16-Bit, 1.5 LSB INL, 250 kSPS PulSAR
Differential ADC in MSOP
AD7687
Data Sheet
FEATURES
TYPICAL APPLICATION CIRCUIT
0.5V TO 5V
VREF
0
IN+
IN–
VREF
2.5 TO 5V
REF VDD VIO
SDI
AD7687
SCK
SDO
GND
1.8V TO VDD
3- OR 4-WIRE INTERFACE
(SPI, DAISY CHAIN, CS)
CNV
0
02972-002
16-bit resolution with no missing codes
Throughput: 250 kSPS
INL: ±0.4 LSB typ, ±1.5 LSB max (±23 ppm of FSR)
Dynamic range: 96.5 dB
SNR: 95.5 dB at 20 kHz
THD: −118 dB at 20 kHz
True differential analog input range
±VREF
0 V to VREF with VREF up to VDD on both inputs
No pipeline delay
Single-supply 2.3 V to 5.5 V operation with
1.8 V/2.5 V/3 V/5 V logic interface
Proprietary serial interface: SPI/QSPI™/MICROWIRE/DSP
compatible
Daisy-chain multiple ADCs and BUSY indicator
Power dissipation
1.35 mW at 2.5 V/100 kSPS, 4 mW at 5 V/100 kSPS, and
1.4 μW at 2.5 V/100 SPS
Standby current: 1 nA
10-lead MSOP and 10-lead, 3 mm × 3 mm LFCSP
Pin-for-pin compatible with AD7685, AD7686, and AD7688
Figure 1.
APPLICATIONS
Battery-powered equipment
Data acquisitions
Instrumentation
Medical instruments
Process controls
GENERAL DESCRIPTION
The AD76871 is a 16-bit, charge redistribution, successive
approximation, analog-to-digital converter (ADC) that operates
from a single power supply, VDD, between 2.3 V to 5.5 V. It
contains a low power, high speed, 16-bit sampling ADC with no
missing codes, an internal conversion clock, and a versatile
serial interface port. The device also contains a low noise, wide
bandwidth, short aperture delay track-and-hold circuit. On the
CNV rising edge, the AD7687 the samples the voltage difference
between IN+ and IN− pins, which can range from −VREF to +VREF.
The reference voltage, VREF, is applied externally and can be set
up to the supply voltage.
The power consumption of the device scales linearly with
throughput.
1
The SPI-compatible serial interface also features the ability to
daisy-chain several ADCs on a single 3-wire bus and provides
an optional BUSY indicator by means of the SDI pin. It is
compatible with 1.8 V, 2.5 V, 3 V, or 5 V logic using the separate
supply VIO.
The AD7687 comes in a 10-lead MSOP or a 10-lead LFCSP
with operation specified from −40°C to +85°C.
Table 1. MSOP, LFCSP/SOT-23 16-Bit PulSAR® ADC
Type
True Differential
Pseudo
Differential/Unipolar
Unipolar
100 kSPS
AD7684
AD7683
250 kSPS
AD7687
AD7685
AD7694
500 kSPS
AD7688
AD7686
AD7680
Protected by U.S. Patent 6,703,961.
Rev. E
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Technical Support
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AD7687
Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1
Driver Amplifier Choice ........................................................... 17
Applications ....................................................................................... 1
Single-to-Differential Driver .................................................... 17
Typical Application Circuit ............................................................. 1
Voltage Reference Input ............................................................ 17
General Description ......................................................................... 1
Power Supply............................................................................... 17
Revision History ............................................................................... 3
Supplying the ADC from the Reference.................................. 18
Specifications..................................................................................... 4
Digital Interface .......................................................................... 18
Timing Specifications .................................................................. 6
CS Mode, 3-Wire Without BUSY Indicator ........................... 19
Absolute Maximum Ratings............................................................ 8
CS Mode, 3-Wire with BUSY Indicator .................................. 20
Thermal Resistance ...................................................................... 8
CS Mode, 4-Wire Without BUSY Indicator ........................... 21
ESD Caution .................................................................................. 8
CS Mode, 4-Wire with BUSY Indicator .................................. 22
Pin Configurations and Function Descriptions ........................... 9
Chain Mode Without BUSY Indicator .................................... 23
Terminology .................................................................................... 10
Chain Mode with BUSY Indicator ........................................... 24
Typical Performance Characteristics ........................................... 11
Applications Information .............................................................. 25
Theory of Operation ...................................................................... 14
Layout .......................................................................................... 25
Circuit Information .................................................................... 14
Evaluating the Performance of the AD7687............................... 25
Converter Operation .................................................................. 14
Outline Dimensions ....................................................................... 26
Typical Connection Diagram ................................................... 15
Ordering Guide .......................................................................... 26
Analog Input ............................................................................... 16
Rev. E | Page 2 of 26
Data Sheet
AD7687
REVISION HISTORY
12/15—Rev. D to Rev. E
Deleted Figure 1; Renumbered Sequentially ................................. 1
Changes to Features Section and General Description Section ....... 1
Change to Signal-to-(Noise + Distortion) Parameter, Table 2 ......... 4
Added Timing Diagrams Section.................................................... 7
Moved Figure 2 and Figure 3 ........................................................... 7
Changes to Table 8 ............................................................................ 9
Changes to Figure 7 Caption, Figure 8 Caption, Figure 10
Caption, and Figure 11 Caption ....................................................11
Added Theory of Operation Section ............................................14
Changes to Analog Input Section .................................................16
Changes to Drive Amplifier Choice Section, Table 10, Figure 30,
Voltage Reference Input Section, and Power Supply Section .........17
Changes to Supplying the ADC from the Reference Section
and Digital Interface Section .........................................................18
Changed CS Mode, 3-Wire, No BUSY Indicator Section to CS
Mode, 3-Wire Without BUSY Indicator Section ........................19
Changes to CS Mode, 3-Wire Without BUSY Indicator Section ...19
Changes to CS Mode, 3-Wire with BUSY Indicator Section ......20
Changed CS Mode, 4-Wire, No BUSY Indicator Section to CS
Mode, 4-Wire Without BUSY Indicator Section ........................21
Changes to CS Mode, 4-Wire Without BUSY Indicator Section ... 21
Changes to CS Mode, 4-Wire with BUSY Indicator Section..........22
Changed Chain Mode, No BUSY Indicator Section to Chain
Mode Without BUSY Indicator Section .......................................23
Changes to Chain Mode Without BUSY Indicator Section,
Figure 42 Caption, and Figure 43 Caption...................................23
Changes to Chain Mode with BUSY Indicator Section .............24
Changes to Layout Section .............................................................25
Changed Application Hints Section to Application
Recommendations Section ............................................................25
Changes to Ordering Guide ...........................................................26
4/15—Rev. C to Rev. D
Added Patent Note, Note 1 .............................................................. 1
Changes to SNR Degradation Equation, Driver Amplifier
Choice Section ................................................................................. 16
Changes to Ordering Guide ........................................................... 26
7/14—Rev. B to Rev. C
Deleted QFN ...................................................................Throughout
Changed Application Diagram Section to Typical Application
Circuit Section ................................................................................... 1
Change to Features Section.............................................................. 1
Added Note 1 ..................................................................................... 1
Changes to Figure 27 ...................................................................... 14
Changes to Evaluating the Performance of the AD7687
Section .............................................................................................. 24
Updated Outline Dimensions........................................................ 25
Changes to Ordering Guide ........................................................... 26
8/11—Rev. A to Rev. B
Changes to Table 7 ............................................................................ 7
Changes to Ordering Guide ........................................................... 26
2/11—Rev. 0 to Rev. A
Deleted QFN in Development Note ............................Throughout
Changes to Table 6 ............................................................................ 7
Added Thermal Resistance Section and Table 7 ........................... 7
Changes to Figure 6 and Table 8 ..................................................... 8
Updated Outline Dimensions........................................................ 25
Changes to Ordering Guide ........................................................... 26
4/05—Revision 0: Initial Version
Rev. E | Page 3 of 26
AD7687
Data Sheet
SPECIFICATIONS
VDD = 2.3 V to 5.5 V, VIO = 2.3 V to VDD, VREF = VDD, TA = −40°C to +85°C, unless otherwise noted.
