40°C to +85°C, AVDD = 3 V, and DVDD = 3 V, unless otherwise noted.
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4-WIRE OPERATION
1/fsample
tACQ
tconv-max
CONVST
tsu-DI-CNV
twl-CNV
DIN
œœ
SCLK
1
DOUT
D17
2
3
D16
D15
16
17
18
D1
D0
œœ
twh-DI-min
td-DI-DO
D2
œœ
td-DI-DOhz
Figure 2. 4-Wire Operation: DIN Functions as Chip Select
Table 2. TIMING REQUIREMENTS: 4-Wire Operation (1)
PARAMETER
tACQ
Acquisition time
tconv
Conversion time
1/fsample
Time between conversions
twh-DI
MIN
TYP
MAX
1200
500
UNIT
ns
8800
ns
10000
ns
Pulse duration: DIN high
10
ns
twl-CNV
Pulse width: CONVST low
20
td-DI-DO
Delay time: DIN low to MSB valid
12.3
ns
td-DI-DOhz
Delay time: DIN high or last SCLK falling edge to DOUT 3-state
13.2
ns
tsu-DI-CNV
Setup time: DIN high to CONVST rising edge
th-DI-CNV
Hold time: DIN high from CONVST rising edge
(1)
6
ns
7.5
ns
0
ns
All specifications are at TA = –40°C to +85°C, AVDD = 3 V, and DVDD = 3 V, unless otherwise noted.
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DAISY-CHAIN OPERATION
1/fsample
tconv-
tACQ
max
CONVST
th-CK-CNV
SCLK
DIN 1 = LOW
1
2
17
18
19
20
35
36
D17
D16
D1
D0
tsu-DI-CK
tsu-CK-CNV
DOUT 1,
DIN 2
D17
D16
D1
D0
DOUT 2
D17
D16
D1
D0
Device 2 Data
Device 1 Data
Figure 3. Daisy-Chain Operation: Two Devices
Table 3. TIMING REQUIREMENTS: Daisy-Chain (1)
PARAMETER
MIN
tACQ
Acquisition time
1200
tconv
Conversion time
500
1/fsample
Time between conversions
tsu-CK-CNV
Setup time: SCLK valid to CONVST rising edge
th-CK-CNV
Hold time: SCLK valid from CONVST rising edge
tsu-DI-CNV
Setup time: DIN low to CONVST rising edge (see Figure 2)
th-DI-CNV
Hold time: DIN low from CONVST rising edge
tsu-DI-CK
Setup time: DIN valid to SCLK falling edge
(1)
TYP
MAX
UNIT
ns
8800
ns
10000
ns
5
ns
5
ns
7.5
ns
0
ns
1.5
ns
All specifications are at TA = –40°C to +85°C, AVDD = 3 V, and DVDD = 3 V, unless otherwise noted.
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EQUIVALENT CIRCUITS
500 µA
IOL
1.4 V
DOUT
20 pF
500 µA
IOH
Figure 4. Load Circuit for Digital Interface Timing
DIN
CONVST
SCLK
VIH
VIL
VOH
VOH
VOL
VOL
SDO
Figure 5. Voltage Levels for Timing
8
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PIN CONFIGURATIONS
DGS PACKAGE
VSSOP-10
(TOP VIEW, Not to Scale)
DRC PACKAGE
SON-10
(TOP VIEW, Not to Scale)
REF
1
10
DVDD
AVDD
2
9
DIN
AINP
3
8
SCLK
AINN
4
7
DOUT
GND
5
6
CONVST
REF
1
10
DVDD
AVDD
2
9
DIN
AINP
3
8
SCLK
AINN
4
7
DOUT
GND
5
6
CONVST
Thermal
PAD
PIN ASSIGNMENTS
PIN NAME
PIN
NUMBER
FUNCTION
AINN
4
Analog input
Inverting analog signal input
AINP
3
Analog input
Noninverting analog signal input
AVDD
2
Analog
CONVST
6
Digital input
Convert input.
This pin also functions as the CS input in 3-wire interface mode. Refer to the
Description and Timing Characteristics sections for more details.
Serial data input.
The DIN level at the start of a conversion selects the mode of operation (such as CS
or daisy-chain mode). This pin also serves as the CS input in 4-wire interface mode.
Refer to the Description and Timing Characteristics sections for more details.
DESCRIPTION
Analog power supply.
This pin must be decoupled to GND with a 1-μF capacitor.
DIN
9
Digital input
DOUT
7
Digital output
Serial data output
DVDD
10
Power supply
Digital interface power supply.
This pin must be decoupled to GND with a 1-μF capacitor.
Device ground.
Note that this pin is a common ground pin for both the analog power supply (AVDD)
and digital I/O supply (DVDD). The reference return line is also internally connected to
this pin.
GND
5
Analog, digital
REF
1
Analog
SCLK
8
Digital input
Clock input for serial interface.
Data output (on DOUT) are synchronized with this clock.
Thermal pad
—
Thermal pad
Exposed thermal pad.
Texas Instruments recommends connecting the thermal pad to the printed circuit
board (PCB) ground.
Positive reference input.
This pin must be decoupled with a 10-μF or larger capacitor.
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TYPICAL CHARACTERISTICS
At TA = +25°C, AVDD = 3 V, DVDD = 3 V, VREF = 5 V, and fSAMPLE = 100 kSPS, unless otherwise noted.
1
Typical Differential Nonlinearity (LSB)
Typical Integral Nonlinearity (LSB)
3
AVDD = 3 V
REF = 2.5 V
TA = 25ƒC
2
1
0
±1
±2
±3
±131072
0.5
0.25
0
-0.25
-0.5
-0.75
-1
±131072
131071
ADC Output Code
1
AVDD = 3 V
REF = 5 V
TA = 25ƒC
2
Typical Differential Nonlinearity (LSB)
Typical Integral Nonlinearity (LSB)
C002
Figure 7. TYPICAL DNL (VREF = 2.5 V)
3
1
0
±1
±2
±3
±131072
131071
ADC Output Code
AVDD = 3 V
REF = 5 V
TA = 25ƒC
0.75
0.5
0.25
0
-0.25
-0.5
-0.75
-1
±131072
131071
ADC Output Code
C003
Figure 8. TYPICAL INL (VREF = 5 V)
C004
Figure 9. TYPICAL DNL (VREF = 5 V)
3
2
AVDD = 3 V
REF = 5 V
2
Differential Nonlinearity (LSB)
Integral Nonlinearity (LSB)
131071
ADC Output Code
C001
Figure 6. TYPICAL INL (VREF = 2.5 V)
1
0
-1
-2
-3
AVDD = 3 V
REF = 5 V
1.5
1
0.5
0
-0.5
-1
-40
-15
10
35
Free-Air Temperature (oC)
60
85
-40
C00
Figure 10. INL vs TEMPERATURE
10
AVDD = 3 V
REF = 2.5 V
TA = 25ƒC
0.75
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-15
10
35
60
Free-Air Temperature (oC)
85
C00
Figure 11. DNL vs TEMPERATURE
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TYPICAL CHARACTERISTICS (continued)
At TA = +25°C, AVDD = 3 V, DVDD = 3 V, VREF = 5 V, and fSAMPLE = 100 kSPS, unless otherwise noted.
