AD AD7608

8-Channel DAS with 18-Bit, Bipolar,
Simultaneous Sampling ADC
AD7608
Data Sheet
FEATURES
APPLICATIONS
8 simultaneously sampled inputs
True bipolar analog input ranges: ±10 V, ±5 V
Single 5 V analog supply and 2.3 V to 5.25 V VDRIVE
Fully integrated data acquisition solution
Analog input clamp protection
Input buffer with 1 MΩ analog input impedance
Second-order antialiasing analog filter
On-chip accurate reference and reference buffer
18-bit ADC with 200 kSPS on all channels
Oversampling capability with digital filter
Flexible parallel/serial interface
SPI/QSPI™/MICROWIRE™/DSP compatible
Pin compatible solutions from 14-bits to 18-bits
Performance
7 kV ESD rating on analog input channels
98 dB SNR, −107 dB THD
Low power: 100 mW
Standby mode: 25 mW
64-lead LQFP package
Power line monitoring and protection systems
Multiphase motor controls
Instrumentation and control systems
Multiaxis positioning systems
Data acquisition systems (DAS)
COMPANION PRODUCTS
External References: ADR421, ADR431
Digital Isolators: ADuM1402, ADuM5000, ADuM5402
Voltage Regulator Design Tool: ADIsimPower, Supervisor
Parametric Search
Complete list of complements on AD7608 product page
Table 1. High Resolution, Bipolar Input, Simultaneous
Sampling DAS Solutions
SingleEnded
Inputs
AD76081
AD7606
AD7606-6
AD7606-4
AD7607
Resolution
18 Bits
16 Bits
14 Bits
True
Differential
Inputs
AD7609
Number of
Simultaneous
Sampling Channels
8
8
6
4
8
FUNCTIONAL BLOCK DIAGRAM
AVCC
CLAMP
CLAMP
V2
CLAMP
V2GND
CLAMP
V3
CLAMP
V3GND
CLAMP
V4
V4GND
CLAMP
CLAMP
V5
CLAMP
V5GND
CLAMP
V6
CLAMP
V6GND
CLAMP
V7
CLAMP
V7GND
CLAMP
V8
CLAMP
V8GND
CLAMP
RFB
1MΩ
RFB
1MΩ
RFB
1MΩ
RFB
1MΩ
RFB
1MΩ
RFB
1MΩ
RFB
1MΩ
RFB
1MΩ
RFB
1MΩ
RFB
1MΩ
RFB
1MΩ
RFB
1MΩ
RFB
1MΩ
RFB
1MΩ
RFB
1MΩ
RFB
SECOND
ORDER LPF
AVCC
REGCAP
REGCAP
2.5V
LDO
2.5V
LDO
T/H
REFCAPB
REFCAPA
REFIN/REFOUT
SECOND
ORDER LPF
T/H
2.5V
REF
SECOND
ORDER LPF
T/H
REF SELECT
AGND
OS 2
OS 1
OS 0
SECOND
ORDER LPF
T/H
SERIAL
8:1
MUX
SECOND
ORDER LPF
T/H
18-BIT
SAR
DIGITAL
FILTER
PARALLEL/
SERIAL
INTERFACE
DOUTA
DOUTB
RD/SCLK
CS
PAR/SER SEL
VDRIVE
SECOND
ORDER LPF
T/H
PARALLEL
DB[15:0]
AD7608
SECOND
ORDER LPF
SECOND
ORDER LPF
T/H
CLK OSC
CONTROL
INPUTS
T/H
AGND
CONVST A CONVST B RESET RANGE
BUSY
FRSTDATA
08938-001
V1
V1GND
1MΩ
Figure 1.
1
Patent pending.
Rev. A
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113 ©2011-2012 Analog Devices, Inc. All rights reserved.
AD7608
Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1
Converter Details ....................................................................... 19
Applications ....................................................................................... 1
Analog Input ............................................................................... 19
Companion Products ....................................................................... 1
ADC Transfer Function ............................................................. 20
Functional Block Diagram .............................................................. 1
Internal/External Reference ...................................................... 21
Revision History ............................................................................... 2
Typical Connection Diagram ................................................... 22
General Description ......................................................................... 3
Power-Down Modes .................................................................. 22
Specifications..................................................................................... 4
Conversion Control ................................................................... 23
Timing Specifications .................................................................. 6
Digital Interface .............................................................................. 24
Absolute Maximum Ratings.......................................................... 10
Parallel Interface (PAR/SER SEL = 0) ...................................... 24
Thermal Resistance .................................................................... 10
Serial Interface (PAR/SER SEL = 1) ......................................... 25
ESD Caution ................................................................................ 10
Reading During Conversion ..................................................... 25
Pin Configuration and Function Descriptions ........................... 11
Digital Filter ................................................................................ 26
Typical Performance Characteristics ........................................... 14
Layout Guidelines....................................................................... 30
Terminology .................................................................................... 18
Outline Dimensions ....................................................................... 32
Theory of Operation ...................................................................... 19
Ordering Guide .......................................................................... 32
REVISION HISTORY
1/12—Rev. 0 to Rev. A
Changes to Analog Input Ranges Section ····································19
4/11—Revision 0: Initial Version
Rev. A | Page 2 of 32
Data Sheet
AD7608
GENERAL DESCRIPTION
The AD7608 is an 18-bit, 8-channel simultaneous sampling,
analog-to-digital data acquisition system (DAS). The part contains
analog input clamp protection, a second-order antialiasing filter,
a track-and-hold amplifier, an 18-bit charge redistribution
successive approximation analog-to-digital converter (ADC), a
flexible digital filter, a 2.5 V reference and reference buffer, and
high speed serial and parallel interfaces.
The AD7608 operates from a single 5 V supply and can
accommodate ±10 V and ±5 V true bipolar input signals while
sampling at throughput rates up to 200 kSPS for all channels.
The input clamp protection circuitry can tolerate voltages up
to ±16.5 V. The AD7608 has 1 MΩ analog input impedance
regardless of sampling frequency. The single supply operation,
on-chip filtering, and high input impedance eliminate the need
for driver op amps and external bipolar supplies. The AD7608
antialiasing filter has a 3 dB cutoff frequency of 22 kHz and
provides 40 dB antialias rejection when sampling at 200 kSPS.
The flexible digital filter is pin driven, yields improvements in
SNR, and reduces the 3 dB bandwidth.
Rev. A | Page 3 of 32
AD7608
Data Sheet
SPECIFICATIONS
VREF = 2.5 V external/internal, AVCC = 4.75 V to 5.25 V, VDRIVE = 2.3 V to 5.25 V; fSAMPLE = 200 kSPS, TA = TMIN to TMAX, unless otherwise noted. 1
Table 2.