Table 2.
Parameter
RESOLUTION
ANALOG INPUT
Voltage Range
Absolute Input Voltage
Common-Mode Input Range
Analog Input CMRR
Leakage Current at 25°C
Input Impedance
ACCURACY
No Missing Codes
Differential Linearity Error
Integral Linearity Error
Transition Noise
Gain Error 2, TMIN to TMAX
Gain Error Temperature Drift
Offset Error2, TMIN to TMAX
Offset Temperature Drift
Power Supply Sensitivity
THROUGHPUT
Conversion Rate
Transient Response
AC ACCURACY
Dynamic Range
Signal-to-Noise Ratio
Spurious-Free Dynamic Range
Total Harmonic Distortion
Signal-to-(Noise + Distortion) Ratio
Test Conditions/Comments
Min
16
IN+ − IN−
IN+ and IN−
IN+ and IN−
fIN = 250 kHz
Acquisition phase
−VREF
−0.1
0
Typ
Max
Unit
Bits
+VREF
VREF + 0.1
VREF/2 + 0.1
V
V
V
dB
nA
VREF/2
65
1
See the Analog Input section
16
−1
−1.5
REF = VDD = 5 V
VDD = 4.5 V to 5.5 V
VDD = 2.3 V to 4.5 V
VDD = 5 V ± 5%
VDD = 4.5 V to 5.5 V
VDD = 2.3 V to 4.5 V
Full-scale step
0
0
VREF = 5 V
fIN = 20 kHz, VREF = 5 V
fIN = 20 kHz, VREF = 2.5 V
fIN = 20 kHz
fIN = 20 kHz
fIN = 20 kHz, VREF = 5 V
fIN = 20 kHz, VREF = 5 V, −60 dB input
fIN = 20 kHz, VREF = 2.5 V
95.8
94
92
Intermodulation Distortion 4
94
92
±0.4
±0.4
0.35
±2
±0.3
±0.1
±0.7
±0.3
±0.05
+1
+1.5
±6
±1.6
±3.5
250
200
1.8
96.5
95.5
92.5
−118
−118
95
36.5
92.5
115
Bits
LSB 1
LSB
LSB
LSB
ppm/°C
mV
mV
ppm/°C
LSB
kSPS
kSPS
µs
dB 3
dB
dB
dB
dB
dB
dB
dB
dB
LSB means least significant bit. With the ±5 V input range, one LSB is 152.6 µV.
See the Terminology section. These specifications do include full temperature range variation but do not include the error contribution from the external reference.
All specifications in dB are referred to a full-scale input FSR. Tested with an input signal at 0.5 dB below full-scale, unless otherwise specified.
4
fIN1 = 21.4 kHz, fIN2 = 18.9 kHz, each tone at −7 dB below full-scale.
1
2
3
Rev. E | Page 4 of 26
Data Sheet
AD7687
VDD = 2.3 V to 5.5 V, VIO = 2.3 V to VDD, VREF = VDD, TA = −40°C to +85°C, unless otherwise noted.
Table 3.
Parameter
REFERENCE
Voltage Range
Load Current
SAMPLING DYNAMICS
−3 dB Input Bandwidth
Aperture Delay
DIGITAL INPUTS
Logic Levels
VIL
VIH
IIL
IIH
DIGITAL OUTPUTS
Data Format
Pipeline Delay
VOL
VOH
POWER SUPPLIES
VDD
VIO
VIO Range
Standby Current 1, 2
Power Dissipation
TEMPERATURE RANGE 3
Specified Performance
Test Conditions/Comments
Min
Typ
0.5
Max
Unit
VDD + 0.3
250 kSPS, REF = 5 V
50
V
µA
VDD = 5 V
2
2.5
MHz
ns
−0.3
0.7 × VIO
−1
−1
+0.3 × VIO
VIO + 0.3
+1
+1
Serial 16-bits twos complement
Conversion results available immediately
after completed conversion
0.4
VIO − 0.3
ISINK = 500 µA
ISOURCE = −500 µA
Specified performance
Specified performance
2.3
2.3
1.8
VDD and VIO = 5 V, 25°C
VDD = 2.5 V, 100 SPS throughput
VDD = 2.5 V, 100 kSPS throughput
VDD = 2.5 V, 200 kSPS throughput
VDD = 5 V, 100 kSPS throughput
VDD = 5 V, 250 kSPS throughput
TMIN to TMAX
1
1.4
1.35
2.7
4
−40
With all digital inputs forced to VIO or GND as required.
During acquisition phase.
3
Contact sales for extended temperature range.
1
2
Rev. E | Page 5 of 26
5.5
VDD + 0.3
VDD + 0.3
50
V
V
µA
µA
V
V
5.5
12.5
V
V
V
nA
µW
mW
mW
mW
mW
+85
°C
AD7687
Data Sheet
TIMING SPECIFICATIONS
−40°C to +85°C, VDD = 4.5 V to 5.5 V, VIO = 2.3 V to 5.5 V or VDD + 0.3 V, whichever is the lowest, unless otherwise stated.
See Figure 2 and Figure 3 for load conditions.
Table 4.
Parameter
CONVERSION TIME: CNV RISING EDGE TO DATA AVAILABLE
ACQUISITION TIME
TIME BETWEEN CONVERSIONS
CNV PULSE WIDTH (CS MODE)
Symbol
tCONV
tACQ
tCYC
tCNVH
SCK PERIOD
CS Mode
Chain Mode
VIO Above 4.5 V
VIO Above 3 V
VIO Above 2.7 V
VIO Above 2.3 V
SCK TIME
Low
High
SCK FALLING EDGE
To Data Remains Valid
To Data Valid Delay
VIO Above 4.5 V
VIO Above 3 V
VIO Above 2.7 V
VIO Above 2.3 V
CNV OR SDI
Low to SDO D15 MSB Valid (CS Mode)
VIO Above 4.5 V
VIO Above 2.7 V
VIO Above 2.3 V
High or Last SCK Falling Edge to SDO High Impedance (CS Mode)
tSCK
SDI
Valid Setup Time from CNV Rising Edge (CS Mode)
Valid Hold Time from CNV Rising Edge (CS Mode)
Valid Setup Time from SCK Falling Edge (Chain Mode)
Valid Hold Time from SCK Falling Edge (Chain Mode)
High to SDO High (Chain Mode with BUSY indicator)
VIO Above 4.5 V
VIO Above 2.3 V
SCK
Valid Setup Time from CNV Rising Edge (Chain Mode)
Valid Hold Time from CNV Rising Edge (Chain Mode)
Min
0.5
1.8
4
10
Typ
Max
2.2
15
ns
17
18
19
20
ns
ns
ns
ns
tSCKL
tSCKH
7
7
ns
ns
tHSDO
tDSDO
5
14
15
16
17
ns
ns
ns
ns
15
18
22
25
ns
ns
ns
ns
tEN
tDIS
tSSDICNV
tHSDICNV
tSSDISCK
tHSDISCK
tDSDOSDI
15
0
3
4
ns
ns
ns
ns
15
26
tSSCKCNV
tHSCKCNV
Rev. E | Page 6 of 26
Unit
µs
µs
µs
ns
5
5
ns
ns
ns
ns
Data Sheet
AD7687
−40°C to +85°C, VDD = 2.3 V to 4.5 V, VIO = 2.3 V to 4.5 V or VDD + 0.3 V, whichever is the lowest, unless otherwise stated.
See Figure 2 and Figure 3 for load conditions.
Table 5.