2
AVDD = 3 V
TA = 25oC
2
Differential Nonlinearity (LSB)
Integral Nonlinearity (LSB)
3
1
0
-1
-2
AVDD = 3 V
TA = 25oC
1.5
1
0.5
0
-0.5
-1
-3
2.5
3
3.5
4
Reference Voltage (V)
4.5
2.5
5
3
Figure 12. INL vs REFERENCE VOLTAGE
4.5
5
C00
Figure 13. DNL vs REFERENCE VOLTAGE
40
60
AVDD = 3 V
REF = 2.5 V
TA = 25oC
AVDD = 3 V
REF = 5 V
TA = 25oC
50
Hits per Code (%)
30
Hits per Code (%)
3.5
4
Reference Voltage (V)
C00
20
40
30
20
10
10
0
0
10
12
14
16
ADC Output Code
18
20
4
Figure 14. DC INPUT HISTOGRAM (VREF = 2.5 V)
8
ADC Output Code
10
C01
Figure 15. DC INPUT HISTOGRAM (VREF = 5 V)
0
0
AVDD = 3 V
REF = 2.5 V
TA = 25ƒC
fIN = 1 kHz
SNR = 95.4 dB
THD = ±119 dB
±40
±60
±80
AVDD = 3 V
REF = 5 V
TA = 25ƒC
fIN = 1 kHz
SNR = 100 dB
THD = ±115 dB
±20
±40
±60
Power (dB)
±20
Power (dB)
6
C00
±100
±120
±80
±100
±120
±140
±140
±160
±160
±180
±180
±200
±200
0
10
20
30
Input Frequency (kHz)
40
50
0
C001
Figure 16. TYPICAL FFT (VREF = 2.5 V)
10
20
30
40
Input Frequency (kHz)
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C002
Figure 17. TYPICAL FFT (VREF = 5 V)
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TYPICAL CHARACTERISTICS (continued)
At TA = +25°C, AVDD = 3 V, DVDD = 3 V, VREF = 5 V, and fSAMPLE = 100 kSPS, unless otherwise noted.
102
Signal-to-Noise and Distortion (dBFS)
Signal-to-Noise Ratio (dBFS)
102
101
100
99
98
97
96
95
fIN = 1 kHz
94
101
100
99
98
97
96
95
fIN = 1 kHz
94
2.5
3
3.5
4
Reference Voltage (V)
4.5
5
2.5
Figure 18. SNR vs REFERENCE VOLTAGE
Total Harmonic Distortion (dBFS)
Effective Number of Bits
4.5
5
C01
-109
fIN = 1 kHz
17.5
17
16.5
16
15.5
15
14.5
fIN = 1 kHz
-111
-113
-115
-117
-119
-121
-123
-125
14
2.5
3
3.5
4
Reference Voltage (V)
4.5
2.5
5
3
C01
Figure 20. ENOB vs REFERENCE VOLTAGE
3.5
4
Reference Voltage (V)
4.5
5
C01
Figure 21. THD vs REFERENCE VOLTAGE
102
126
fIN = 1 kHz
124
Signal-to-Noise Ratio (dBFS)
Spurious-Free Dynamic Range (dBFS)
3.5
4
Reference Voltage (V)
Figure 19. SINAD vs REFERENCE VOLTAGE
18
122
120
118
116
114
112
fIN = 1 kHz
101
100
99
98
97
96
95
94
110
2.5
3
3.5
4
Reference Voltage (V)
4.5
5
-40
C01
Figure 22. SFDR vs REFERENCE VOLTAGE
12
3
C01
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-15
10
35
Free-Air Temperature (oC)
60
85
C01
Figure 23. SNR vs TEMPERATURE
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TYPICAL CHARACTERISTICS (continued)
At TA = +25°C, AVDD = 3 V, DVDD = 3 V, VREF = 5 V, and fSAMPLE = 100 kSPS, unless otherwise noted.
18
fIN = 1 kHz
101
fIN = 1 kHz
17.5
Effective Number of Bits
Signal-to-Noise and Distortion (dBFS)
102
100
99
98
97
96
95
17
16.5
16
15.5
15
14.5
94
14
-40
-15
10
35
Free-Air Temperature (oC)
60
85
-40
Figure 24. SINAD vs TEMPERATURE
Spurious-Free Dynamic Range (dBFS)
Total Harmonic Distortion (dBFS)
fIN = 1 kHz
-113
-115
-117
-119
-121
-123
-125
60
85
C02
126
fIN = 1 kHz
124
122
120
118
116
114
112
110
-40
-15
10
35
Free-Air Temperature (oC)
60
85
-40
-15
C02
Figure 26. THD vs TEMPERATURE
10
35
Free-Air Temperature (oC)
60
85
C02
Figure 27. SFDR vs TEMPERATURE
1
2.5
Power Consumption (mW)
Analog Supply Current (mA)
10
35
Free-Air Temperature (oC)
Figure 25. ENOB vs TEMPERATURE
-109
-111
-15
C01
0.75
0.5
0.25
0
2
1.5
1
0.5
0
-40
-15
10
35
60
Free-Air Temperature (oC)
85
Figure 28. SUPPLY CURRENT vs TEMPERATURE
-40
C00
-15
10
35
60
Free-Air Temperature (oC)
85
C00
Figure 29. POWER CONSUMPTION vs TEMPERATURE
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TYPICAL CHARACTERISTICS (continued)
At TA = +25°C, AVDD = 3 V, DVDD = 3 V, VREF = 5 V, and fSAMPLE = 100 kSPS, unless otherwise noted.
1
Power Consumption (mW)
Analog Supply Current (mA)
0.4
0.3
0.2
0.1
0.8
0.6
0.4
0.2
0
0
0
20
40
60
Throughput (kSPS)
80
0
100
Figure 30. SUPPLY CURRENT vs THROUGHPUT
40
60
Throughput (kSPS)
200
4
175
3
150
2
125
1
100
75
100
C00
0
-1
50
-2
25
-3
-4
0
-40
-15
10
35
Free-Air Temperature (oC)
60
-40
85
-15
C03
Figure 32. POWER-DOWN CURRENT vs TEMPERATURE
10
35
Free-Air Temperature (oC)
60
85
C03
Figure 33. OFFSET vs TEMPERATURE
0.01
18000
AVDD = 3 V
REF = 5 V
TA = 25oC
15000 Devices
16000
0.006
14000
12000
Frequency
Gain Error (%FS)
80
Figure 31. POWER CONSUMPTION vs THROUGHPUT
Offset (mV)
Power-Down Current (mA)
20
C00
0.002
-0.002
10000
8000
6000
4000
-0.006
2000
-0.01
0
-40
-15
10
35
Free-Air Temperature (oC)
60
-0.01
-0.005
0
Gain Error (% FS)
C03
Figure 34. GAIN ERROR vs TEMPERATURE
14
85
0.005
0.01
C03
Figure 35. TYPICAL DISTRIBUTION OF GAIN ERROR
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TYPICAL CHARACTERISTICS (continued)
At TA = +25°C, AVDD = 3 V, DVDD = 3 V, VREF = 5 V, and fSAMPLE = 100 kSPS, unless otherwise noted.