Parameter
DYNAMIC PERFORMANCE
Signal-to-Noise Ratio (SNR) 2, 3
Signal-to-(Noise + Distortion) (SINAD)2
Dynamic Range
Total Harmonic Distortion (THD)2
Peak Harmonic or Spurious Noise (SFDR)2
Intermodulation Distortion (IMD)2
Second-Order Terms
Third-Order Terms
Channel-to-Channel Isolation2
ANALOG INPUT FILTER
Full Power Bandwidth
tGROUP DELAY
DC ACCURACY
Resolution
Differential Nonlinearity2
Integral Nonlinearity2
Total Unadjusted Error (TUE)
Positive Full-Scale Error2, 5
Positive Full-Scale Error Drift
Positive Full-Scale Error Matching2
Bipolar Zero Code Error2,
6
Bipolar Zero Code Error Drift
Bipolar Zero Code Error Matching2
Negative Full-Scale Error2, 5
Negative Full-Scale Error Drift
Negative Full-Scale Error Matching2
Test Conditions/Comments
fIN = 1 kHz sine wave unless otherwise noted
Oversampling by 16; ±10 V range; fIN = 130 Hz
Oversampling by 16; ±5 V range; fIN = 130 Hz
No oversampling; ±10 V range
No oversampling; ±5 V range
No oversampling; ±10 V range
No oversampling; ±5 V range
No oversampling; ±10 V range
No oversampling; ±5 V range
Min
Typ
98
95.5
89.5
88.5
88.5
88
99.5
97.5
90.9
90
90.5
89.5
91.5
90.5
−107
−108
Max
−95
Unit
dB
dB
dB
dB
dB
dB
dB
dB
dB
dB
fa = 1 kHz, fb = 1.1 kHz
fIN on unselected channels up to 160 kHz
−110
−106
−95
dB
dB
dB
−3 dB, ±10 V range
−3 dB, ±5 V range
−0.1 dB, ±10 V range
−0.1 dB, ±5 V range
±10 V range
±5 V range
23
15
10
5
11
15
kHz
kHz
kHz
kHz
µs
µs
No missing codes
±10 V range
±5 V range
External reference
Internal reference
External reference
Internal reference
±10 V range
±5 V range
±10 V range
± 5 V range
±10 V range
± 5 V range
±10 V range
±5 V range
External reference
Internal reference
External reference
Internal reference
±10 V range
±5 V range
Rev. A | Page 4 of 32
18
±0.75
±2.5
±15
±40
±15
±40
±2
±7
12
30
±3.5
±3.5
10
5
3
21
±15
±40
±4
±8
12
30
−0.99/+2.6
±7.5
±128
95
128
±24
±48
30
65
±128
95
128
Bits
LSB 4
LSB
LSB
LSB
LSB
LSB
ppm/°C
ppm/°C
LSB
LSB
LSB
LSB
µV/°C
µV/°C
LSB
LSB
LSB
LSB
ppm/°C
ppm/°C
LSB
LSB
Data Sheet
Parameter
ANALOG INPUT
Input Voltage Ranges
Analog Input Current
Input Capacitance 7
Input Impedance
REFERENCE INPUT/OUTPUT
Reference Input Voltage Range
DC Leakage Current
Input Capacitance7
Reference Output Voltage
Reference Temperature Coefficient
LOGIC INPUTS
Input High Voltage (VINH)
Input Low Voltage (VINL)
Input Current (IIN)
Input Capacitance (CIN)7
LOGIC OUTPUTS
Output High Voltage (VOH)
Output Low Voltage (VOL)
Floating-State Leakage Current
Floating-State Output Capacitance7
Output Coding
CONVERSION RATE
Conversion Time
Track-and-Hold Acquisition Time
Throughput Rate
POWER REQUIREMENTS
AVCC
VDRIVE
ITOTAL
Normal Mode (Static)
Normal Mode (Operational)8
Standby Mode
Shutdown Mode
Power Dissipation
Normal Mode (Static)
Normal Mode (Operational) 8
Standby Mode
Shutdown Mode
AD7608
Test Conditions/Comments
Min
RANGE = 1
RANGE = 0
10 V; see Figure 28
5 V; see Figure 28
Typ
Max
Unit
±10
±5
V
V
µA
µA
pF
MΩ
2.525
±1
V
µA
pF
V
5.4
2.5
5
1
2.475
REF SELECT = 1
REFIN/REFOUT
2.5
7.5
2.49/
2.505
±10
ppm/°C
0.9 × VDRIVE
0.1 × VDRIVE
±2
V
V
µA
pF
0.2
±20
V
V
µA
pF
5
ISOURCE = 100 µA
ISINK = 100 µA
VDRIVE − 0.2
±1
5
Twos complement
All eight channels included; see Table 3
4
1
200
µs
µs
kSPS
5.25
5.25
V
V
16
20
5
2
22
27
8
11
mA
mA
mA
µA
80
100
25
10
115.5
142
42
58
mW
mW
mW
µW
Per channel, all eight channels included
4.75
2.3
Digital inputs = 0 V or VDRIVE
fSAMPLE = 200 kSPS
fSAMPLE = 200 kSPS
Temperature range for B version is −40°C to +85°C.
See the Terminology section.
3
This specification applies when reading during a conversion or after a conversion. If reading during a conversion in parallel mode with VDRIVE = 5 V, SNR typically reduces by 1.5 dB
and THD by 3 dB.
4
LSB means least significant bit. With ±5 V input range, 1 LSB = 38.14 µV. With ±10 V input range, 1 LSB = 76.29 µV.
5
These specifications include the full temperature range variation and contribution from the internal reference buffer but do not include the error contribution from
the external reference.
6
Bipolar zero code error is calculated with respect to the analog input voltage.
7
Sample tested during initial release to ensure compliance.
8
Operational power/current figure includes contribution when running in oversampling mode.
1
2
Rev. A | Page 5 of 32
AD7608
Data Sheet
TIMING SPECIFICATIONS
AVCC = 4.75 V to 5.25 V, VDRIVE = 2.3 V to 5.25 V, VREF = 2.5 V external reference/internal reference, TA = TMIN to TMAX, unless otherwise noted. 1
Table 3.
Parameter
PARALLEL/SERIAL/BYTE MODE
tCYCLE
Limit at TMIN, TMAX
Min
Typ Max
Unit
5
µs
10.5
µs
µs
tWAKE-UP STANDBY
4.15
9.1
18.8
39
78
158
315
100
µs
µs
µs
µs
µs
µs
µs
µs
tWAKE-UP SHUTDOWN
Internal Reference
30
ms
External Reference
13
ms
5
Description
1/throughput rate
Parallel mode, reading during or after conversion; or serial mode: VDRIVE =
3.3 V to 5.25 V, reading during a conversion using DOUTA and DOUTB lines
Serial mode reading during conversion; VDRIVE = 2.7 V
Serial mode reading after a conversion; VDRIVE = 2.3 V, DOUTA and DOUTB lines
Conversion time
Oversampling off
Oversampling by 2
Oversampling by 4
Oversampling by 8
Oversampling by 16
Oversampling by 32
Oversampling by 64
STBY rising edge to CONVST x rising edge; power-up time from
standby mode
tCONV
3.45
7.87
16.05
33
66
133
257
tRESET
tOS_SETUP
tOS_HOLD
t1
t2
t3
t4
t5 2
t6
t7
PARALLEL/BYTE READ
OPERATION
t8
t9
t10
50
20
20
25
ns
ns
ns
ns
ns
ns
ns
ms
ns
ns
0
0
ns
ns
40
25
25
0
0.5
25
10F9F
t11
t12
4
E
A
STBY rising edge to CONVST x rising edge; power-up time from
shutdown mode
STBY rising edge to CONVST x rising edge; power-up time from
shutdown mode
RESET high pulse width
BUSY to OS x pin setup time
BUSY to OS x pin hold time
CONVST x high to BUSY high
Minimum CONVST x low pulse
Minimum CONVST x high pulse
BUSY falling edge to CS falling edge setup time
Maximum delay allowed between CONVST A, CONVST B rising edges
Maximum time between last CS rising edge and BUSY falling edge