Parameter
CONVERSION TIME: CNV RISING EDGE TO DATA AVAILABLE
ACQUISITION TIME
TIME BETWEEN CONVERSIONS
CNV PULSE WIDTH (CS MODE)
Symbol
tCONV
tACQ
tCYC
tCNVH
SCK PERIOD
CS Mode
Chain Mode
VIO Above 3 V
VIO Above 2.7 V
VIO Above 2.3 V
SCK TIME
Low
High
SCK FALLING EDGE
To Data Remains Valid
To Data Valid Delay
VIO Above 3 V
VIO Above 2.7 V
VIO Above 2.3 V
CNV OR SDI
Low to SDO D15 MSB Valid (CS Mode)
VIO Above 2.7 V
VIO Above 2.3 V
High or Last SCK Falling Edge to SDO High Impedance (CS Mode)
tSCK
Min
0.7
1.8
5
10
Typ
Max
3.2
Unit
µs
µs
µs
ns
25
ns
29
35
40
ns
ns
ns
tSCKL
tSCKH
12
12
ns
ns
tHSDO
tDSDO
5
24
30
35
ns
ns
ns
18
22
25
ns
ns
ns
36
ns
ns
ns
ns
ns
tEN
tDIS
SDI
Valid Setup Time from CNV Rising Edge (CS Mode)
Valid Hold Time from CNV Rising Edge (CS Mode)
Valid Setup Time from SCK Falling Edge (Chain Mode)
Valid Hold Time from SCK Falling Edge (Chain Mode)
High to SDO High (Chain Mode with BUSY indicator)
SCK
Valid Setup Time from CNV Rising Edge (Chain Mode)
Valid Hold Time from CNV Rising Edge (Chain Mode)
tSSDICNV
tHSDICNV
tSSDISCK
tHSDISCK
tDSDOSDI
30
0
5
4
tSSCKCNV
tHSCKCNV
5
8
ns
ns
Timing Diagrams
70% VIO
IOL
30% VIO
tDELAY
tDELAY
1.4V
TO SDO
CL
50pF
IOH
2V OR VIO – 0.5V1
0.8V OR 0.5V2
0.8V OR 0.5V2
12V IF VIO ABOVE 2.5V, VIO – 0.5V IF VIO BELOW 2.5V.
20.8V IF VIO ABOVE 2.5V, 0.5V IF VIO BELOW 2.5V.
02972-003
500µA
2V OR VIO – 0.5V1
Figure 2. Load Circuit for Digital Interface Timing
Figure 3. Voltage Levels for Timing
Rev. E | Page 7 of 26
02972-004
500µA
AD7687
Data Sheet
ABSOLUTE MAXIMUM RATINGS
THERMAL RESISTANCE
Table 6.
Parameter
Analog Inputs
IN+1, IN−1
REF
Supply Voltages
VDD, VIO to GND
VDD to VIO
Digital Inputs to GND
Digital Outputs to GND
Storage Temperature Range
Junction Temperature
Lead Temperature Range
1
Rating
θJA is specified for the worst-case conditions, that is, a device
soldered in a circuit board for surface-mount packages.
GND − 0.3 V to VDD + 0.3 V
or ±130 mA
GND − 0.3 V to VDD + 0.3 V
Table 7. Thermal Resistance
Package Type
10-Lead LFCSP
10-Lead MSOP
−0.3 V to +7 V
±7 V
−0.3 V to VIO + 0.3 V
−0.3 V to VIO + 0.3 V
−65°C to +150°C
150°C
JEDEC J-STD-20
ESD CAUTION
See the Analog Input section.
Stresses at or above those listed under Absolute Maximum
Ratings may cause permanent damage to the product. This is a
stress rating only; functional operation of the product at these
or any other conditions above those indicated in the operational
section of this specification is not implied. Operation beyond
the maximum operating conditions for extended periods may
affect product reliability.
Rev. E | Page 8 of 26
θJA
84
200
θJC
2.96
44
Unit
°C/W
°C/W
Data Sheet
AD7687
PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
9
SDI
IN+ 3
TOP VIEW
(Not to Scale)
8
SCK
IN– 4
GND 5
7
6
SDO
CNV
REF 1
VDD 2
IN+ 3
IN– 4
GND 5
10 VIO
AD7687
9 SDI
TOP VIEW
(Not to Scale)
8 SCK
7 SDO
6 CNV
NOTES
1. FOR THE LFCSP ONLY, THE EXPOSED
PADDLE MUST BE CONNECTED TO GND.
Figure 4. 10-Lead MSOP Pin Configuration
02972-006
10 VIO
AD7687
02972-005
REF 1
VDD 2
Figure 5. 10-Lead LFCSP Pin Configuration
Table 8. Pin Function Descriptions
Pin No.
1
Mnemonic
REF
Type1
AI
2
3
4
5
6
VDD
IN+
IN−
GND
CNV
P
AI
AI
P
DI
7
SDO
DO
8
SCK
DI
9
SDI
DI
10
VIO
P
EPAD
N/A
Function
Reference Input Voltage. The REF range is from 0.5 V to VDD, referred to the GND pin. Place a 10 μF
decoupling capacitor as close to the pin as possible.
Power Supply.
Differential Positive Analog Input.
Differential Negative Analog Input.
Power Supply Ground.
Convert Input. This input has multiple functions. On its leading edge, it initiates a conversion and selects
the interface mode: chain or CS (depending on the state of SDI). In CS mode, CNV enables the SDO pin
when low. In chain mode, the data is read while CNV is high.
Serial Data Output. The conversion result is output on this pin. It is synchronized to SCK. SDO also acts as
the BUSY indicator if the feature is enabled.
Serial Data Clock Input. This input primarily shifts data out on SDO when data is valid. In chain mode, the
state of SCK determines if the BUSY indicator feature is enabled. If SCK is low during the CNV rising edge,
the BUSY feature is disabled. If it is high during the CNV rising edge, the BUSY feature is enabled.
Serial Data Input. This input serves multiple functions. It selects the interface mode of the ADC as follows:
Chain mode is selected if SDI is low during the CNV rising edge. In this mode, SDI is used as a data input to
daisy chain the conversion results of two or more ADCs onto a single SDO line. The digital data level on SDI
is output on SDO with a delay of 16 SCK cycles.
CS mode is selected if SDI is high during the CNV rising edge. In this mode, either SDI or CNV can enable
the serial output signals when low, and if SDI or CNV is low when the conversion is complete, the BUSY
indicator feature is enabled.
Input/Output Interface Digital Power. Nominally at the same supply as the host interface (1.8 V, 2.5 V, 3 V,
or 5 V).
For the LFCSP only, the exposed paddle must be connected to GND.
1
AI means analog input, DI means digital input, DO means digital output, P means power, and N/A means not applicable.
Rev. E | Page 9 of 26
AD7687
Data Sheet
TERMINOLOGY
Integral Nonlinearity Error (INL)
INL refers to the deviation of each individual code from a line
drawn from negative full scale through positive full scale. The
point used as negative full scale occurs ½ LSB before the first
code transition. Positive full scale is defined as a level 1½ LSB
beyond the last code transition. Measure the deviation from the
middle of each code to the true straight line (see Figure 25).
Differential Nonlinearity Error (DNL)
In an ideal ADC, code transitions are 1 LSB apart. The DNL is
the maximum deviation from this ideal value. It is often specified
in terms of resolution for which no missing codes are guaranteed.
Zero Error
Zero error is the difference between the ideal midscale voltage,
that is, 0 V, from the actual voltage producing the midscale
output code, that is, 0 LSB.
Gain Error
The first transition (from 100…00 to 100…01) should occur at
a level ½ LSB above nominal negative full scale (−4.999924 V
for the ±5 V range). The last transition (from 011…10 to
011…11) should occur for an analog voltage 1½ LSB below the
nominal full scale (+4.999771 V for the ±5 V range.) The gain
error is the deviation of the difference between the actual level
of the last transition and the actual level of the first transition
from the difference between the ideal levels.
Spurious-Free Dynamic Range (SFDR)
SFDR is the difference, in decibels (dB), between the rms
amplitude of the input signal and the peak spurious signal.
Effective Number of Bits (ENOB)
ENOB is a measurement of the resolution with a sine wave
input. It is related to SINAD by the following formula
ENOB = (SINADdB − 1.76)/6.02
and is expressed in bits.
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of the first five harmonic
components to the rms value of a full-scale input signal and is
expressed in dB.
Dynamic Range
Dynamic range is the ratio of the rms value of the full scale to
the total rms noise measured with the inputs shorted together.
The value for dynamic range is expressed in dB.
Signal-to-Noise Ratio (SNR)
SNR is the ratio of the rms value of the actual input signal to the
rms sum of all other spectral components below the Nyquist
frequency, excluding harmonics and dc. The value for SNR is
expressed in dB.