16000
8000
AVDD = 3 V
REF = 5 V
TA = 25oC
15000 Devices
7000
12000
5000
Frequency
Frequency
6000
AVDD = 3 V
REF = 5 V
TA = 25ƒC
15000 Devices
14000
4000
3000
10000
8000
6000
2000
4000
1000
2000
0
0
-4
-3
-2
-1
0
1
Offset (mV)
2
3
±1.0
4
C03
Figure 36. TYPICAL DISTRIBUTION OF OFFSET ERROR
0.0
0.5
1.0
±0.5
Differential Nonlinearity Min and Max (LSB)
1.5
C038
Figure 37. TYPICAL DISTRIBUTION OF DIFFERENTIAL
NONLINEARITY (Minimum and Maximum)
14000
12000
Frequency
10000
AVDD = 3 V
REF = 5 V
TA = 25ƒC
15000 Devices
8000
6000
4000
2000
0
±3.0 ±2.5 ±2.0 ±1.5 ±1.0 ±0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0
Integral Nonlinearity Min and Max (LSB)
C039
Figure 38. TYPICAL DISTRIBUTION OF INTEGRAL
NONLINEARITY (Minimum and Maximum)
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OVERVIEW
The ADS8887 is a high-speed, successive approximation register (SAR), analog-to-digital converter (ADC) from
a 16- and 18-bit product family. This compact device features high performance. Power consumption is
inherently low and scales linearly with sampling speed. The architecture is based on charge redistribution, which
inherently includes a sample-and-hold (S/H) function.
The ADS8887 supports a true-differential analog input across two pins (INP and INN). When a conversion is
initiated, the differential input on these pins is sampled on the internal capacitor array. While a conversion is in
progress, both the INP and INN inputs are disconnected from the internal circuit.
The ADS8887 uses an internal clock to perform conversions. The device reconnects the sampling capacitors to
the INP and INN pins after conversion and then enters an acquisition phase. During the acquisition phase, the
device is powered down and the conversion result can be read.
The device digital output is available in SPI-compatible format, which makes interfacing with microprocessors,
digital signal processors (DSPs), or field-programmable gate arrays (FPGAs) easy.
ANALOG INPUT
As shown in Figure 39, the device features a differential analog input. Both positive and negative inputs are
individually sampled on 55-pF sampling capacitors and the device converts for the voltage difference between
the two sampled values: VINP – VINN.
AVDD
REF
DVDD
REF
CONVST
AINP
Sample
and
Hold
AINN
SCLK
SAR
ADC
ADC
SPI
DOUT
DIN
AGND
REFM
DGND
GND
GND
Figure 39. Detailed Block Diagram
16
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Most differential input SAR ADCs prohibit the input common-mode voltage, VCM (that is, the average voltage
between the inputs), at AINP or AINM from varying more than approximately 10% beyond the mid-scale input
value. As shown in Figure 40, the device has a unique common-mode voltage detection and rejection block that
does not have this restriction and thus allows VCM to be set to any value between 0 V and VREF without
degrading device performance.
REF
AINP
+
±
2
Binary
Search
Algorithm
(SAR)
+
±
I
N
T
E
R
F
A
C
E
AINM
Common Mode Voltage
Detection and Rejection
Block
Figure 40. Conceptual Diagram: True Differential Input Structure
Table 4 shows the full-scale input range of the device as a function of input common-mode voltage. The device
offers a maximum dynamic range for VCM = VREF / 2. The differential input with wide common-mode range allows
connecting differential signals from sensors without any signal conditioning.
Table 4. Full-Scale Input Range
ABSOLUTE INPUT RANGE
VCM
VCM < VREF / 2
VAINP
VAINN
0 to 2 × VCM
0 to 2 x VCM
FULL SCALE INPUT RANGE (VFS)
(–2 x VCM) to (2 x VCM)
VCM = VREF / 2
0 to VREF
0 to VREF
(–VREF) to (VREF)
VCM > VREF / 2
(2 × VCM – VREF) to VREF
(2 × VCM – VREF) to VREF
(–2 × (VCM – VREF)) to (2 × (VCM – VREF))
Figure 41 shows an equivalent circuit of the input sampling stage. The sampling switch is represented by a 96-Ω
resistance in series with the ideal switch. Refer to the ADC Input Driver section for more details on the
recommended driving circuits.
Device in Hold Mode
96
AINP
4 pF
55 pF
REF
4 pF
55 pF
96
GND
GND
AINN
Figure 41. Input Sampling Stage Equivalent Circuit
Figure 39 and Figure 41 illustrate electrostatic discharge (ESD) protection diodes to REF and GND from both
analog inputs. Make sure that these diodes do not turn on by keeping the analog inputs within the specified
range.
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REFERENCE
The device operates with an external reference voltage and switches binary-weighted capacitors onto the
reference terminal (REF pin) during the conversion process. The switching frequency is proportional to the
internal conversion clock frequency but the dynamic charge requirements are a function of the absolute value of
the input voltage and reference voltage. This dynamic load must be supported by a reference driver circuit
without degrading the noise and linearity performance of the device. During the acquisition process, the device
automatically powers down and does not take any dynamic current from the external reference source. The basic
circuit diagram for such a reference driver circuit for precision ADCs is shown in Figure 42. Refer to the ADC
Reference Driver section for more details on the application circuits.
RREF_FLT
Buffer
CREF_FLT
RBUF_FLT
Voltage
Reference
REF
CBUF_FLT
ADC
Figure 42. Reference Driver Schematic
CLOCK
The device uses an internal clock for conversion. Conversion duration may vary but is bounded by the minimum
and maximum value of tconv, as specified in the Timing Characteristics section. An external SCLK is only used for
a serial data read operation. Data are read after a conversion completes and when the device is in acquisition
phase for the next sample.
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ADC TRANSFER FUNCTION
The ADS8887 is a unipolar, differential input device. The device output is in twos compliment format.
Figure 43 shows ideal characteristics for the device. The full-scale range for the ADC input (AINP – AINN) is
equal to twice the reference input voltage to the ADC (2 × VREF). The LSB for the ADC is given by Equation 1.
1 LSB = [2 × (VREF / 218)]
(1)
ADC Code (Hex)
1FFFF
00000
3FFFF
20001
20000
±VREF
+ 1 LSB
±1 LSB
0
VREF
± 1 LSB
VIN
Differential Analog Input
(AINP AINN)
Figure 43. Differential Transfer Characteristics
DIGITAL INTERFACE
The ADS8887 is a low pin-count device. However, the device offers three different options for interfacing with the
digital host.
These options can be broadly classified as being either CS mode (in either a 3- or 4-wire interface) or daisychain mode. The device operates in CS mode if DIN is high at the CONVST rising edge. If DIN is low at the
CONVST rising edge, or if DIN and CONVST are connected together, the device operates in daisy-chain mode.
The 3-wire interface in CS mode is useful for applications that need galvanic isolation on-board. The 4-wire
interface in CS mode allows the user to sample the analog input independent of the serial interface timing and,
therefore, allows easier control of an individual device while having multiple, similar devices on-board. The daisychain mode is provided to hook multiple devices in a chain similar to a shift register and is useful in reducing
component count and the number of signal traces on the board.