Minimum delay between RESET low to CONVST x high
E
A
A
E
A
A
E
A
A
E
A
A
CS to RD setup time
CS to RD hold time
RD low pulse width
VDRIVE above 4.75 V
VDRIVE above 3.3 V
VDRIVE above 2.7 V
VDRIVE above 2.3 V
RD high pulse width
CS high pulse width (see Figure 5); CS and RD linked
E
E
A
A
A
A
E
E
A
A
A
A
E
A
A
16
21
25
32
15
22
ns
ns
ns
ns
ns
ns
E
A
A
E
A
E
A
Rev. A | Page 6 of 32
A
E
A
A
A
Data Sheet
Parameter
t13
AD7608
Limit at TMIN, TMAX
Min
Typ Max
Unit
E
ns
ns
ns
ns
t143
E
A
A
16
21
25
32
22
ns
ns
ns
ns
ns
ns
ns
23.5
17
14.5
11.5
MHz
MHz
MHz
MHz
6
6
E
A
t18
A
E
A
A
E
A
A
SERIAL READ OPERATION
fSCLK
Frequency of serial read clock
VDRIVE above 4.75 V
VDRIVE above 3.3 V
VDRIVE above 2.7 V
VDRIVE above 2.3 V
Delay from CS until DOUTA/DOUTB three-state disabled/delay from CS until
MSB valid
VDRIVE above 4.75 V
VDRIVE above 3.3 V
VDRIVE = 2.3 V to 2.7 V
Data access time after SCLK rising edge
VDRIVE above 4.75 V
VDRIVE above 3.3 V
VDRIVE above 2.7 V
VDRIVE above 2.3 V
SCLK low pulse width
SCLK high pulse width
SCLK rising edge to DOUTA/DOUTB valid hold time
CS rising edge to DOUTA/DOUTB three-state enabled
E
E
A
A
15
20
30
ns
ns
ns
17
23
27
34
ns
ns
ns
ns
ns
ns
22
ns
t19 3
1F
t20
t21
t22
t23
A
A
16
20
25
30
t15
t16
t17
Description
Delay from CS until DB[15:0] three-state disabled
VDRIVE above 4.75 V
VDRIVE above 3.3 V
VDRIVE above 2.7 V
VDRIVE above 2.3 V
Data access time after RD falling edge
VDRIVE above 4.75 V
VDRIVE above 3.3 V
VDRIVE above 2.7 V
VDRIVE above 2.3 V
Data hold time after RD falling edge
CS to DB[15:0] hold time
Delay from CS rising edge to DB[15:0] three-state enabled
0.4 tSCLK
0.4 tSCLK
7
FRSTDATA OPERATION
t24
A
E
A
A
Delay from CS falling edge until FRSTDATA three-state disabled
VDRIVE above 4.75 V
VDRIVE above 3.3 V
VDRIVE above 2.7 V
VDRIVE above 2.3 V
Delay from CS falling edge until FRSTDATA high, serial mode
VDRIVE above 4.75 V
VDRIVE above 3.3 V
VDRIVE above 2.7 V
VDRIVE above 2.3 V
Delay from RD falling edge to FRSTDATA high
VDRIVE above 4.75 V
VDRIVE above 3.3 V
VDRIVE above 2.7 V
VDRIVE above 2.3 V
E
A
A
15
20
25
30
t25
15
20
25
30
ns
ns
ns
ns
ns
ns
ns
ns
ns
t26
E
A
A
E
A
16
20
25
30
ns
ns
ns
ns
Rev. A | Page 7 of 32
A
A
AD7608
Data Sheet
Limit at TMIN, TMAX
Min
Typ Max
Parameter
t27
Unit
Description
Delay from RD falling edge to FRSTDATA low
VDRIVE = 3.3 V to 5.25 V
VDRIVE = 2.3 V to 2.7 V
Delay from 16th SCLK falling edge to FRSTDATA low
VDRIVE = 3.3 V to 5.25 V
VDRIVE = 2.3 V to 2.7 V
Delay from CS rising edge until FRSTDATA three-state enabled
E
A
A
19
24
ns
ns
17
22
24
ns
ns
ns
t28
t29
E
A
A
Sample tested during initial release to ensure compliance. All input signals are specified with tR = tF = 5 ns (10% to 90% of VDD) and timed from a voltage level of 1.6 V.
The delay between the CONVST x signals was measured as the maximum time allowed while ensuring a <40 LSB performance matching between channel sets.
3
A buffer is used on the data output pins for these measurements, which is equivalent to a load of 20 pF on the output pins.
1
2
Timing Diagrams
t5
CONVST A/
CONVST B
tCYCLE
CONVST A/
CONVST B
t2
t3
tCONV
t1
BUSY
t4
CS
t7
08938-002
tRESET
RESET
Figure 2.CONVST x Timing—Reading After a Conversion
t5
CONVST A/
CONVST B
tCYCLE
CONVST A/
CONVST B
t2
t3
tCONV
t1
BUSY
t6
CS
t7
08938-003
tRESET
RESET
Figure 3. CONVST x Timing—Reading During a Conversion
CS
t8
t16
t13
t14
DATA:
DB[15:0]
FRSTDATA
V1
[17:2]
INVALID
t24
t26
V1
[1:0]
t17
t15
V2
[17:2]
V2
[1:0]
V8
[17:2]
t27
V8
[1:0]
t29
Figure 4. Parallel Mode Separate CS and RD Pulses
E
A
Rev. A | Page 8 of 32
E
A
A
A
08938-004
RD
t9
t11
t10
Data Sheet
AD7608
t12
CS, RD
t16
t13
V1
[17:2]
V1
[1:0]
V2
[17:2]
V2
[1:0]
V7
[17:2]
V7
[1:0]
V8
[17:2]
t17
V8
[1:0]
08938-005
DATA:
DB[15:0]
FRSTDATA
Figure 5. CS and RD Linked Parallel Mode
E
A
E
A
A
A
CS
t21
SCLK
t19
t18
DOUTA,
DOUTB
t20
DB17
t22
DB16
DB15
t25
DB1
t23
DB0
t29
08938-006
t28
FRSTDATA
Figure 6. Serial Read Operation (Channel 1)
Rev. A | Page 9 of 32
AD7608
Data Sheet
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
Table 4.
Parameter
AVCC to AGND
VDRIVE to AGND
Analog Input Voltage to AGND1
Digital Input Voltage to AGND
Digital Output Voltage to AGND
REFIN to AGND
Input Current to Any Pin Except Supplies1
Operating Temperature Range
B Version
Storage Temperature Range
Junction Temperature
Pb/SN Temperature, Soldering
Reflow (10 sec to 30 sec)
Pb-Free Temperature, Soldering Reflow
ESD (All Pins Except Analog Inputs)
ESD (Analog Input Pins Only)
1
Rating
−0.3 V to +7 V
−0.3 V to AVCC + 0.3 V
±16.5 V
−0.3 V to VDRIVE + 0.3 V
−0.3 V to VDRIVE + 0.3 V
−0.3 V to AVCC + 0.3 V
±10 mA
−40°C to +85°C
−65°C to +150°C
150°C
240 (+0)°C
260 (+0)°C
2 kV
7 kV
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
THERMAL RESISTANCE
θJA is specified for the worst-case conditions, that is, a device
soldered in a circuit board for surface-mount packages. These
specifications apply to a 4-layer board.
Table 5. Thermal Resistance
Package Type
64-Lead LQFP
ESD CAUTION
Transient currents of up to 100 mA do not cause SCR latch-up.
Rev. A | Page 10 of 32
θJA
45
θJC
11
Unit
°C /W
Data Sheet
AD7608
64 63 62 61 60 59 58
V1GND
V1
V2
V3
V2GND
V4
V3GND
V5
V4GND
V6
V5GND
V6GND
V7
V7GND
V8
V8GND
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
57 56 55 54 53 52 51 50 49
48 AVCC
AVCC 1
ANALOG INPUT
PIN 1
AGND 2
OS 0 3
DECOUPLING CAPACITOR PIN
47 AGND
46 REFGND
POWER SUPPLY
OS 1 4
45 REFCAPB
GROUND PIN
OS 2 5
44 REFCAPA
PAR/SER SEL 6
DATA OUTPUT
REFERENCE INPUT/OUTPUT
42 REFIN/REFOUT
TOP VIEW
(Not to Scale)
RANGE 8
DIGITAL INPUT
43 REFGND
AD7608
STBY 7
DIGITAL OUTPUT
41 AGND
CONVST A 9
40 AGND
CONVST B 10
39 REGCAP
RESET 11
38 AVCC
RD/SCLK 12
37 AVCC
36 REGCAP
CS 13
BUSY 14
35 AGND
FRSTDATA 15
DB0 16
34 REF SELECT
33 DB15
08938-007
DB14
DB13
DB12
DB11
DB10
DB9
AGND
DB7/DOUTA
DB8/DOUTB
VDRIVE
DB6
DB5
DB4
DB3
DB2
DB1
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Figure 7. Pin Configuration
Table 6. Pin Function Descriptions
Pin No.