Signal-to-(Noise + Distortion) Ratio (SINAD)
SINAD is the ratio of the rms value of the actual input signal to
the rms sum of all other spectral components below the Nyquist
frequency, including harmonics but excluding dc. The value for
SINAD is expressed in dB.
Aperture Delay
Aperture delay is the measure of the acquisition performance. It
is the time between the rising edge of the CNV input and when
the input signal is held for a conversion.
Transient Response
Transient response is the time required for the ADC to acquire its
input accurately after a full-scale step function is applied.
Rev. E | Page 10 of 26
Data Sheet
AD7687
TYPICAL PERFORMANCE CHARACTERISTICS
1.5
1.5
POSITIVE DNL = +0.27LSB
NEGATIVE DNL = –0.24LSB
1.0
1.0
0.5
0.5
DNL (LSB)
0
0
–0.5
–1.0
–1.0
02972-001
–0.5
–1.5
0
16384
32768
CODE
49152
02972-009
INL (LSB)
POSITIVE INL = +0.32LSB
NEGATIVE INL = –0.41LSB
–1.5
65535
0
16384
Figure 6. Integral Nonlinearity vs. Code
32768
CODE
49152
65535
Figure 9. Differential Nonlinearity vs. Code
300000
250000
VDD = REF = 5V
258680
VDD = REF = 2.5V
200403
250000
200000
200000
COUNTS
COUNTS
150000
150000
100000
100000
0
1049
1391
0
0
41
42
43
45
44
CODE IN HEX
46
02972-007
0
0
0
0
60
44
45
46
29918
18
0
0
4A
4B
4C
0
47
Figure 7. Histogram of a DC Input at the Code Center, VDD = REF = 5 V
49
47
48
CODE IN HEX
Figure 10. Histogram of a DC Input at the Code Center, VDD = REF = 2.5 V
0
0
8192 POINT FFT
VDD = REF = 5V
fS = 250kSPS
fIN = 2.1kHz
SNR = 95.5dB
THD = –118.3dB
2nd HARMONIC = –130dB
3rd HARMONIC = –122.7dB
–60
–80
–100
–120
02972-008
–140
–160
–180
0
20
40
60
80
FREQUENCY (kHz)
100
–40
–60
–80
–100
–120
–140
02972-011
–40
32768 POINT FFT
VDD = REF = 2.5V
fS = 250kSPS
fIN = 2kHz
SNR = 92.8dB
THD = –115.9dB
2nd HARMONIC = –124dB
3rd HARMONIC = –119dB
–20
AMPLITUDE (dB of Full Scale)
–20
AMPLITUDE (dB of Full Scale)
30721
02972-010
50000
50000
–160
–180
120
0
Figure 8. FFT Plot, VDD = REF = 5 V
20
40
60
80
FREQUENCY (kHz)
Figure 11. FFT Plot, VDD = REF = 2.5 V
Rev. E | Page 11 of 26
100
120
AD7687
Data Sheet
17.0
100
–100
–105
SNR
16.0
95
SINAD
THD, SFDR (dB)
ENOB (Bits)
15.0
90
–115
THD
SFDR
–120
14.0
70
2.3
2.7
3.1
3.5
3.9
4.3
4.7
REFERENCE VOLTAGE (V)
13.0
5.5
5.1
–125
–130
2.3
Figure 12. SNR, SINAD, and ENOB vs. Reference Voltage
02972-015
85
02972-012
SNR, SINAD (dB)
–110
ENOB
2.7
3.1
3.5
3.9
4.3
4.7
REFERENCE VOLTAGE (V)
5.1
5.5
Figure 15. THD, SFDR vs. Reference Voltage
100
–60
VREF = 5V, –10dB
95
–70
VREF = 2.5V, –10dB
VREF = 5V, –1dB
–80
VREF = 2.5V, –1dB
85
THD (dB)
SINAD (dB)
90
80
VREF = 2.5V, –1dB
–90
VREF = 5V, –1dB
–100
VREF = 2.5V, –10dB
75
70
0
50
100
FREQUENCY (kHz)
150
VREF = 5V, –10dB
02972-016
02972-013
–110
–120
200
0
50
Figure 13. SINAD vs. Frequency
100
FREQUENCY (kHz)
200
150
Figure 16. THD vs. Frequency
100
–90
VREF = 5V
95
–100
THD (dB)
90
VREF = 5V
–110
VREF = 2.5V
85
–35
–15
5
25
45
65
TEMPERATURE (°C)
85
105
–130
–55
125
Figure 14. SNR vs. Temperature
02972-017
80
–55
–120
02972-014
SNR (dB)
VREF = 2.5V
–35
–15
5
25
45
65
TEMPERATURE (°C)
Figure 17. THD vs. Temperature
Rev. E | Page 12 of 26
85
105
125
Data Sheet
AD7687
100
1000
99
fS = 100kSPS
VDD = 5V
OPERATING CURRENT (µA)
98
97
VREF = 5V
SNR (dB)
96
95
94
VREF = 2.5V
93
750
VDD = 2.5V
500
250
91
90
–10
–8
–6
–4
INPUT LEVEL (dB)
–2
02972-021
02972-018
92
VIO
0
–55
0
–35
–15
5
25
45
65
TEMPERATURE (°C)
85
105
125
Figure 21. Operating Current vs. Temperature
Figure 18. SNR vs. Input Level
6
1000
750
500
250
02972-019
VIO
0
2.3
2.7
3.1
3.5
3.9
4.3
SUPPLY (V)
4.7
5.1
GAIN ERROR
2
0
–2
OFFSET ERROR
–4
–6
–55
5.5
–35
–15
5
25
45
65
TEMPERATURE (°C)
85
105
125
Figure 22. Offset Error and Gain Error vs. Temperature
Figure 19. Operating Current vs. Supply
25
1000
VDD = 2.5V, 85°C
20
tDSDO DELAY (ns)
750
500
250
15
VDD = 2.5V, 25°C
10
VDD = 5V, 85°C
VDD = 5V, 25°C
5
VDD = 3.3V, 85°C
VDD + VIO
0
–55
–35
–15
5
25
45
65
TEMPERATURE (°C)
85
02972-020
POWER-DOWN CURRENT (nA)
4
105
02972-023
OPERATING CURRENT (µA)
VDD
02972-022
OFFSET ERROR AND GAIN ERROR (LSB)
fS = 100kSPS
VDD = 3.3V, 25°C
0
0
125
20
40
60
80
SDO CAPACITIVE LOAD (pF)
100
Figure 23. tDSDO Delay vs. Capacitance Load and Supply
Figure 20. Power-Down Current vs. Temperature
Rev. E | Page 13 of 26
120
AD7687
Data Sheet
THEORY OF OPERATION
IN+
SWITCHES CONTROL
MSB
32,768C 16,384C
LSB
4C
2C
C
SW+
C
BUSY
REF
COMP
GND
32,768C 16,384C
4C
2C
C
MSB
CONTROL
LOGIC
OUTPUT CODE
C
LSB
SW–
02972-024
CNV
IN–
Figure 24. ADC Simplified Schematic
CIRCUIT INFORMATION
CONVERTER OPERATION
The AD7687 is a fast, low power, single-supply, precise 16-bit
ADC using a successive approximation architecture.
The AD7687 is a successive approximation ADC based on a
charge redistribution DAC. Figure 24 shows the simplified
schematic of the ADC. The capacitive DAC consists of two
identical arrays of 16 binary weighted capacitors, which are
connected to the two comparator inputs.
The AD7687 is capable of converting 250,000 samples per
second (250 kSPS) and powers down between conversions.
When operating at 100 SPS, for example, it typically consumes
1.35 µW, which is ideal for battery-powered applications.
The AD7687 provides the user with an on-chip track-and-hold
and does not exhibit any pipeline delay or latency, making it
ideal for multiple multiplexed channel applications.
The AD7687 is specified for use from 2.3 V to 5.5 V and can be
interfaced to any of the 1.8 V to 5 V digital logic family. It is
housed in a 10-lead MSOP or in a tiny 10-lead LFCSP that saves
space and allows flexible configurations.
It is pin-for-pin-compatible with the AD7685, AD7686, and
AD7688.