CS Mode
CS mode is selected if DIN is high at the CONVST rising edge. There are two different interface options available
in this mode: 3-wire CS mode, and 4-wire CS mode. The following sections discuss these interface options in
detail.
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3-Wire CS Mode
This interface option is most useful when a single ADC is connected to an SPI-compatible digital host. In this
interface option, DIN can be connected to DVDD and CONVST functions as CS (as shown in Figure 44). As
shown in Figure 45, a CONVST rising edge forces DOUT to 3-state, samples the input signal, and causes the
device to enter a conversion phase. Conversion is done with the internal clock and continues regardless of the
state of CONVST. As a result, CONVST (functioning as CS) can be pulled low after the start of the conversion to
select other devices on the board. However, CONVST must return high before the minimum conversion time
(tconv-min) elapses and is held high until the maximum possible conversion time (tconv-max) elapses.
DVDD
DIN
CONVST
CNV
SCLK
CLK
DOUT
SDI
ADC
Digital Host
Figure 44. Connection Diagram: 3-Wire CS Mode (DIN = 1)
1/fsample
DIN = HIGH
CONVST = 1
CONVST
SCLK
1
DOUT
D17
2
3
D16
D15
œœ
16
17
18
D1
D0
œœ
tACQ
tconv-max
tconv-min
ADC
STATE
D2
œœ
Acquiring
Sample N
Conversion Result of Sample N Clocked-out
while Acquiring Sample N+1
Converting
Sample N
End-of-Conversion
Figure 45. Interface Timing Diagram: 3-Wire CS Mode (DIN = 1)
When conversion is complete, the device enters an acquisition phase and powers down. CONVST (functioning
as CS) can be brought low after the maximum conversion time (tconv-max) elapses. On the CONVST falling edge,
DOUT comes out of 3-state and the device outputs the MSB of the data. The lower data bits are output on
subsequent SCLK falling edges. Data are valid on both SCLK edges. Data are valid on both edges of SCLK and
can be captured on either edge. However, a digital host capturing data on the SCLK falling edge can achieve a
faster reading rate (provided th_CK_DO is acceptable). DOUT goes to 3-state after the 18th SCLK falling edge or
when CONVST goes high, whichever occurs first.
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4-Wire CS Mode
This interface option is useful when one or more ADCs are connected to an SPI-compatible digital host.
Figure 46 shows the connection diagram for single ADC, Figure 48 shows the connection diagram for two ADCs.
CS
CNV
DIN
CONVST
DOUT
SDI
SCLK
CLK
ADC
Digital Host
Figure 46. Connection Diagram: Single ADC with 4-Wire CS Mode
In this interface option, DIN is controlled by the digital host and functions as CS. As shown in Figure 47, with DIN
high, a CONVST rising edge selects CS mode, forces DOUT to 3-state, samples the input signal, and causes the
device to enter a conversion phase. In this interface option, CONVST must be held at a high level from the start
of the conversion until all data bits are read. Conversion is done with the internal clock and continues regardless
of the state of DIN. As a result, DIN (functioning as CS) can be pulled low to select other devices on the board.
However, DIN must be pulled high before the minimum conversion time (tconv-min) elapses and remains high until
the maximum possible conversion time (tconv-max) elapses.
1/fsample
tconv-max
tACQ
tconv-min
CONVST
DIN = 1
DIN
SCLK
1
2
DOUT
D17
D16
œœ
17
18
œœ
ADC
STATE
End-ofConversion
Acquiring
Sample N
D1
D0
œœ
Read Sample N
Converting
Sample N
Acquiring Sample N+1
Figure 47. Interface Timing Diagram: Single ADC with 4-Wire CS Mode
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When conversion is complete, the device enters acquisition phase and powers down. DIN (functioning as CS)
can be brought low after the maximum conversion time (tconv-max) elapses. On the DIN falling edge, DOUT comes
out of 3-state and the device outputs the MSB of the data. The lower data bits are output on subsequent SCLK
falling edges. Data are valid on both SCLK edges. Data are valid on both edges of SCLK and can be captured
on either edge. However, a digital host capturing data on the SCLK falling edge can achieve a faster reading rate
(provided th_CK_DO is acceptable). DOUT goes to 3-state after the 18th SCLK falling edge or when DIN goes high,
whichever occurs first.
As shown in Figure 48, multiple devices can be hooked together on the same data bus. In this case, as shown in
Figure 49, the DIN of the second device (functioning as CS for the second device) can go low after the first
device data are read and the DOUT of the first device is in 3-state.
Care must be taken so that CONVST and DIN are not both low together at any time during the cycle.
CS1
CS2
CNV
CONVST
DIN
CONVST
DIN
DOUT
DOUT
SCLK
SDI
SCLK
CLK
ADC #1
ADC #2
Digital Host
Figure 48. Connection Diagram: Two ADCs with 4-Wire CS Mode
1/fsample
tconv-max
tACQ
tconv-min
CONVST
DIN = 1
DIN
(ADC 1)
DIN = 1
DIN
(ADC 2)
SCLK
1
2
DOUT
D17
D16
œœ
17
18
19
20
D0
D17
D16
ADC
STATE
Acquiring
Sample N
Converting
Sample N
36
œœ
œœ
End-ofConversion
œœ
35
D1
D1
D0
œœ
œœ
Read Sample N
ADC 1
Read Sample N
ADC 2
Acquiring Sample N+1
Figure 49. Interface Timing Diagram: Two ADCs with 4-Wire CS Mode
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DAISY-CHAIN MODE
Daisy-chain mode is selected if DIN is low at the time of a CONVST rising edge or if DIN and CONVST are
connected together. The following section discusses this interface option in detail.
Daisy-Chain Mode
This interface option is most useful in applications where multiple ADC devices are used but the digital host has
limited interfacing capability. Figure 50 shows a connection diagram with N ADCs connected in the daisy-chain.
The CONVST pins of all ADCs in the chain are connected together and are controlled by a single pin of the
digital host. Similarly, the SCLK pins of all ADCs in the chain are connected together and are controlled by a
single pin of the digital host. The DIN pin for ADC 1 is connected to GND. The DOUT pin of ADC 1 is connected
to the DIN pin of ADC 2, and so on. The DOUT pin of the last ADC in the chain (ADC N) is connected to the SDI
pin of the digital host.