1, 37, 38, 48
Type 1
P
Mnemonic
AVCC
2, 26, 35,
40, 41, 47
P
AGND
5, 4, 3
DI
OS [2: 0]
6
DI
12F1F
PAR/SER SEL
E
A
A
Description
Analog Supply Voltage 4.75 V to 5.25 V. This supply voltage is applied to the internal front-end
amplifiers and to the ADC core. These supply pins should be decoupled to AGND.
Analog Ground. This pin is the ground reference point for all analog circuitry on the AD7608. All
analog input signals and external reference signals should be referred to these pins. All six of these
AGND pins should connect to the AGND plane of a system.
Oversampling Mode Pins. Logic inputs. These inputs are used to select the oversampling ratio. OS 2
is the MSB control bit, while OS 0 is the LSB control bit. See the Digital Filter section for further
details on the oversampling mode of operation and Table 8 for oversampling bit decoding.
Parallel/Serial Interface Selection Input. Logic input. If this pin is tied to a logic low, the parallel
interface is selected. If this pin is tied to a logic high, the serial interface is selected. In serial mode,
the RD/SCLK pin functions as the serial clock input. The DB7/DOUTA and DB8/DOUTB pins function as
serial data outputs. When the serial interface is selected, DB[15:9] and DB[6:0] pins should be tied to
GND.
Standby Mode Input. This pin is used to place the AD7608 into one of two power-down modes: standby
mode or shutdown mode. The power-down mode entered depends on the state of the RANGE pin
as shown in Table 7. When in standby mode, all circuitry, except the on-chip reference regulators,
and regulator buffers, is powered down. When in shutdown mode, all circuitry is powered down.
Analog Input Range Selection. Logic input. The polarity on this pin determines the input range of
the analog input channels. If this pin is tied to a logic high, the analog input range is ±10 V for all
channels. If this pin is tied to a logic low, the analog input range is ±5 V for all channels. A logic
change on this pin has an immediate effect on the analog input range. Changing this pin during
a conversion is not recommended. See the Analog Input section for more details.
Conversion Start Input A, Conversion Start Input B. Logic inputs. These logic inputs are used to
initiate conversions on the analog input channels. For simultaneous sampling of all input channels,
CONVST A and CONVST B can be shorted together and a single convert start signal applied.
Alternatively, CONVST A can be used to initiate simultaneous sampling for V1, V2, V3, and V4, and
CONVST B can be used to initiate simultaneous sampling on the other analog inputs (V5, V6, V7, and
V8). This is only possible when oversampling is not switched on.
When the CONVST A or CONVST B pin transitions from low to high, the front-end track-and-hold
circuitry for their respective analog inputs is set to hold. This function allows a phase delay to be
created inherently between the sets of analog inputs.
E
A
7
DI
8
DI
RANGE
9, 10
DI
CONVST A,
CONVST B
STBY
E
A
A
Rev. A | Page 11 of 32
AD7608
Data Sheet
Pin No.
11
Type 1
DI
12
DI
12F1F
Mnemonic
RESET
RD/SCLK
E
A
A
Description
Reset Input. When set to logic high, the rising edge of RESET resets the AD7608. Once tWAKE-UP has
elapsed, the part should receive a RESET pulse after power up. The RESET high pulse should be
typically 100 ns wide. If a RESET pulse is applied during a conversion, the conversion is aborted. If
a RESET pulse is applied during a read, the contents of the output registers resets to all zeros.
Parallel Data Read Control Input when Parallel Interface is Selected (RD)/Serial Clock Input when the
Serial Interface is Selected (SCLK). When both CS and RD are logic low in parallel mode, the output
bus is enabled.
In parallel mode, two RD pulses are required to read the full 18 bits of conversion results from each
channel. The first RD pulse outputs DB[17:2], the second RD pulse outputs DB[1:0].
In serial mode, this pin acts as the serial clock input for data transfers. The CS falling edge takes the
data output lines, DOUTA and DOUTB, out of three-state and clocks out the MSB of the conversion
result. The rising edge of SCLK clocks all subsequent data bits onto the DOUTA and DOUTB serial data
outputs. For further information, see the Conversion Control section.
Chip Select. This active low logic input frames the data transfer. When both CS and RD are logic low
in parallel mode, the output bus, DB[15:0], is enabled and the conversion result is output on the
parallel data bus lines. In serial mode, the CS is used to frame the serial read transfer and clock out
the MSB of the serial output data.
Busy Output. This pin transitions to a logic high after both CONVST A and CONVST B rising edges
and indicates that the conversion process has started. The BUSY output remains high until the
conversion process for all channels is complete. The falling edge of BUSY signals that the conversion
data is being latched into the output data registers and is available to be read after a Time t4. Any
data read while BUSY is high must be complete before the falling edge of BUSY occurs. Rising edges
on CONVST A or CONVST B have no effect while the BUSY signal is high.
Digital Output. The FRSTDATA output signal indicates when the first channel, V1, is being read back
on either the parallel or serial interface. When the CS input is high, the FRSTDATA output pin is in
three-state. The falling edge of CS takes FRSTDATA out of three-state. In parallel mode, the falling
edge of RD corresponding to the result of V1 then sets the FRSTDATA pin high indicating that the
result from V1 is available on the output data bus. The FRSTDATA output returns to a logic low
following the third falling edge of RD. In serial mode, FRSTDATA goes high on the falling edge of CS
as this clocks out the MSB of V1 on DOUTA. It returns low on the 18th SCLK falling edge after the CS
falling edge. See the Conversion Control section for more details.
Parallel Output Data Bits, DB6 to DB0. When PAR/SER SEL = 0, these pins act as three-state parallel
digital output pins. When CS and RD are low, these pins are used to output DB8 to DB2 of the
conversion result during the first RD pulse and output 0 during the second RD pulse. When PAR/SER
SEL = 1, these pins should be tied to GND.
Logic Power Supply Input. The voltage (2.3 V to 5.25 V) supplied at this pin determines the
operating voltage of the interface. This pin is nominally at the same supply as the supply of the host
interface (that is, DSP and FPGA).
Parallel Output Data Bit 7 (DB7)/Serial Interface Data Output Pin (DOUTA). When PAR/SER SEL = 0, this
pin acts as a three-state parallel digital output pin. When CS and RD are low, this pin is used to
output DB9 of the conversion result. When PAR/SER SEL = 1, this pin functions as DOUTA and outputs
serial conversion data. See the Conversion Control section for further details.
Parallel Output Data Bit 8 (DB8)/Serial Interface Data Output Pin (DOUTB). When PAR/SER SEL = 0, this
pin acts as a three-state parallel digital output pin. When CS and RD are low, this pin is used to
output DB10 of the conversion result. When PAR/SER SEL = 1, this pin functions as DOUTB and
outputs serial conversion data. See the Conversion Control section for further details.
Parallel Output Data Bits, DB13 to DB9. When PAR/SER SEL = 0, these pins act as three-state parallel
digital output pins. When CS and RD are low, these pins are used to output DB15 to DB11 of the
conversion result during the first RD pulse and output zero during the second RD pulse. When
PAR/SER SEL = 1, these pins should be tied to GND.