During the acquisition phase, terminals of the array tied to the
input of the comparator are connected to GND via SW+ and
SW−. All independent switches are connected to the analog
inputs. Thus, the capacitor arrays function as sampling
capacitors and acquire the analog signal on the IN+ and IN−
inputs. When the acquisition phase is complete and the CNV
input goes high, a conversion phase is initiated. When the
conversion phase begins, SW+ and SW− open first. The two
capacitor arrays are then disconnected from the inputs and
connected to the GND input. Therefore, the differential voltage
between the inputs IN+ and IN− captured at the end of the
acquisition phase is applied to the comparator inputs, causing
the comparator to become unbalanced. By switching each
element of the capacitor array between GND and REF, the
comparator input varies by binary weighted voltage steps
(VREF/2, VREF/4…VREF/65536). The control logic toggles these
switches, starting with the MSB, to bring the comparator back
into a balanced condition. After the completion of this process,
the device returns to the acquisition phase and the control logic
generates the ADC output code and a BUSY signal indicator.
Because the AD7687 has an on-board conversion clock, the
serial clock, SCK, is not required for the conversion process.
Rev. E | Page 14 of 26
Data Sheet
AD7687
Transfer Functions
Table 9. Output Codes and Ideal Input Voltages
Description
FSR − 1 LSB
Midscale + 1 LSB
Midscale
Midscale − 1 LSB
−FSR + 1 LSB
−FSR
011...111
011...110
011...101
1
2
Analog Input
VREF = 5 V
+4.999847 V
+152.6 µV
0V
−152.6 µV
−4.999847 V
−5 V
Digital Output Code
Hexadecimal
7FFF1
0001
0000
FFFF
8001
80002
This is also the code for an overranged analog input (VIN+ − VIN− above
VREF − VGND).
This is also the code for an underranged analog input (VIN+ − VIN− below
−VREF + VGND).
TYPICAL CONNECTION DIAGRAM
100...010
Figure 26 shows an example of the recommended connection
diagram for the AD7687 when multiple supplies are available.
100...001
100...000
–FSR
–FSR + 1 LSB
+FSR – 1 LSB
+FSR – 1.5 LSB
–FSR + 0.5 LSB
ANALOG INPUT
02972-025
Figure 25. ADC Ideal Transfer Function
≥7V
REF1
5V
10µF2
100nF
≥7V
1.8V TO VDD
100nF
33Ω
REF
0 TO VREF
VDD
IN+
3
≤ –2V
≥7V
VIO
SDI
2.7nF
SCK
AD7687
4
IN–
33Ω
3- OR 4-WIRE INTERFACE5
SDO
CNV
GND
VREF TO 0
3
≤ –2V
2.7nF
4
1SEE VOLTAGE INPUT REFERENCE SECTION FOR REFERENCE SELECTION.
2C
REF IS USUALLY A 10µF CERAMIC CAPACITOR (X5R).
3SEE DRIVER AMPLIFIER CHOICE SECTION.
4OPTIONAL FILTER. SEE ANALOG INPUT SECTION.
5SEE DIGITAL INTERFACE FOR MOST CONVENIENT INTERFACE MODE.
Figure 26. Typical Connection Diagram with Multiple Supplies
Rev. E | Page 15 of 26
02972-026
ADC CODE (TWOS COMPLEMENT)
Figure 25 and Table 9 show the ideal transfer characteristic for
the AD7687.
AD7687
Data Sheet
ANALOG INPUT
Figure 27 shows an equivalent circuit of the input structure of
the AD7687.
The two diodes, D1 and D2, provide ESD protection for the
analog inputs IN+ and IN−. Take care to ensure that the analog
input signal never exceeds the supply rails by more than 0.3 V
because this causes these diodes to begin to forward-bias and
start conducting current. These diodes can handle a forwardbiased current of 130 mA maximum. These overvoltage
conditions can occur if the supplies of the input buffer (U1)
differ from VDD. In such a case, use an input buffer with a
short-circuit current limitation to protect the device.
VDD
D1
IN+
OR IN–
CIN
D2
02972-027
CPIN
RIN
GND
Figure 27. Equivalent Analog Input Circuit
During the acquisition phase, the impedance of the analog
inputs (IN+ or IN−) can be modeled as a parallel combination
of capacitor, CPIN, and the network formed by the series connection
of RIN and CIN. CPIN is primarily the pin capacitance. RIN is
typically 3 kΩ and is a lumped component made up of some
serial resistors and the on resistance of the switches. CIN is
typically 30 pF and is mainly the ADC sampling capacitor.
During the conversion phase, where the switches are opened,
the input impedance is limited to CPIN. RIN and CIN make a
1-pole, low-pass filter that reduces undesirable aliasing effects
and limits the noise.
If the source impedance of the driving circuit is sufficiently low,
the AD7687 can be driven directly. Large source impedances
significantly affect the ac performance, especially the total
harmonic distortion (THD). The maximum source impedance
depends on the amount of THD that can be tolerated by the
AD7687. The THD degrades as a function of the source
impedance and the maximum input frequency, as shown in
Figure 29.
–60
The analog input structure allows the sampling of the true
differential signal between IN+ and IN−. This differential input
scheme allows for rejection of common-mode signals. Figure 28
shows the typical CMRR over frequency.
–70
THD (dB)
–80
90
VDD = 5V
–90
RS = 250Ω
80
–100
RS = 100Ω
70
RS = 50Ω
RS = 33Ω
–120
60
02972-029
–110
0
25
50
FREQUENCY (kHz)
75
100
Figure 29. THD vs. Analog Input Frequency and Source Resistance
50
02972-028
CMRR (dB)
VDD = 2.5V
40
1
10
100
FREQUENCY (kHz)
1000
Figure 28. Analog Input CMRR vs. Frequency
Rev. E | Page 16 of 26
Data Sheet
AD7687
DRIVER AMPLIFIER CHOICE
SINGLE-TO-DIFFERENTIAL DRIVER
Although the AD7687 is easy to drive, consider the following
when selecting a driver amplifier.
For applications using a single-ended analog signal, either bipolar
or unipolar, a single-ended-to-differential driver (like the one
shown in Figure 30) allows for a differential input into the part.
When provided a single-ended input signal, this configuration
produces a differential ±VREF with midscale at VREF/2.
SNRLOSS
53
= 20log
532 + π f −3dB (2 Ne N )2
2
ANALOG INPUT
(±10V, ±5V, ..)
VREF
10kΩ
590Ω
10kΩ
U1
VREF
10µF
100nF
590Ω
IN+ REF
590Ω
VREF
10kΩ
10kΩ
U2
AD7687
IN–
100nF
02972-030
The noise generated by the driver amplifier needs to be kept as low
as possible in order to preserve the SNR and transition noise
performance of the AD7687. The AD7687 has a noise much lower
than most of the other 16-bit ADCs and, therefore, can be driven
by a noisier op amp while preserving the same or better system
performance. The noise coming from the driver is filtered by the
AD7687 analog input circuit 1-pole, low-pass filter made by RIN
and CIN or by an external filter. Because the typical noise of the
AD7687 is 53 µV rms, the SNR degradation due to the amplifier is
Figure 30. Single-Ended-to-Differential Driver Circuit
where:
f–3dB is either the input bandwidth in MHz of the AD7687 (2 MHz)
or the cutoff frequency of an external filter, if one is used.
N is the noise gain of the amplifier (for example, +1 in buffer
configuration).
eN is the equivalent input noise voltage of the op amp, in nV/√Hz.
VOLTAGE REFERENCE INPUT
For ac applications, ensure that the THD performance of the driver
is commensurate with the AD7687 and that the driver exceeds
the THD vs. frequency shown in Figure 16.
For optimum performance, drive the REF pin with a low output
impedance amplifier (such as the AD8031 or the AD8605) as a
reference buffer with a 10 µF (X5R, 0805 size) ceramic chip
decoupling capacitor.
For multichannel multiplexed applications, the driver amplifier
and the AD7687 analog input circuit must settle a full-scale step
onto the capacitor array at a 16-bit level (0.0015%, 15 ppm). Settling
at 0.1% to 0.01% is more commonly specified in the amplifier
data sheet. This can differ significantly from the settling time at
a 16-bit level and must be verified prior to driver selection.