CNV
CONVST
DIN
DOUT
SCLK
CONVST
DIN
CONVST
DOUT
}
DIN
SCLK
CONVST
DOUT
DIN
SCLK
SDI
DOUT
SCLK
CLK
ADC 1
ADC 2
}
ADC N
ADC N1
Digital Host
Figure 50. Connection Diagram: Daisy-Chain Mode (DIN = 0)
As shown in Figure 51, the device DOUT pin is driven low when DIN and CONVST are low together. With DIN
low, a CONVST rising edge selects daisy-chain mode, samples the analog input, and causes the device to enter
a conversion phase. In this interface option, CONVST must remain high from the start of the conversion until all
data bits are read. When started, the conversion continues regardless of the state of SCLK, however SCLK must
be low at the CONVST rising edge.
tconv-min
1/fsample
tconv-max
tACQ
CONVST
SCLK
1
2
œœ
17
18
19
20
D17
D16
œœ
35
36
DIN 1 = LOW
œœ
DOUT 1
DIN 2
D17
D16
D1
D0
D1
D0
œœ
œœ
D17
DOUT 2
œœ
œœ
End-ofConversion
ADC
STATE
D16
Acquiring
Sample N
Converting
Sample N
D1
D0
œœ
Read Sample N
ADC 2
Read Sample N
ADC 1
Acquiring Sample N+1
Figure 51. Interface Timing Diagram: For Two devices in Daisy-Chain Mode
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At the end of conversion, every ADC in the chain loads its own conversion result into the internal, 18-bit, shift
register and also outputs the MSB bit of this conversion result on its own DOUT pin. All ADCs enter an
acquisition phase and power-down. On every subsequent SCLK falling edge, the internal shift register of each
ADC latches the data available on its DIN pin and shifts out the next bit of data on its DOUT pin. Therefore, the
digital host receives the data of ADC N, followed by the data of ADC N–1, and so on (in MSB-first fashion). A
total of 18 x N SCLK falling edges are required to capture the outputs of all N devices in the chain. Data are valid
on both SCLK edges. Data are valid on both edges of SCLK and can be captured on either edge. However, a
digital host capturing data on the SCLK falling edge can achieve a faster reading rate (provided th_CK_DO is
acceptable).
POWER SUPPLY
The device has two separate power supplies: AVDD and DVDD. The internal circuits of the device operate on
AVDD; DVDD is used for the digital interface. AVDD and DVDD can be independently set to any value within the
permissible range.
Decouple the AVDD and DVDD pins with GND, using individual 1-µF decoupling capacitors placed in close
proximity to the pin, as shown in Figure 52.
Digital
Supply
REF
Analog
Supply
AVDD
1 µF
DVDD
DIN
AINP
SCLK
AINN
DOUT
GND
CONVST
1 µF
Figure 52. Supply Decoupling
POWER SAVING
The device has an auto power-down feature that powers down the internal circuitry at the end of every
conversion. Referring to Figure 53, the input signal is acquired on the sampling capacitors when the device is in
a power-down state (tacq); at the same time, the result for the previous conversion is available for reading. The
device powers up on the start of the next conversion. During conversion phase (tconv), the device also consumes
current from the reference source (connected to pin REF).
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tTHROUGHPUT
Device Phase
tCONV
tACQ
œœ
IREF
tACQ
~50000X
~50000X
œœ
IAVDD
tCONV
2 * tTHROUGHPUT
~1200X
~1200X
~2X
IAVG(AVDD+REF)
Figure 53. Power Scaling with Throughput
The conversion time, tconv, is independent of the SCLK frequency. When operating the device at speeds lower
than the maximum rated throughput, the conversion time, tconv, does not change; the device spends more time in
power-down state. Therefore, as shown in Figure 54, the device power consumption from the AVDD supply and
the external reference source is directly proportional to the speed of operation. Extremely low AVDD power-down
current (50 nA, typical) and extremely low external reference leakage current (250 nA, typical), make this device
ideal for very low throughput applications (such as pulsed measurements).
Analog Supply Current (mA)
0.4
0.3
0.2
0.1
0
0
20
40
60
Throughput (kSPS)
80
100
C00
Figure 54. Power Scaling with Throughput
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APPLICATION INFORMATION
The two primary circuits required to maximize the performance of a high-precision, successive approximation
register (SAR), analog-to-digital converter (ADC) are the input driver and the reference driver circuits. This
section details some general principles for designing these circuits, followed by some application circuits
designed using the ADS8887.
ADC REFERENCE DRIVER
The external reference source to the ADS8887 must provide low-drift and very accurate voltage for the ADC
reference input and support the dynamic charge requirements without affecting the noise and linearity
performance of the device. The output broadband noise of most references can be in the order of a few 100
μVRMS. Therefore, to prevent any degradation in the noise performance of the ADC, the output of the voltage
reference must be appropriately filtered by using a low-pass filter with a cutoff frequency of a few hundred Hertz.
After band-limiting the noise of the reference circuit, the next important step is to design a reference buffer that
can drive the dynamic load posed by the reference input of the ADC. The reference buffer must regulate the
voltage at the reference pin such that the value of VREF stays within the 1-LSB error at the start of each
conversion. This condition necessitates the use of a large capacitor, CBUF_FLT (refer to Figure 42) for regulating
the voltage at the reference input of the ADC. The amplifier selected to drive the reference pin should have an
extremely low offset and temperature drift with a low output impedance to drive the capacitor at the ADC
reference pin without any stability issues.
Reference Driver Circuit for VREF = 4.5 V
The application circuit in Figure 55 shows the schematic of a complete reference driver circuit that generates a
voltage of 4.5 V dc using a single 5-V supply. This circuit is suitable to drive the reference of the ADS8887 at
higher sampling rates up to 100 kSPS. The 4.5-V reference voltage in this design is generated by the highprecision, low-noise REF5045 circuit. The output broadband noise of the reference is heavily filtered by a lowpass filter with a 3-dB cutoff frequency of 160 Hz.
20 k
1 µF
-
AVDD
REF5045
Vin
+
Vout
1k
+
1k
+
OPA333
AVDD
+
THS4281
1 µF
0.2
AVDD
Temp
Gnd
1 µF
AVDD
REF AVDD
V
+
AINP
10 µF
Trim
CONVST
ADS8887
AINM
GND
1 µF
Figure 55. Schematic of Reference Driver Circuit with VREF = 4.5 V
The reference buffer is designed with the THS4281 and OPA333 in a composite architecture to achieve superior
dc and ac performance at a reduced power consumption, compared to using a single high-performance amplifier.
The THS4281 is a high-bandwidth amplifier with a very low output impedance of 1 Ω at a frequency of 1 MHz.
The low output impedance makes the THS4281 a good choice for driving a high capacitive load to regulate the
voltage at the reference input of the ADC. The high offset and drift specifications of the THS4281 are corrected
by using a dc-correcting amplifier (OPA333) inside the feedback loop. The composite scheme inherits the
extremely low offset and temperature drift specifications of the OPA333.
For step-by-step design procedure, circuit schematics, bill of materials, PCB files, simulation results, and test
results, using a similar device, refer to 18-Bit Data Acquisition (DAQ) Block Optimized for 1-μs Full-Scale Step
Response (SLAU512).
26
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Reference Driver Circuit for VREF = 2.5 V in Ultralow Power, Lower Throughput Applications
The application circuit in shows the schematic of a complete reference driver circuit that generates a voltage of
2.5 V dc using a single 3.3-V supply. This ultralow power reference block is suitable to drive the ADS8887 for
power-sensitive applications at a relatively lower throughput. This design uses the high-precision REF3325 circuit
that provides an accurate 2.5-V reference voltage at an extremely low quiescent current of 5 µA. The output
broadband noise of the reference is heavily filtered by a low-pass filter with a 3-dB cutoff frequency of 16 Hz.