Parallel Output Data Bit 14 (DB14). When PAR/SER SEL = 0, this pin act as three-state parallel digital
output pin. When CS and RD are low, this pin is used to output DB16 of the conversion result during the
first RD pulse and DB0 of the same conversion result during the second RD pulse. When PAR/SER
SEL = 1, this pins should be tied to GND.
Parallel Output Data Bit 15 (DB15). When PAR/SER SEL = 0, this pin acts as three-state parallel digital
output pin. This pin is used to output DB17 of the conversion result during the first RD pulse and
DB1 of the same conversion result during the second RD pulse. When PAR/SER SEL = 1, this pins
should be tied to GND.
E
A
A
E
E
A
A
A
A
E
A
A
E
E
A
A
A
A
E
A
A
13
DI
CS
E
A
E
A
E
A
A
A
E
A
A
14
DO
BUSY
15
DO
FRSTDATA
E
A
A
E
A
A
E
A
A
E
E
A
A
A
A
E
A
A
22 to 16
DO
DB[6:0]
E
A
A
E
E
A
A
A
A
E
E
A
A
23
P
VDRIVE
24
DO
DB7/DOUTA
A
E
A
A
A
E
A
A
E
E
A
A
A
A
E
A
A
25
DO
DB8/DOUTB
E
A
A
E
A
A
E
A
A
E
A
A
31 to 27
DO
DB[13:9]
E
A
E
A
E
A
A
A
A
E
A
E
A
A
A
E
A
32
DO/DI
DB14
A
E
A
A
E
A
A
E
A
A
E
A
33
DO/DI
DB15
E
A
E
A
A
A
E
A
A
E
A
E
A
Rev. A | Page 12 of 32
E
A
A
A
A
A
Data Sheet
AD7608
Pin No.
34
Type 1
DI
Mnemonic
REF SELECT
36, 39
P
REGCAP
42
REF
REFIN/
REFOUT
43, 46
44, 45
REF
REF
49, 51, 53,
55, 57, 59,
61, 63
50, 52, 54,
56, 58, 60,
62, 64
AI
REFGND
REFCAPA,
REFCAPB
V1 to V8
AI/
GND
V1GND to
V8GND
1
12F1F
Description
Internal/External Reference Selection Input. Logic input. If this pin is set to logic high then the
internal reference is selected and is enabled, if this pin is set to logic low then the internal reference
is disabled and an external reference voltage must be applied to the REFIN/REFOUT pin.
Decoupling Capacitor Pins for Voltage Output from Internal Regulator. These output pins should be
decoupled separately to AGND using a 1 μF capacitor. The voltage on these output pins is in the
range of 2.5 V to 2.7 V.
Reference Input/Reference Output. The on-chip reference of 2.5 V is available on this pin for external
use if the REF SELECT pin is set to a logic high. Alternatively, the internal reference can be disabled
by setting the REF SELECT pin to a logic low and an external reference of 2.5 V can be applied to this
input. See the Internal/External Reference section. Decoupling is required on this pin for both the
internal or external reference options. A 10 µF capacitor should be applied from this pin to ground
close to the REFGND pins.
Reference Ground Pins. These pins should be connected to AGND.
Reference Buffer Output Force/Sense Pins. These pins must be connected together and decoupled
to AGND using a low ESR 10 μF ceramic capacitor.
Analog Inputs. These pins are single-ended analog inputs. The analog input range of these channels
is determined by the RANGE pin.
Analog Input Ground Pins. These pins correspond to the V1 to V8 analog input pins. Connect all
analog input AGND pins to the AGND plane of a system.
Refers to classification of pin type; P denotes power, AI denotes analog input, REF denotes reference, DI denotes digital input, DO denotes digital output.
Rev. A | Page 13 of 32
AD7608
Data Sheet
TYPICAL PERFORMANCE CHARACTERISTICS
4.0
3.5
AVCC, VDRIVE = 5V
INTERNAL REFERENCE
fSAMPLE = 200 kSPS
TA = 25°C
±10V RANGE
SNR = 91.23dB
SINAD = 91.17dB
THD = 108.69dB
16384 POINT FFT
fIN = 1kHz
60k
70k
80k
90k
100k
Figure 12. Typical DNL, ±10 V Range
–80
–100
–2.0
–2.5
–3.0
–140
–3.5
–4.0
AVCC, VDRIVE = 5V
INTERNAL REFERENCE
fSAMPLE = 200 kSPS
TA = 25°C
±5V RANGE
0
–120
–160
0
1k
2k
3k
4k
5k
INPUT FREQUENCY (Hz)
6k
08938-109
AMPLITUDE (dB)
–60
INL (LSB)
AVCC, VDRIVE = 5V
INTERNAL REFERENCE
fSAMPLE = 12.5 kSPS
TA = 25°C
±10V RANGE
SNR = 100.26dB
SINAD = 100.15dB
THD = –115.21dB
16384 POINT FFT
fIN = 131Hz
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
Figure 10. FFT Over Sampling by 16, ±10 V Range
25,000
0
–40
08938-010
CODE
Figure 9. FFT Plot, ±5 V Range
–20
08938-011
50k
INPUT FREQUENCY (Hz)
CODE
Figure 13. Typical INL, ±5 V Range
Rev. A | Page 14 of 32
08938-012
40k
250,000
30k
250,000
20k
225,000
10k
200,000
0
250,000
262,144
–160
08938-009
0
–140
200,000
–120
AVCC, VDRIVE = 5V
INTERNAL REFERENCE
fSAMPLE = 200 kSPS
TA = 25°C
±10V RANGE
175,000
–100
175,000
–80
150,000
AMPLITUDE (dB)
–60
DNL (LSB)
AVCC, VDRIVE = 5V
INTERNAL REFERENCE
fSAMPLE = 200 kSPS
TA = 25°C
±5V RANGE
SNR = 90.46dB
SINAD = 90.43dB
THD = 110.74dB
16384 POINT FFT
fIN = 1kHz
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–0.6
–0.7
–0.8
–0.9
–1.0
25,000
0
–40
225,000
CODE
Figure 11. Typical INL, ±10 V Range
Figure 8. FFT Plot, ±10 V Range
–20
225,000
100k
200,000
90k
175,000
80k
150,000
70k
125,000
60k
125,000
50k
100,000
40k
INPUT FREQUENCY (Hz)
100,000
30k
75,000
20k
75,000
10k
150,000
0
0
08938-008
–160
125,000
–3.5
–4.0
100,000
–140
75,000
–120
–2.0
–2.5
–3.0
–100
50,000
–1.0
–1.5
50,000
–80
0
–0.5
50,000
SNR (dB)
–60
1.0
0.5
25,000
–40
2.0
1.5
INL (LSB)
–20
AVCC, VDRIVE = 5V
INTERNAL REFERENCE
fSAMPLE = 200 kSPS
TA = 25°C
±10V RANGE
3.0
2.5
0
AD7608
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–0.6
–0.7
–0.8
–0.9
–1.0
40
AVCC, VDRIVE = 5V