Table 10. Recommended Driver Amplifiers.
Amplifier
AD8021
AD8022
AD8031
AD8519
AD8605, AD8615
AD8655
ADA4841-2
ADA4941-1
OP184
Typical Application
Very low noise and high frequency
Low noise and high frequency
High frequency and low power
Small, low power and low frequency
5 V single-supply, low power
5 V single-supply, low noise
Very low noise, small, and low power
Very low noise, low power single-ended-todifferential
Low power, low noise, and low frequency
The AD7687 voltage reference input, REF, has a dynamic input
impedance and must therefore be driven by a low impedance
source with sufficient decoupling between the REF and GND
pins (as explained in the Layout section).
If an unbuffered reference voltage is used, the decoupling value
depends on the reference used. For instance, a 22 µF (X5R,
1206 size) ceramic chip capacitor is appropriate for optimum
performance using a low temperature drift ADR431, ADR433,
ADR434, or ADR435 reference.
If desired, smaller reference decoupling capacitor values down
to 2.2 µF can be used with a minimal impact on performance,
especially DNL.
Regardless, there is no need for an additional lower value ceramic
decoupling capacitor (for example, 100 nF) between the REF
and GND pins.
POWER SUPPLY
The AD7687 is specified for use over a wide operating range of
2.3 V to 5.5 V. Unlike other low voltage converters, it has a low
enough noise to design a 16-bit resolution system with low
voltage supplies while maintaining respectable performance. It
uses two power supply pins: a core supply, VDD, and a digital
input/output interface supply, VIO. VIO allows direct interface
with any logic between 1.8 V and VDD. VIO and VDD can be
powered by the same source, reducing the number of supplies
required in the overall design. The AD7687 is independent of
power supply sequencing between VIO and VDD.
Rev. E | Page 17 of 26
AD7687
Data Sheet
Additionally, it is resistant to power supply variations over a wide
frequency range. Figure 31 shows the power supply rejection
ration (PSRR) of the device over frequency.
5V
5V
10
5V
10k
100
1F
95
AD8031
10F
1F
1
VDD = 5V
90
REF
VDD
VIO
1OPTIONAL
75
VDD = 2.5V
70
REFERENCE BUFFER AND FILTER.
02972-033
AD7687
80
Figure 33. Example of Application Circuit
65
DIGITAL INTERFACE
60
Though the AD7687 has a reduced number of pins, it offers
flexibility in its serial interface modes.
02972-031
PSRR (dB)
85
55
50
1
10
100
FREQUENCY (kHz)
1000
10000
Figure 31. PSRR vs. Frequency
The AD7687 powers down automatically at the end of each
conversion phase, and consequentially its power consumption
scales linearly with the sampling rate, as shown in Figure 32.
This makes the device ideal for low sampling rate (even a few
SPS) and low battery-powered applications.
When in CS mode, the AD7687 is compatible with SPI, QSPI,
digital hosts, and DSPs, such as the Blackfin® processors or the
high performance, mixed-signal DSP family. In this mode, the
AD7687 uses either a 3-wire or a 4-wire interface. A 3-wire
interface using the CNV, SCK, and SDO signals minimizes
wiring connections and is useful, for instance, in isolated
applications. A 4-wire interface using the SDI, CNV, SCK, and
SDO signals allows CNV, which initiates the conversions, to be
independent of the readback timing (SDI). This is useful in low
jitter sampling or simultaneous sampling applications.
1000
When in chain mode, the AD7687 provides a daisy chain
feature using the SDI input for cascading multiple ADCs on a
single data line similar to a shift register.
VDD = 2.5V
10
The mode in which the device operates depends on the SDI
level when the CNV rising edge occurs. The CS mode is
selected if SDI is high, and the chain mode is selected if SDI is
low. The SDI hold time is such that when SDI and CNV are
connected together, the chain mode is always selected.
VIO
0.1
0.001
10
02972-032
OPERATING CURRENT (A)
VDD = 5V
100
1000
10000
SAMPLING RATE (SPS)
100000
1000000
Figure 32. Operating Currents vs. Sampling Rate
SUPPLYING THE ADC FROM THE REFERENCE
With its low operating current, the AD7687 can be supplied
directly by the reference circuitry (see Figure 33). The reference
line is driven by one of the following:
The system power supply directly.
A reference voltage with enough current output capability,
such as the ADR435.
A reference buffer, such as the AD8031, which can also
filter the system power supply (see Figure 33).
The initial state of SDO on power up is indeterminate. Therefore,
to put SDO in a known state, initiate a conversion and clock out
all data bits.
In either mode, the AD7687 offers the option of forcing a start
bit in front of the data bits. Use this start bit as a BUSY signal
indicator to interrupt the digital host and trigger the data
reading. Otherwise, without a BUSY indicator, the user must
time out the maximum conversion time prior to readback.
The BUSY indicator feature is enabled
Rev. E | Page 18 of 26
In the CS mode if CNV or SDI is low when the ADC
conversion ends (see Figure 37 and Figure 41).
In the chain mode if SCK is high during the CNV rising
edge (see Figure 45).
Data Sheet
AD7687
CS MODE, 3-WIRE WITHOUT BUSY INDICATOR
both SCK edges. Although the rising edge can capture the data,
a digital host using the SCK falling edge allows a faster reading
rate (provided it has an acceptable hold time). After the 16th
SCK falling edge, or when CNV goes high, whichever is earlier,
SDO returns to high impedance.
This mode is usually used when a single AD7687 is connected
to an SPI-compatible digital host. Figure 34 shows the connection
diagram and Figure 35 gives the corresponding timing.
With SDI tied to VIO, a rising edge on CNV initiates a conversion,
selects the CS mode, and forces SDO to high impedance. Once
a conversion is initiated, it continues to completion irrespective
of the state of CNV. For instance, it can be useful to bring CNV
low to select other SPI devices, such as analog multiplexers, but
CNV must be returned high before the minimum conversion
time and held high until the maximum conversion time to
avoid the generation of the BUSY signal indicator (see tCONV in
Table 5). When the conversion is complete, the AD7687 enters
the acquisition phase and powers down. When CNV goes low,
the MSB is output onto SDO. The remaining data bits are then
clocked by subsequent SCK falling edges. The data is valid on
CONVERT
DIGITAL HOST
CNV
VIO
SDI
AD7687
DATA IN
SDO
02972-034
SCK
CLK
Figure 34. CS Mode, 3-Wire Without BUSY Indicator
Connection Diagram (SDI High)
SDI = 1
tCYC
tCNVH
CNV
ACQUISITION
tCONV
tACQ
CONVERSION
ACQUISITION
tSCK
tSCKL
1
2
14
3
tHSDO
16
tSCKH
tDSDO
tEN
SDO
15
D15
D14
D13
tDIS
D1
D0
Figure 35. CS Mode, 3-Wire Without BUSY Indicator Serial Interface Timing (SDI High)
Rev. E | Page 19 of 26
02972-035
SCK
AD7687
Data Sheet
bits are then clocked out, MSB first, by subsequent SCK falling
edges. The data is valid on both SCK edges. Although the rising
edge can capture the data, a digital host using the SCK falling
edge allows a faster reading rate (provided it has an acceptable
hold time). After the optional 17th SCK falling edge, or when
CNV goes high, whichever is earlier, SDO returns to high
impedance.
CS MODE, 3-WIRE WITH BUSY INDICATOR
This mode is usually used when a single AD7687 is connected
to an SPI-compatible digital host having an interrupt input.
Figure 36 shows the connection diagram and Figure 37 gives
the corresponding timing.
With SDI tied to VIO, a rising edge on CNV initiates a conversion,
selects the CS mode, and forces SDO to high impedance. SDO
is maintained in high impedance until the completion of the
conversion irrespective of the state of CNV. Prior to the minimum
conversion time, CNV can be used to select other SPI devices,
such as analog multiplexers, but CNV must be returned low
before the minimum conversion time and held low until the
maximum conversion time to guarantee the generation of the
BUSY signal indicator (see tCONV in Table 5). When the conversion
is complete, SDO goes from high to low impedance. With a
pull-up on the SDO line, this transition can be used as an
interrupt signal to initiate the data reading controlled by the
digital host. When using this option, select the value of the pullup resistor such that it maintains an appropriate rise time on the
SDO line for the application. This is a function of the resistance
of the pull-up and the capacitance of the SDO line. The AD7687
then enters the acquisition phase and powers down. The data
If multiple AD7687 devices are selected at the same time, the
SDO output pin handles this contention without damage or
induced latch-up. Keep this contention as short as possible to
limit extra power dissipation.