The reference buffer is designed using the low-power OPA333 that can operate from a 3.3-V supply at an
extremely low quiescent current of 28 µA. The wideband noise contribution from the amplifier is limited by a lowpass filter of a cutoff frequency equal to 1.5 kHz, formed by a 4.7-Ω resistor in combination with a 22-µF
capacitor. The 4.7-Ω series resistor creates an additional drop in the reference voltage, which is corrected by a
dual-feedback configuration.
For step-by-step design procedure, circuit schematics, bill of materials, PCB files, simulation results, and test
results, using a similar device, refer to 18-Bit, 10kSPS Data Acquisition (DAQ) Block Optimized for Ultra Low
Power < 1mW (SLAU514).
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ADC INPUT DRIVER
The input driver circuit for a high-precision ADC mainly consists of two parts: a driving amplifier and a fly-wheel
RC filter. The amplifier is used for signal conditioning of the input voltage and its low output impedance provides
a buffer between the signal source and the switched capacitor inputs of the ADC. The RC filter helps attenuate
the sampling charge injection from the switched-capacitor input stage of the ADC and functions as an antialiasing
filter to band-limit the wideband noise contributed by the front-end circuit. Careful design of the front-end circuit is
critical to meet the linearity and noise performance of a high-precision, 18-bit ADC such as the ADS8887.
Input Amplifier Selection
Selection criteria for the input amplifiers is highly dependent on the input signal type as well as the performance
goals of the data acquisition system. Some key amplifier specifications to consider while selecting an appropriate
amplifier to drive the inputs of the ADC are:
• Small-signal bandwidth. Select the small-signal bandwidth of the input amplifiers to be as high as possible
after meeting the power budget of the system. Higher bandwidth reduces the closed-loop output impedance
of the amplifier, thus allowing the amplifier to more easily drive the low cutoff frequency RC filter (refer to the
Antialiasing Filter section) at the inputs of the ADC. Higher bandwidth also minimizes the harmonic distortion
at higher input frequencies. In order to maintain the overall stability of the input driver circuit, the amplifier
bandwidth should be selected as described in Equation 2:
•
§
·
1
¸¸
Unity Gain Bandwidth t 4 u ¨¨
© 2S u RFLT u CFLT ¹
(2)
Noise. Noise contribution of the front-end amplifiers should be as low as possible to prevent any degradation
in SNR performance of the system. As a rule of thumb, to ensure that the noise performance of the data
acquisition system is not limited by the front-end circuit, the total noise contribution from the front-end circuit
should be kept below 20% of the input-referred noise of the ADC. Noise from the input driver circuit is bandlimited by designing a low cutoff frequency RC filter, as explained in Equation 3.
2
§ V 1 _ AM P_ PP ·
S
¨
¸
NG u 2 u ¨ f
en2 _ RM S u u f3dB
¸
6
.
6
2
¨
¸
©
¹
d
§ SNR dB ·
¸
20
¹
¨
1 VREF
u
u 10 ©
5
2
where:
•
•
•
•
•
V1 / f_AMP_PP is the peak-to-peak flicker noise in µVRMS,
en_RMS is the amplifier broadband noise density in nV/√Hz,
f–3dB is the 3-dB bandwidth of the RC filter, and
NG is the noise gain of the front-end circuit, which is equal to '1' in a buffer configuration.
THD AMP d THD ADC 10 dB
•
28
(3)
Distortion. Both the ADC and the input driver introduce nonlinearity in a data acquisition block. As a rule of
thumb, to ensure that the distortion performance of the data acquisition system is not limited by the front-end
circuit, the distortion of the input driver should be at least 10 dB lower than the distortion of the ADC, as
shown in Equation 4.
(4)
Settling Time. For dc signals with fast transients that are common in a multiplexed application, the input signal
must settle within an 18-bit accuracy at the device inputs during the acquisition time window. This condition is
critical to maintain the overall linearity performance of the ADC. Typically, the amplifier data sheets specify
the output settling performance only up to 0.1% to 0.001%, which may not be sufficient for the desired 18-bit
accuracy. Therefore, the settling behavior of the input driver should always be verified by TINA™-SPICE
simulations before selecting the amplifier.
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Antialiasing Filter
Converting analog-to-digital signals requires sampling an input signal at a constant rate. Any higher frequency
content in the input signal beyond half the sampling frequency is digitized and folded back into the low-frequency
spectrum. This process is called aliasing. Therefore, an analog, antialiasing filter must be used to remove the
harmonic content from the input signal before being sampled by the ADC. An antialiasing filter is designed as a
low-pass, RC filter, for which the 3-dB bandwidth is optimized based on specific application requirements. For dc
signals with fast transients (including multiplexed input signals), a high-bandwidth filter is designed to allow
accurately settling the signal at the inputs of the ADC during the small acquisition time window. For ac signals,
the filter bandwidth should be kept low to band-limit the noise fed into the input of the ADC, thereby increasing
the signal-to-noise ratio (SNR) of the system.
Besides filtering the noise from the front-end drive circuitry, the RC filter also helps attenuate the sampling
charge injection from the switched-capacitor input stage of the ADC. A differential capacitor, CFLT, is connected
across the inputs of the ADC (as shown in Figure 56). This capacitor helps reduce the sampling charge injection
and provides a charge bucket to quickly charge the internal sample-and-hold capacitors during the acquisition
process. As a rule of thumb, the value of this capacitor should be at least 10 times the specified value of the
ADC sampling capacitance. For the ADS8887, the input sampling capacitance is equal to 59 pF, thus the value
of CFLT should be greater than 590 pF. The capacitor should be a COG- or NPO-type because these capacitor
types have a high-Q, low-temperature coefficient, and stable electrical characteristics under varying voltages,
frequency, and time.
RFLT ”22
f 3 dB
2S u R FLT
1
R FLT u C FLT
CFLT •590 pF
V
AINP
+
ADS8887
AINM
GND
RFLT ”22
Figure 56. Antialiasing Filter
Note that driving capacitive loads can degrade the phase margin of the input amplifiers, thus making the amplifier
marginally unstable. To avoid amplifier stability issues, series isolation resistors (RFLT) are used at the output of
the amplifiers. A higher value of RFLT is helpful from the amplifier stability perspective, but adds distortion as a
result of interactions with the nonlinear input impedance of the ADC. Distortion increases with source impedance,
input signal frequency, and input signal amplitude. Therefore, the selection of RFLT requires balancing the stability
and distortion of the design. For the ADS8887, TI recommends limiting the value of RFLT to a maximum of 22 Ω
in order to avoid any significant degradation in linearity performance. The tolerance of the selected resistors can
be chosen as 1% because the use of a differential capacitor at the input balances the effects resulting from any
resistor mismatch.
The input amplifier bandwidth should be much higher than the cutoff frequency of the antialiasing filter. TI
strongly recommends performing a SPICE simulation to confirm that the amplifier has more than 40° phase
margin with the selected filter. Simulation is critical because even with high-bandwidth amplifiers, some amplifiers
might require more bandwidth than others to drive similar filters. If an amplifier has less than a 40° phase margin
with 22-Ω resistors, using a different amplifier with higher bandwidth or reducing the filter cutoff frequency with a
larger differential capacitor is advisable.