INTERNAL REFERENCE
fSAMPLE = 200 kSPS
TA = 25°C
±5V RANGE
PFS ERROR
24
16
NFS ERROR
8
0
–8
–16
–24
±10V RANGE
AVCC, VDRIVE = 5V
EXTERNAL REFERENCE
08938-013
250,000
262,144
225,000
200,000
175,000
150,000
125,000
100,000
75,000
50,000
25,000
–32
–40
–40
–25
–10
35
50
65
80
10
80
60
8
PFS/NFS ERROR (%FS)
±10V RANGE
20
±5V RANGE
0
–20
6
4
AVCC, VDRIVE = 5V
fSAMPLE = 200 kSPS
TA = 25°C
EXTERNAL REFERENCE
SOURCE RESISTANCE IS MATCHED ON
THE VxGND INPUT
±10V AND ±5V RANGE
2
–40
0
200kSPS
AVCC, VDRIVE = 5V
EXTERNAL REFERENCE
–25
–10
5
20
35
50
65
80
TEMPERATURE (°C)
–2
08938-017
–60
0
20k
40k
60k
80k
100k
120k
SOURCE RESISTANCE (Ω)
08938-019
40
NFS ERROR (LSB)
20
Figure 17. NFS/PFS Error Matching
Figure 14. Typical DNL, ±5 V Range
–80
–40
5
TEMPERATURE (°C)
CODE
08938-018
NFS/PFS CHANNEL MATCHING (LSB)
32
0
DNL (LSB)
Data Sheet
Figure 18. PFS/NFS Error vs. Source Resistance
Figure 15. NFS Error vs. Temperature
80
105
60
100
0
±5V RANGE
–20
±10V RANGE
95
90
–40
200kSPS
AVCC, VDRIVE = 5V
EXTERNAL REFERENCE
–60
–80
–40
–25
–10
5
20
35
50
TEMPERATURE (°C)
65
80
OS × 64
OS × 32
OS × 16
OS × 8
OS × 4
OS × 2
NO OS
85
AVCC, VDRIVE = 5V
fSAMPLE CHANGES WITH OS RATE
TA = 25°C
INTERNAL REFERENCE
±10V RANGE
80
10
100
1k
INPUT FREQUENCY (Hz)
Figure 16. PFS Error vs. Temperature
10k
100k
08938-119
SNR (dB)
20
08938-118
PFS ERROR (LSB)
40
Figure 19. SNR vs. Input Frequency for Different Oversampling Rates, ±10 V Range
Rev. A | Page 15 of 32
AD7608
Data Sheet
105
4.0
90
AVCC, VDRIVE = 5V
fSAMPLE CHANGES WITH OS RATE
TA = 25°C
INTERNAL REFERENCE
±5V RANGE
1k
10k
0
±5V RANGE
–0.8
–1.6
±10V RANGE
–2.4
200kSPS
AVCC, VDRIVE = 5V
EXTERNAL REFERENCE
100k
INPUT FREQUENCY (Hz)
–4.0
–40
–25
–10
5
20
35
50
65
80
TEMPERATURE (°C)
Figure 20. SNR vs. Input Frequency for Different Oversampling Rates, ±5 V Range
Figure 23. Bipolar Zero Code Error vs. Temperature
–40
BIPOLAR ZERO CODE ERROR MATCHING (LSB)
±10V RANGE
AVCC, VDRIVE = 5V
–50 f
SAMPLE = 200kSPS
RSOURCE MATCHED ON Vx AND VxGND INPUTS
–60
–80
105kΩ
48.7kΩ
23.7kΩ
10kΩ
5kΩ
1.2kΩ
100Ω
51Ω
0Ω
–90
–100
–110
–120
1k
10k
100k
INPUT FREQUENCY (Hz)
08938-021
THD (dB)
–70
CHANNEL-TO-CHANNEL ISOLATION (dB)
–60
–80
105kΩ
48.7kΩ
23.7kΩ
10kΩ
5kΩ
1.2kΩ
100Ω
51Ω
0Ω
10k
INPUT FREQUENCY (Hz)
100k
08938-122
THD (dB)
–70
–120
1k
4
±10V RANGE
0
–4
–8
200kSPS
AVCC, VDRIVE = 5V
EXTERNAL REFERENCE
–12
–16
–40
–25
–10
5
20
35
50
65
80
–50
±5V RANGE
AVCC, VDRIVE = 5V
–50 f
SAMPLE = 200kSPS
RSOURCE MATCHED ON Vx AND VxGND INPUTS
–110
±5V RANGE
8
Figure 24. Bipolar Zero Code Error Matching Between Channels
–40
–100
12
TEMPERATURE (°C)
Figure 21. THD vs. Input Frequency for Various Source Impedances, ±10 V Range
–90
16
08938-024
100
0.8
–3.2
80
10
1.6
Figure 22. THD vs. Input Frequency for Various Source Impedances, ±5 V Range
Rev. A | Page 16 of 32
AVCC, VDRIVE = 5V
INTERNAL REFERENCE
AD7608 RECOMMENDED DECOUPLING USED
fSAMPLE = 150kSPS
–70 T = 25°C
A
INTERFERER ON ALL UNSELECTED CHANNELS
–80
–60
–90
±10V RANGE
–100
±5V RANGE
–110
–120
–130
–140
0
20
40
60
80
100
120
NOISE FREQUENCY (kHz)
Figure 25. Channel-to-Channel Isolation
140
160
08938-025
OS × 64
OS × 32
OS × 16
OS × 8
OS × 4
OS × 2
NO OS
85
08938-120
SNR (dB)
95
2.4
08938-023
BIPOLAR ZERO CODE ERROR (LSB)
3.2
100
Data Sheet
AD7608
110
22
20
±5V RANGE
95
90
AVCC, VDRIVE = 5V
TA = 25 °C
INTERNAL REFERENCE
fSAMPLE SCALES WITH OS RATIO
fIN SCALES WITH OS RATIO
85
80
NO OS
OS × 2
OS × 4 OS × 8 OS × 16 OS × 32 OS × 64
OVERSAMPLING RATIO
18
16
14
12
AVCC, VDRIVE = 5V
10 TA = 25°C
INTERNAL REFERENCE
fSAMPLE VARIES WITH OS RATE
8
NO OS
OS2
OS4
OS8
AVCC = 5.25V
AVCC = 5V
2.5000
2.4995
AVCC = 4.75V
2.4990
2.4980
–40
–25
–10
5
20
35
50
65
80
TEMPERATURE (°C)
08938-129
2.4985
Figure 27. Reference Output Voltage vs. Temperature for Different
Supply Voltages
AVCC, VDRIVE = 5V
6 fSAMPLE = 200kSPS
4
2
0
–2
–4
–6
–6
–4
–2
0
2
INPUT VOLTAGE (V)
4
6
8
10
08938-028
+85°C
+25°C
–40°C
–8
120
±10V RANGE
110
±5V RANGE
100
90
80
AVCC, VDRIVE = 5V
INTERNAL REFERENCE
AD7608 RECOMMENDED DECOUPLING USED
fSAMPLE = 200kSPS
TA = 25°C
70
60
0
100
200
300
400
500
600
Figure 28. Analog Input Current vs. Input Voltage Across Temperature
Rev. A | Page 17 of 32
700
800
AVCC NOISE FREQUENCY (kHz)
Figure 30. PSRR
8
–8
130
900 1000 1100
08938-130
POWER SUPPLY REJECTION RATIO (dB)
2.5005
REFOUT VOLTAGE (V)
OS64
140
2.5010
INPUT CURRENT (µA)
OS32
Figure 29. Supply Current vs. Oversampling Rate
Figure 26. Dynamic Range vs. Oversampling Ratio
–10
–10
OS16
OVERSAMPLING RATIO
08938-027
AVCC SUPPLY CURRENT (mA)
±10V RANGE
100
08938-026
DYNAMIC RANGE (dB)
105
AD7608
Data Sheet
TERMINOLOGY
Integral Nonlinearity
The maximum deviation from a straight line passing through
the endpoints of the ADC transfer function. The endpoints of
the transfer function are zero scale, at ½ LSB below the first
code transition; and full scale, at ½ LSB above the last code
transition.
Differential Nonlinearity
The difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Bipolar Zero Code Error
The deviation of the midscale transition (all 1s to all 0s) from
the ideal, which is 0 V − ½ LSB.