CONVERT
VIO
DIGITAL HOST
CNV
VIO
47kΩ
AD7687
DATA IN
SDO
SCK
IRQ
02972-036
SDI
CLK
Figure 36. CS Mode, 3-Wire with BUSY Indicator
Connection Diagram (SDI High)
SDI = 1
tCYC
tCNVH
CNV
ACQUISITION
tCONV
tACQ
CONVERSION
ACQUISITION
tSCK
tSCKL
1
2
3
tHSDO
15
16
17
tSCKH
tDSDO
SDO
tDIS
D15
D14
D1
D0
Figure 37. CS Mode, 3-Wire with BUSY Indicator Serial Interface Timing (SDI High)
Rev. E | Page 20 of 26
02972-037
SCK
Data Sheet
AD7687
time and held high until the maximum conversion time to
avoid the generation of the BUSY signal indicator. When the
conversion is complete, the AD7687 enters the acquisition
phase and powers down. Each ADC result can be read by
bringing low its SDI input, which consequently outputs the
MSB onto SDO. The remaining data bits are then clocked by
subsequent SCK falling edges. The data is valid on both SCK
edges. Although the rising edge can capture the data, a digital
host using the SCK falling edge allows a faster reading rate
(provided it has an acceptable hold time). After the 16th SCK
falling edge, or when SDI goes high, whichever is earlier, SDO
returns to high impedance and another AD7687 can be read.
CS MODE, 4-WIRE WITHOUT BUSY INDICATOR
This mode is usually used when multiple AD7687 devices are
connected to an SPI-compatible digital host.
Figure 38 shows a connection diagram example using two
AD7687 devices and Figure 39 gives the corresponding timing.
With SDI high, a rising edge on CNV initiates a conversion,
selects the CS mode, and forces SDO to high impedance. In this
mode, CNV must be held high during the conversion phase and
the subsequent data readback (if SDI and CNV are low, SDO is
driven low). Prior to the minimum conversion time, SDI can be
used to select other SPI devices, such as analog multiplexers,
but SDI must be returned high before the minimum conversion
CS2
CS1
CONVERT
AD7687
SDI
SDO
AD7687
SDO
SCK
SCK
02972-038
SDI
DIGITAL HOST
CNV
CNV
DATA IN
CLK
Figure 38. CS Mode, 4-Wire Without BUSY Indicator Connection Diagram
tCYC
CNV
ACQUISITION
tCONV
tACQ
CONVERSION
ACQUISITION
tSSDICNV
SDI(CS1)
tHSDICNV
SDI(CS2)
tSCK
tSCKL
SCK
1
2
14
3
tHSDO
16
17
18
D1
D0
D15
D14
30
31
32
D1
D0
tDSDO
tEN
D15
D14
D13
tDIS
02972-039
SDO
15
tSCKH
Figure 39. CS Mode, 4-Wire Without BUSY Indicator Serial Interface Timing
Rev. E | Page 21 of 26
AD7687
Data Sheet
When using this option, select the value of the pull-up resistor
such that it maintains an appropriate rise time on the SDO line
for the application. This is a function of the resistance of the
pull-up and the capacitance of the SDO line. The AD7687 then
enters the acquisition phase and powers down. The data bits are
then clocked out, MSB first, by subsequent SCK falling edges.
The data is valid on both SCK edges. Although the rising edge
can capture the data, a digital host using the SCK falling edge
allows a faster reading rate (provided it has an acceptable hold
time). After the optional 17th SCK falling edge, or SDI going
high, whichever is earlier, the SDO returns to high impedance.
CS MODE, 4-WIRE WITH BUSY INDICATOR
This mode is usually used when a single AD7687 is connected to
an SPI-compatible digital host, which has an interrupt input, and it
is desired to keep CNV, which is used to sample the analog input,
independent of the signal used to select the data reading. This
requirement is particularly important in applications where low
jitter on CNV is desired.
Figure 40 shows the connection diagram and Figure 41 gives
the corresponding timing.
With SDI high, a rising edge on CNV initiates a conversion,
selects the CS mode, and forces SDO to high impedance. In this
mode, CNV must be held high during the conversion phase and
the subsequent data readback (if SDI and CNV are low, SDO is
driven low). Prior to the minimum conversion time, SDI can
select other SPI devices, such as analog multiplexers, but SDI
must be returned low before the minimum conversion time and
held low until the maximum conversion time to guarantee the
generation of the BUSY signal indicator. When the conversion
is complete, SDO goes from high to low impedance. With a
pull-up on the SDO line, this transition can act as an interrupt
signal to initiate the data readback controlled by the digital host.
CS1
CONVERT
VIO
DIGITAL HOST
CNV
47k
AD7687
DATA IN
SDO
SCK
IRQ
02972-040
SDI
CLK
Figure 40. CS Mode, 4-Wire with BUSY Indicator Connection Diagram
tCYC
CNV
ACQUISITION
tCONV
tACQ
CONVERSION
ACQUISITION
tSSDICNV
SDI
tSCK
tHSDICNV
tSCKL
1
2
3
tHSDO
15
16
17
tSCKH
tDSDO
tDIS
tEN
SDO
D15
D14
D1
Figure 41. CS Mode, 4-Wire with BUSY Indicator Serial Interface Timing
Rev. E | Page 22 of 26
D0
02972-041
SCK
Data Sheet
AD7687
and powers down. The remaining data bits stored in the
internal shift register are then shifted out by subsequent SCK
falling edges. For each ADC, SDI feeds the input of the internal
shift register; these data bits are also shifted in by the SCK
falling edge. Each of the N ADCs in the chain outputs its data
MSB first. The data is valid on both SCK edges. Although the
rising edge can capture the data, a digital host using the SCK
falling edge allows a faster reading rate and, consequently, more
AD7687 devices in the chain (provided the digital host has an
acceptable hold time). After the 16 × Nth SCK falling edge or
CNV rising edge, whichever is earlier, SDO is driven low again.
The maximum conversion rate can be reduced due to the total
readback time. For example, using a digital host with a 3 ns setup time and 3 V interface, up to eight AD7687 devices daisychained on a 3-wire port can be run at a maximum effective
conversion rate of 220 kSPS.
CHAIN MODE WITHOUT BUSY INDICATOR
Use this mode to daisy-chain multiple AD7687 devices on a
3-wire serial interface. This feature is useful for reducing
component count and wiring connections, for isolated
multiconverter applications, or for systems with a limited
interfacing capacity (for example). Data readback is analogous
to clocking a shift register.
Figure 42 shows a connection diagram example using two
AD7687 devices and Figure 43 gives the corresponding timing.
When SDI and CNV are low, SDO is driven low. With SDI and
SCK low, a rising edge on CNV initiates a conversion, selects
the chain mode, and disables the BUSY indicator. In this mode,
CNV is held high during the conversion phase and the subsequent
data readback. When the conversion is complete, the MSB is
output onto SDO and the AD7687 enters the acquisition phase
CONVERT
SDI
CNV
AD7687
SDO
DIGITAL HOST
AD7687
SDI
A
B
SCK
SCK
SDO
DATA IN
02972-042
CNV
CLK
Figure 42. Chain Mode Without BUSY Indicator Connection Diagram
SDIA = 0
tCYC
CNV
ACQUISITION
tCONV
tACQ
CONVERSION
ACQUISITION
tSCK
tSCKL
tSSCKCNV
SCK
1
2
3
14
15
tSSDISCK
tHSCKCNV
16
17
18
DA15
DA14
30
31
32
D A1
DA0
tSCKH
SDOA = SDIB
DA15
DA14
DA13
DA1
DA0
DB15
DB14
DB13
D B1
DB0
tHSDO
tDSDO
SDOB
Figure 43. Chain Mode Without BUSY Indicator Serial Interface Timing
Rev. E | Page 23 of 26
02972-043
tHSDISC
tEN
AD7687
Data Sheet
trigger the data readback controlled by the digital host. The
AD7687 then enters the acquisition phase and powers down.