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APPLICATION CIRCUIT EXAMPLES
This section describes some common application circuits using the ADS8887. These data acquisition (DAQ)
blocks are optimized for specific input types and performance requirements of the system. For simplicity, powersupply decoupling capacitors are not shown in these circuit diagrams; refer to the Power Supply section for
suggested guidelines.
DAQ Circuit for a 10-µs, Full-Scale Step Response
The application circuit shown in Figure 57 is optimized for using the ADS8887 at the maximum-specified
throughput of 100 kSPS for a full-scale step input voltage. Such step input signals are common in multiplexed
applications when switching between different channels. In a worst-case scenario, one channel is at the negative
full-scale (NFS) and the other channel is at the positive full-scale (PFS) voltage, in which case the step size is
the full-scale range (FSR) of the ADC when the MUX channel is switched.
In such applications, the primary design requirement is to ensure that the full-scale step input signal settles to 18bit accuracy at the ADC inputs. This condition is critical to achieve the excellent linearity specifications of the
ADC. Therefore, the bandwidth of the antialiasing RC filter should be large enough to allow optimal settling of the
input signal during the ADC acquisition time. The filter capacitor helps reduce the sampling charge injection at
the ADC inputs, but degrades the phase margin of the driving amplifier, thereby leading to stability issues.
Amplifier stability is maintained by the series isolation resistor. Therefore, the component values of the
antialiasing filter should be carefully selected to meet the settling requirements of the system as well as to
maintain the stability of the input driving amplifiers.
REFERENCE DRIVE CIRCUIT
20 k
1 µF
THS4281
+
-
1k
+
AVDD
OPA333
0.2
1 µF
+
1k
+
REF5045
Vout
Vin
AVDD
AVDD
10 µF
Temp
1 µF
Trim
Gnd
1 µF
+
AVDD
VIN+
+
AVDD
+
OPA364
22
REF AVDD
VCM
V
+
AINP
470 pF
CONVST
ADS8887
AINM
GND
+
VIN-
-
CONVST
22
OPA364
+
+
AVDD
INPUT DRIVER
18-Bit 100-kSPS SAR
ADC
Figure 57. DAQ Circuit for 10-µs, Full-Scale Step Response
For step-by-step design procedure, circuit schematics, bill of materials, PCB files, simulation results, and test
results, using a similar device, refer to 18-Bit Data Acquisition (DAQ) Block Optimized for 1-μs Full-Scale Step
Response (SLAU512).
30
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Low-Power DAQ Circuit for Excellent Dynamic Performance at 100 kSPS
The application circuit shown in Figure 58 is optimized for using the ADS8887 at the maximum specified
throughput of 100 kSPS for a full-scale sinusoidal signal of 1-kHz frequency. This circuit achieves excellent
dynamic performance for the lowest power consumption. The differential ac input signal is processed through
low-noise and low-power amplifiers configured as unity-gain buffers and a low-pass, RC filter before being fed
into the ADC.
In such applications, the input driver must be low in power and noise as well as able to support rail-to-rail input
and output swing with a single supply. A high amplifier bandwidth is also preferred to help attenuate highfrequency distortion. However, oftentimes bandwidth and noise are traded off with the power consumption of the
amplifier. This circuit uses the OPA313 as the front-end driving amplifier because this device offers relatively low
noise density at ultra-low quiescent currents of 50 µA per channel.
The noise contribution from the front-end amplifier is band-limited by the 3-dB bandwidth of the RC filter, which is
designed to be 30 kHz in this application. To maintain good phase margin, RFLT is selected as 47 Ω instead of
the 22-Ω recommended maximum. The result is a slight degradation in linearity, which is a common tradeoff for
very low-power applications.
REFERENCE DRIVE CIRCUIT
20 k
1 µF
THS4281
+
-
1k
+
AVDD
OPA333
0.2
1 µF
+
1k
+
REF5045
Vout
Vin
AVDD
AVDD
10 µF
Temp
1 µF
Trim
Gnd
1 µF
+
AVDD
VIN+
+
AVDD
+
OPA313
47
REF AVDD
VCM
V
+
AINP
56 nF
CONVST
ADS8887
AINM
GND
+
VIN-
-
CONVST
47
OPA313
+
+
AVDD
INPUT DRIVER
18-Bit 100-kSPS SAR
ADC
Figure 58. DAQ Circuit for Lowest Power and Excellent Dynamic Performance at 100 kSPS
For step-by-step design procedure, circuit schematics, bill of materials, PCB files, simulation results, and test
results, using a similar device, refer to 18-Bit, 1-MSPS Data Acquisition (DAQ) Block Optimized for Lowest
Power (SLAU513).
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DAQ Circuit for Lowest Distortion and Noise Performance at 100 kSPS
This section describes two application circuits (Figure 59 and Figure 60) that are optimized for using the
ADS8887 with lowest distortion and noise performance at a throughput of 100 kSPS. In both applications, the
input signal is processed through a high-bandwidth, low-distortion, fully-differential amplifier (FDA) designed in an
inverting gain configuration and a low-pass RC filter before being fed into the ADC.
As a rule of thumb, the distortion from the input driver should be at least 10 dB less than the ADC distortion. The
distortion resulting from variation in the common-mode signal is eliminated by using the FDA in an inverting gain
configuration that establishes a fixed common-mode level for the circuit. This configuration also eliminates the
requirement of a rail-to-rail swing at the amplifier input. Therefore, these circuits use the low-power THS4521 as
an input driver, which provides exceptional ac performance because of its extremely low-distortion and highbandwidth specifications.
In addition, the components of the antialiasing filter are such that the noise from the front-end circuit is kept low
without adding distortion to the input signal.
Differential Input Configuration
The circuit in Figure 59 shows a fully-differential DAQ block optimized for low distortion and noise using the
THS4521 and ADS8887. This front-end circuit configuration requires a differential signal at the input of the FDA
and provides a differential output to drive the ADC inputs. The common-mode voltage of the input signal provided
to the ADC is set by the VOCM pin of the THS4521 (not shown in Figure 59). To use the complete dynamic range
of the ADC, VOCM can be set to VREF / 2 by using a simple resistive divider. However, note that the ADS8887
allows the common-mode input voltage (VCM) to be set to any value in the range of 0 V to VREF.
REFERENCE DRIVE CIRCUIT
20 k 1 µF
THS4281
+
0.2
-
1k +
AVDD
OPA333
1 µF
+
1k +
REF5045
Vout
Vin
AVDD
AVDD
10 µF
Temp
1 µF
Trim
Gnd
1 µF
1K 1K AVDD
AVDD
VIN+
VCM
+
10
+
10 nF
REF AVDD
V
+
CONVST
ADS8887
AINM
10
GND
CONVST
+
VIN-
AINP
THS4521
+
-
1K 1K INPUT DRIVER
18-Bit 100-kSPS
SAR ADC
Figure 59. Differential Input DAQ Circuit for Lowest Distortion and Noise at 100 kSPS
For step-by-step design procedure, circuit schematics, bill of materials, PCB files, simulation results, and test
results, using a similar device, refer to 18-Bit, 1-MSPS Data Acquisition (DAQ) Block Optimized for Lowest
Distortion and Noise (SLAU515).