Bipolar Zero Code Error Match
The absolute difference in bipolar zero code error between any
two input channels.
Positive Full-Scale Error
The deviation of the actual last code transition from the ideal
last code transition (10 V − 1½ LSB (9.99988) and 5 V − 1½ LSB
(4.99994)) after bipolar zero code error is adjusted out. The
positive full-scale error includes the contribution from the
internal reference buffer.
Positive Full-Scale Error Match
The absolute difference in positive full-scale error between any
two input channels.
Negative Full-Scale Error
The deviation of the first code transition from the ideal first
code transition (−10 V + ½ LSB (−9.99996) and −5 V + ½ LSB
(−4.99998)) after the bipolar zero code error is adjusted out.
The negative full-scale error includes the contribution from
the internal reference buffer.
Negative Full-Scale Error Match
The absolute difference in negative full-scale error between any
two input channels.
Signal-to-(Noise + Distortion) Ratio
The measured ratio of signal-to-(noise + distortion) at the
output of the ADC. The signal is the rms amplitude of the
fundamental. Noise is the sum of all nonfundamental signals
up to half the sampling frequency (fS/2, excluding dc).
The ratio depends on the number of quantization levels in
the digitization process; the more levels, the smaller the
quantization noise.
The theoretical signal-to-(noise + distortion) ratio for an ideal
N-bit converter with a sine wave input is given by
Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB
Thus, for an 18-bit converter, the signal-to-(noise + distortion)
is 110.12 dB.
Total Harmonic Distortion (THD)
The ratio of the rms sum of the harmonics to the fundamental.
For the AD7608, it is defined as
THD (dB) =
20log
V2 2 + V3 2 + V4 2 + V5 2 + V6 2 + V7 2 + V8 2 + V9 2
V1
where:
V1 is the rms amplitude of the fundamental.
V2 to V9 are the rms amplitudes of the second through ninth
harmonics.
Peak Harmonic or Spurious Noise
The ratio of the rms value of the next largest component in the
ADC output spectrum (up to fS/2, excluding dc) to the rms value
of the fundamental. Normally, the value of this specification is
determined by the largest harmonic in the spectrum, but for
ADCs where the harmonics are buried in the noise floor, it is
determined by a noise peak.
Intermodulation Distortion (IMD)
With inputs consisting of sine waves at two frequencies, fa and fb,
any active device with nonlinearities creates distortion products
at sum and difference frequencies of mfa ± nfb, where m, n = 0,
1, 2, 3. Intermodulation distortion terms are those for which
neither m nor n is equal to 0. For example, the second-order
terms include (fa + fb) and (fa − fb), and the third-order terms
include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb).
The calculation of the intermodulation distortion is per the
THD specification, where it is the ratio of the rms sum of the
individual distortion products to the rms amplitude of the
sum of the fundamentals expressed in decibels (dB).
Power Supply Rejection Ratio (PSRR)
Variations in power supply affect the full-scale transition but not
the converter’s linearity. PSR is the maximum change in fullscale transition point due to a change in power supply voltage
from the nominal value. The PSR ratio (PSRR) is defined as the
ratio of the power in the ADC output at full-scale frequency, f,
to the power of a 100 mV p-p sine wave applied to the ADC’s
VDD and VSS supplies of Frequency fS.
PSRR (dB) = 10 log (Pf/PfS)
where:
Pf is equal to the power at Frequency f in the ADC output.
PfS is equal to the power at Frequency fS coupled onto the AVCC
supply.
Channel-to-Channel Isolation
Channel-to-channel isolation is a measure of the level of crosstalk
between all input channels. It is measured by applying a full-scale
sine wave signal, up to 160 kHz, to all unselected input channels
and then determining the degree to which the signal attenuates
in the selected channel with a 1 kHz sine wave signal applied (see
Figure 25).
Rev. A | Page 18 of 32
Data Sheet
AD7608
THEORY OF OPERATION
CONVERTER DETAILS
Analog Input Clamp Protection
The AD7608 is a data acquisition system that employs a high
speed, low power, charge redistribution, successive approximation analog-to-digital converter (ADC) and allows the
simultaneous sampling of eight analog input channels. The
analog inputs on the AD7608 can accept true bipolar input
signals. The RANGE pin is used to select either ±10 V or
±5 V as the input range. The AD7608 operates from a single
5 V supply.
Figure 31 shows the analog input structure of the AD7608. Each
AD7608 analog input contains clamp protection circuitry. Despite
single 5 V supply operation, this analog input clamp protection
allows for an input overvoltage up to ±16.5 V.
ANALOG INPUT
Vx
CLAMP
VxGND
CLAMP
1MΩ
SECONDORDER
LPF
RFB
08938-029
The AD7608 contains input clamp protection, input signal
scaling amplifiers, a second-order antialiasing filter, track-andhold amplifiers, an on-chip reference, reference buffers, a high
speed ADC, a digital filter, and high speed parallel and serial
interfaces. Sampling on the AD7608 is controlled using the
CONVST x signals.
RFB
1MΩ
Figure 31. Analog Input Circuitry
Figure 32 shows the voltage vs. current characteristic of the
clamp circuit. For input voltages of up to ±16.5 V, no current
flows in the clamp circuit. For input voltages that are above
±16.5 V, the AD7608 clamp circuitry turns on.
30
Analog Input Ranges
When in a power-down mode, it is recommended to tie the
analog inputs to GND. As per the input clamp protection section,
the overvoltage clamp protection is recommended for use in
transient overvoltage conditions and should not remain active
for extended periods. Stressing the analog inputs outside of the
conditions mentioned here may degrade the Bipolar Zero Code
error and THD performance of the AD7608.
10
0
–10
–20
–40
–25
–20
–15
–10
–5
0
5
10
15
20
25
SOURCE VOLTAGE (V)
08938-030
AVCC, VDRIVE = 5V
TA = 25 °C
–30
Figure 32. Input Protection Clamp Profile
A series resistor should be placed on the analog input channels
to limit the current to ±10 mA for input voltages above ±16.5 V.
In an application where there is a series resistance on an analog
input channel, Vx, a corresponding resistance is required on the
analog input GND channel, VxGND (see Figure 33). If there is
no corresponding resistor on the VxGND channel, an offset
error occurs on that channel.
Analog Input Impedance
The analog input impedance of the AD7608 is 1 MΩ. This is
a fixed input impedance that does not vary with the AD7608
sampling frequency. This high analog input impedance eliminates the need for a driver amplifier in front of the AD7608,
allowing for direct connection to the source or sensor. With
the need for a driver amplifier eliminated, bipolar supplies
(which are often a source of noise in a system) can be
removed from the signal chain.
Rev. A | Page 19 of 32
RFB
AD7608
ANALOG
INPUT
SIGNAL
R
R C
Vx
VxGND
CLAMP
CLAMP
1MΩ
1MΩ
RFB
Figure 33. Input Resistance Matching on the Analog Input
08938-031
During normal operation, the applied analog input voltage
should remain within the analog input range selected via the
range pin. A RESET pulse must be applied after power-up to
ensure the analog input channels are configured for the range
selected.
20
INPUT CLAMP CURRENT
The AD7608 can handle true bipolar, single-ended input voltages.
The logic level on the RANGE pin determines the analog input
range of all analog input channels. If this pin is tied to a logic
high, the analog input range is ±10 V for all channels. If this pin
is tied to a logic low, the analog input range is ±5 V for all
channels. A logic change on the RANGE pin has an immediate
effect on the analog input range; how-ever, there is typically
a settling time of approximately 80 µs, in addition to the normal
acquisition time requirement. The recommended practice is to
hardwire the RANGE pin according to the desired input range
for the system signals.