The data bits stored in the internal shift register are then
clocked out, MSB first, by subsequent SCK falling edges. For
each ADC, SDI feeds the input of the internal shift register;
these data bits are also shifted in by the SCK falling edge. Each
of the N ADCs in the chain outputs its data MSB first. The data
is valid on both SCK edges. Although the rising edge can capture
the data, a digital host using the SCK falling edge allows a faster
reading rate and, consequently, more AD7687 devices in the
chain (provided the digital host has an acceptable hold time).
After the optional (16 × N) + 1th SCK falling edge or CNV
rising edge, whichever is earlier, SDO is driven low again. The
maximum conversion rate may be reduced due to the total
readback time. For example, using a digital host with a 3 ns setup time and 3 V interface, up to eight AD7687 devices daisychained on a 3-wire port can be run at a maximum effective
conversion rate of 220 kSPS.
CHAIN MODE WITH BUSY INDICATOR
This mode can also be used to daisy-chain multiple AD7687
devices on a 3-wire serial interface while providing a BUSY
indicator. This feature is useful for reducing component count
and wiring connections, for isolated multiconverter applications
or for systems with a limited interfacing capacity (for example).
Data readback is analogous to clocking a shift register.
Figure 44 shows a connection diagram example using three
AD7687 devices, and Figure 45 gives the corresponding timing.
When SDI and CNV are low, SDO is driven low. With SDI low
and SCK high, a rising edge on CNV initiates a conversion,
selects the chain mode, and enables the BUSY indicator feature.
In this mode, CNV is held high during the conversion phase
and the subsequent data readback. When all ADCs in the chain
have completed their conversions, the SDO pin of the ADC
closest to the digital host (see the AD7687 C in Figure 44) is driven
high. This transition on SDO can act as a BUSY indicator to
CONVERT
SDI
AD7687
CNV
SDO
SDI
AD7687
DIGITAL HOST
CNV
SDO
SDI
AD7687
A
B
C
SCK
SCK
SCK
DATA IN
SDO
IRQ
02972-044
CNV
CLK
Figure 44. Chain Mode with BUSY Indicator Connection Diagram
tCYC
ACQUISITION
tCONV
tACQ
ACQUISITION
CONVERSION
tSSCKCNV
SCK
tSCKH
1
tHSCKCNV
2
tSSDISCK
tEN
SDOA = SDIB
3
4
tSCK
15
16
17
18
19
31
32
33
34
35
tSCKL
tHSDISC
tDSDOSDI
DA15 DA14 DA13
D A1
DA0
DB15 DB14 DB13
DB 1
DB0 DA15 DA14
DA1
DA 0
49
DC15 DC14 DC13
DC1
DC0 DB15 DB14
DB1
DB0 DA15 DA14
tDSDOSDI
tDSDOSDI
SDOC
48
tDSDOSDI
tHSDO
tDSDO
SDOB = SDIC
47
tDSDOSDI
Figure 45. Chain Mode with BUSY Indicator Serial Interface Timing
Rev. E | Page 24 of 26
DA1
DA0
02972-045
CNV = SDIA
Data Sheet
AD7687
APPLICATIONS INFORMATION
LAYOUT
Providing a steady and stable reference voltage to the AD7687 is
critical for device operation. Prioritize design tasks aimed at
preventing voltage fluctuations at this node. Decouple the REF
pin, which has a dynamic input impedance, with minimal parasitic
inductances (see the Converter Operation Section). Achieve this
by placing the reference decoupling ceramic capacitor as close as
physically possible the REF and GND pins and connecting it
with wide, low impedance traces.
02972-046
Limiting sources of noise on the analog signal nodes is
imperative in high precision ADC systems. The digital lines
controlling the AD7687 have the potential to radiate noise that
can couple into the analog signals; therefore, ensure that these
two types of signals are separated and confined to different
areas of boards housing the AD7687. Never allow fast switching
signals (such as CNV or clocks) to run near analog signal paths,
and avoid physical crossover of digital and analog signals. Do
not route digital lines under the AD7687 without a ground
plane providing adequate isolation between the two. To
facilitate these design tasks, the analog and digital pins are
located on separate sides of the device (see Figure 46).
Figure 46. Example of Layout of the AD7687 (Top Layer)
Finally, decouple the power supply pins of the AD7687 (VDD
and VIO) with ceramic capacitors (typically 100 nF) placed
close to the AD7687 and connected using short and wide traces
to provide low impedance paths and reduce the effect of glitches
on the power supply lines.
Figure 46 and Figure 47 show an example of a layout following
these rules.
02972-047
Printed circuit boards (PCBs) housing the AD7687 must contain
at least one ground plane. Connecting analog and digital ground
on the board is not required; however, connecting these planes
underneath the AD7687 is recommended.
Figure 47. Example of Layout of the AD7687 (Bottom Layer)
EVALUATING THE PERFORMANCE OF THE AD7687
The EVAL-AD7687SDZ evaluation board documentation
outlines other recommended layouts for the AD7687. The
evaluation board package includes a fully assembled and tested
evaluation board, documentation, and software for controlling
the board from a PC via the EVAL-SDP-CB1Z.
Rev. E | Page 25 of 26
AD7687
Data Sheet
OUTLINE DIMENSIONS
3.10
3.00
2.90
10
3.10
3.00
2.90
1
5.15
4.90
4.65
6
5
PIN 1
IDENTIFIER
0.50 BSC
0.95
0.85
0.75
15° MAX
1.10 MAX
0.70
0.55
0.40
0.23
0.13
6°
0°
0.30
0.15
091709-A
0.15
0.05
COPLANARITY
0.10
COMPLIANT TO JEDEC STANDARDS MO-187-BA
Figure 48. 10-Lead Mini Small Outline Package [MSOP]
(RM-10)
Dimensions shown in millimeters
2.48
2.38
2.23
3.10
3.00 SQ
2.90
0.50 BSC
10
6
1.74
1.64
1.49
EXPOSED
PAD
0.50
0.40
0.30
1
5
BOTTOM VIEW
TOP VIEW
0.80
0.75
0.70
SEATING
PLANE
0.30
0.25
0.20
0.05 MAX
0.02 NOM
COPLANARITY
0.08
0.20 MIN
PIN 1
INDICATOR
(R 0.15)
FOR PROPER CONNECTION OF
THE EXPOSED PAD, REFER TO
THE PIN CONFIGURATION AND
FUNCTION DESCRIPTIONS
SECTION OF THIS DATA SHEET.
0.20 REF
02-05-2013-C
PIN 1 INDEX
AREA
Figure 49. 10-Lead Lead Frame Chip Scale Package [LFCSP_WD]
3 mm × 3 mm Body, Very Very Thin, Dual Lead
(CP-10-9)
Dimensions shown in millimeters
ORDERING GUIDE
Model 1, 2, 3
AD7687BRMZ
AD7687BRMZRL7
AD7687BCPZ-R2
AD7687BCPZRL7
EVAL-AD7687SDZ
EVAL-SDP-CB1Z
Integral Nonlinearity
±1.5 LSB
±1.5 LSB
±1.5 LSB
±1.5 LSB
Temperature Range
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
Package Description
10-Lead MSOP, Tube
10-Lead MSOP, Reel
10-Lead LFCSP_WD, Reel
10-Lead LFCSP_WD, Reel
Evaluation Board
Controller Board
Package
Option
RM-10
RM-10
CP-10-9
CP-10-9
Ordering
Quantity
50
1,000
250
1,500
Branding
C3Q
C3Q
C3Q
#C03
Z = RoHS Compliant Part, # denotes RoHS compliant product, may be top or bottom marked.
The EVAL-AD7687SDZ can be used as a standalone evaluation board or in conjunction with the EVAL-SDP-CB1Z for evaluation and/or demonstration purposes.
3
The EVAL-SDP-CB1Z allows a PC to control and communicate with all Analog Devices, Inc. evaluation boards ending in the SDZ designator.
1
2
©2005–2015 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D02972-0-12/15(E)
Rev. E | Page 26 of 26
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