32
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Single-Ended to Differential Configuration
The circuit in Figure 60 shows a single-ended to differential DAQ block optimized for low distortion and noise
using the THS4521 and the ADS8887. This front-end circuit configuration requires a single-ended ac signal at
the input of the FDA and provides a fully-differential output to drive the ADC inputs. The common-mode voltage
of the input signal provided to the ADC is set by the VOCM pin of the THS4521 (not shown in Figure 60). To use
the complete dynamic range of the ADC, VOCM can be set to VREF / 2 by using a simple resistive divider.
However, note that the ADS8887 allows the common-mode input voltage (VCM) to be set to any value in the
range of 0 V to VREF.
REFERENCE DRIVE CIRCUIT
20 k
1 µF
THS4281
+
-
1k
+
AVDD
OPA333
0.2
1 µF
+
1k
+
REF5045
Vout
Vin
AVDD
AVDD
10 µF
Temp
1 µF
Trim
Gnd
1 µF
1K
1K
AVDD
AVDD
VIN+
+
10
+
REF AVDD
10 nF
CONVST
ADS8887
AINM
10
1K
V
+
AINP
THS4521
+
-
GND
1K
INPUT DRIVER
CONVST
18-Bit 100-kSPS
SAR ADC
Figure 60. Single-Ended to Differential DAQ Circuit for Lowest Distortion and Noise at 100 kSPS
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Ultralow-Power DAQ Circuit at 10 kSPS
The data acquisition circuit shown in Figure 61 is optimized for using the ADS8887 at a reduced throughput of 10
kSPS with ultralow-power consumption (< 1 mW) targeted at portable and battery-powered applications.
In order to save power, this circuit is operated on a single 3.3-V supply. The circuit uses extremely low-power,
dual amplifiers (such as the OPA2333) with a maximum quiescent current of 28 µA per channel to drive the ADC
inputs. The input amplifiers are configured in a modified unity-gain buffer configuration. The filter capacitor at the
ADC inputs attenuates the sampling charge-injection noise from the ADC but effects the stability of the input
amplifiers by degrading the phase margin. This attenuation requires a series isolation resistor to maintain
amplifier stability. The value of the series resistor is directly proportional to the open-loop output impedance of
the driving amplifier to maintain stability, which is high (in the order of kΩ) in the case of low-power amplifiers
such as the OPA333. Therefore, a high value of 1 kΩ is selected for the series resistor at the ADC inputs.
However, this series resistor creates an additional voltage drop in the signal path, thereby leading to linearity and
distortion issues. The dual-feedback configuration used in Figure 61 corrects for this additional voltage drop and
maintains system performance at ultralow-power consumption.
1k REFERENCE DRIVE CIRCUIT
1 µF
REF3325
IN
AVDD
4.7
OPA333
10 k + +
OUT
GND
22 µF
1 µF
AVDD
10 k AVDD
10 nF
REF AVDD
AINP
CONVST
ADS8887
AINM
-
1k GND
CONVST
+
OPA333
+ +
VIN+
VCM
AVDD
4.7 nF
VIN+
+ +
1k OPA333
ADS8887 running at 10 kSPS
-
10 nF
10 k INPUT DRIVER
Figure 61. Ultralow-Power DAQ Circuit at 10 kSPS
For step-by-step design procedure, circuit schematics, bill of materials, PCB files, simulation results, and test
results, using a similar device, refer to 18-Bit, 10kSPS Data Acquisition (DAQ) Block Optimized for Ultra Low
Power < 1mW (SLAU514).
34
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SBAS613A – MAY 2013 – REVISED DECEMBER 2013
REVISION HISTORY
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Original (May 2013) to Revision A
Page
•
Changed Wide Common-Mode Voltage Range Features bullet .......................................................................................... 1
•
Changed sub-bullets of AC and DC Performance Features bullet ....................................................................................... 1
•
Changed Full-Scale Step Settling Features bullet ................................................................................................................ 1
•
Deleted last two Applications bullets .................................................................................................................................... 1
•
Changed Description section ................................................................................................................................................ 1
•
Changed front page graphic ................................................................................................................................................. 1
•
Added Family Information, Absolute Maximum Ratings, and Thermal Information tables ................................................... 2
•
Added Electrical Characteristics table .................................................................................................................................. 3
•
Added Timing Characteristics section .................................................................................................................................. 5
•
Added Pin Configurations section ......................................................................................................................................... 9
•
Added Typical Characteristics section ................................................................................................................................ 10
•
Added Overview section ..................................................................................................................................................... 16
•
Added Application Information section ............................................................................................................................... 26
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PACKAGE OPTION ADDENDUM
www.ti.com
15-Jan-2014
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
Lead/Ball Finish
MSL Peak Temp
(2)
(6)
(3)
Op Temp (°C)
Device Marking
(4/5)
ADS8887IDGS
ACTIVE
VSSOP
DGS
10
80
Green (RoHS
& no Sb/Br)
CU NIPDAU
Level-2-260C-1 YEAR
-40 to 85
8887
ADS8887IDGSR
ACTIVE
VSSOP
DGS
10
2500
Green (RoHS
& no Sb/Br)
CU NIPDAU
Level-2-260C-1 YEAR
-40 to 85
8887
ADS8887IDRCR
ACTIVE
SON
DRC
10
3000
Green (RoHS
& no Sb/Br)
CU NIPDAU
Level-2-260C-1 YEAR
-40 to 85
8887
ADS8887IDRCT
ACTIVE
SON
DRC
10
250
Green (RoHS
& no Sb/Br)
CU NIPDAU
Level-2-260C-1 YEAR
-40 to 85
8887
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability
information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that
lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between
the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight
in homogeneous material)
(3)
MSL, Peak Temp. - The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
(4)
There may be additional marking, which relates to the logo, the lot trace code information, or the environmental category on the device.
(5)
Multiple Device Markings will be inside parentheses. Only one Device Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation
of the previous line and the two combined represent the entire Device Marking for that device.
(6)
Lead/Ball Finish - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead/Ball Finish values may wrap to two lines if the finish
value exceeds the maximum column width.
Addendum-Page 1
Samples
PACKAGE OPTION ADDENDUM
www.ti.com
15-Jan-2014
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Addendum-Page 2
PACKAGE MATERIALS INFORMATION
www.ti.com
7-Jan-2014
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device
ADS8887IDGSR
Package Package Pins
Type Drawing
VSSOP
DGS
10
SPQ
Reel
Reel
A0
Diameter Width (mm)
(mm) W1 (mm)
2500
330.0
12.4
Pack Materials-Page 1
5.3
B0
(mm)
K0
(mm)
P1
(mm)
3.4
1.4
8.0
W
Pin1
(mm) Quadrant
12.0
Q1
PACKAGE MATERIALS INFORMATION
www.ti.com
7-Jan-2014
*All dimensions are nominal
Device
Package Type
Package Drawing
Pins
SPQ
Length (mm)
Width (mm)
Height (mm)
ADS8887IDGSR
VSSOP
DGS
10
2500
367.0
367.0
35.0
Pack Materials-Page 2
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