AD7608
Data Sheet
Analog Input Antialiasing Filter
An analog antialiasing filter (a second-order Butterworth) is also
provided on the AD7608. Figure 34 and Figure 35 show the
frequency and phase response, respectively, of the analog
antialiasing filter. In the ±5 V range, the −3 dB frequency is
typically 15 kHz. In the ±10 V range, the −3 dB frequency is
typically 23 kHz.
5
±5V RANGE
–15
–20
–25
–30
–35
±10V RANGE
–40
+25
+85
0.1dB
10,303Hz
9619Hz
9326Hz
3dB
24,365Hz
23,389Hz
22,607Hz
±5V RANGE
–40
+25
+85
0.1dB
5225Hz
5225Hz
4932Hz
3dB
16,162Hz
15,478Hz
14,990Hz
–40
100
1k
10k
08938-135
ATTENUATION (dB)
–5
–10
±10V RANGE
AVCC, VDRIVE = 5V
fSAMPLE = 200kSPS
TA = 25°C
100k
INPUT FREQUENCY (Hz)
Figure 34. Analog Antialiasing Filter Frequency Response
16
ADC TRANSFER FUNCTION
±5V RANGE
12
±10V RANGE
10
VIN
× 131,072 ×
10V
VIN
±5V CODE =
× 131,072 ×
5V
±10V CODE =
8
6
011...111
011...110
2
ADC CODE
4
AVCC, VDRIVE = 5V
fSAMPLE = 200kSPS
TA = 25°C
0
–2
100
1k
10k
INPUT FREQUENCY (Hz)
100k
08938-033
PHASE DELAY (µs)
The conversion clock for the part is internally generated, and
the conversion time for all channels is 4 µs on the AD7608. The
BUSY signal returns low after all eight conversions to indicate the
end of the conversion process. On the falling edge of BUSY, the
track-and-hold amplifiers return to track mode. New data can
be read from the output register via the parallel, parallel byte, or
serial interface after BUSY goes low; or, alternatively, data from
the previous conversion can be read while BUSY is high. Reading
data from the AD7608 while a conversion is in progress has little
effect on performance and allows a faster throughput to be
achieved. In parallel mode at VDRIVE > 3.3 V, the SNR is reduced
by ~1.5 dB when reading during a conversion.
The output coding of the AD7608 is twos complement. The
designed code transitions occur midway between successive
integer LSB values, that is, 1/2 LSB, 3/2 LSB. The LSB size is
FSR/262,144 for the AD7608. The ideal transfer characteristic
for the AD7608 is shown in Figure 36.
18
14
The end of the conversion process across all eight channels is
indicated by the falling edge of BUSY; and it is at this point that the
track-and-holds return to track mode, and the acquisition time
for the next set of conversions begins.
000...001
000...000
111...111
REF
2.5V
REF
2.5V
LSB =
+FS – (–FS)
218
100...010
100...001
100...000
–FS + 1/2LSB
Figure 35. Analog Antialiasing Filter Phase Response
0V – 1LSB +FS – 3/2LSB
ANALOG INPUT
Track-and-Hold Amplifiers
The track-and-hold amplifiers on the AD7608 allow the ADC
to accurately acquire an input sine wave of full-scale amplitude
to 18-bit resolution. The track-and-hold amplifiers sample
their respective inputs simultaneously on the rising edge of
CONVST x. The aperture time for track-and-hold (that is, the
delay time between the external CONVST x signal and the
+FS
±10V RANGE +10V
±5V RANGE +5V
MIDSCALE
0V
0V
–FS
–10V
–5V
LSB
76.29µV
38.15µV
08938-034
0
track-and-hold actually going into hold) is well matched, by design,
across all eight track-and-holds on one device and from device
to device. This matching allows more than one AD7608 device
to be sampled simultaneously in a system.
Figure 36. AD7608 Transfer Characteristic
The LSB size is dependent on the analog input range selected.
Rev. A | Page 20 of 32
Data Sheet
AD7608
INTERNAL/EXTERNAL REFERENCE
Internal Reference Mode
The AD7608 contains an on-chip 2.5 V band gap reference. The
REFIN/REFOUT pin allows access to the 2.5 V reference that
generates the on-chip 4.5 V reference internally, or it allows an
external reference of 2.5 V to be applied to the AD7608. An
externally applied reference of 2.5 V is also gained up to 4.5 V,
using the internal buffer. This 4.5 V buffered reference is the
reference used by the SAR ADC.
One AD7608 device, configured to operate in the internal reference mode, can be used to drive the remaining AD7608 devices,
which are configured to operate in external reference mode (see
Figure 39). The REFIN/REFOUT pin of the AD7608, configured
in internal reference mode, should be decoupled using a 10 µF
ceramic decoupling capacitor. The other AD7608 devices,
configured in external reference mode, should use at least a
100 nF decoupling capacitor on their REFIN/REFOUT pins.
REFIN/REFOUT
SAR
10µF
REFCAPA
2.5V
REF
Figure 37. Reference Circuitry
The AD7608 contains a reference buffer configured to gain the
REF voltage up to ~4.5 V, as shown in Figure 37. The REFCAPA
and REFCAPB pins must be shorted together externally, and a
ceramic capacitor of 10 μF applied to REFGND, to ensure that
the reference buffer is in closed-loop operation. The reference
voltage available at the REFIN/REFOUT pin is 2.5 V.
AD7608
AD7608
AD7608
REF SELECT
REF SELECT
REF SELECT
REFIN/REFOUT
REFIN/REFOUT
REFIN/REFOUT
100nF
100nF
100nF
08938-037
ADR421
0.1µF
Figure 38. Single External Reference Driving Multiple AD7608 REFIN Pins
External Reference Mode
One ADR421 external reference can be used to drive the
REFIN/REFOUT pins of all AD7608 devices (see Figure 38).
In this configuration, each REFIN/REFOUT pin of the AD7608
should be decoupled with at least a 100 nF decoupling capacitor.
VDRIVE
AD7608
AD7608
AD7608
REF SELECT
REF SELECT
REF SELECT
REFIN/REFOUT
REFIN/REFOUT
REFIN/REFOUT
+
10µF
100nF
100nF
Figure 39. Internal Reference Driving Multiple AD7608 REFIN Pins
Rev. A | Page 21 of 32
08938-036
When the AD7608 is configured in external reference mode,
the REFIN/REFOUT pin is a high input impedance pin. For
applications using multiple AD7608 devices, the following
configurations are recommended, depending on the application
requirements.
REFCAPB
BUF
08938-035
The REF SELECT pin is a logic input pin that allows the user to
select between the internal reference or an external reference.
If this pin is set to logic high, the internal reference is selected
and enabled. If this pin is set to logic low, the internal reference
is disabled and an external reference voltage must be applied
to the REFIN/REFOUT pin. The internal reference buffer is
always enabled. After a reset, the AD7608 operates in the reference
mode selected by the REF SELECT pin. Decoupling is required
on the REFIN/REFOUT pin for both the internal and external
reference options. A 10 µF ceramic capacitor is required on the
REFIN/REFOUT pin.
AD7608
Data Sheet
TYPICAL CONNECTION DIAGRAM
The power-down mode is selected through the state of the
RANGE pin when the STBY pin is low. Table 7 shows the
configurations required to choose the desired power-down
mode. When the AD7608 is placed in standby mode, the
current consumption is 8 mA maximum and power-up time
is approximately 100 µs because the capacitor on the REFCAPA
and REFCAPB pins must charge up. In standby mode, the
on-chip reference and regulators remain powered up, and the
amplifiers and ADC core are powered down.
E
Figure 40 shows the typical connection diagram for the AD7608.
There are four AVCC supply pins on the part, and each of the
four pins should be decoupled using a 100 nF capacitor at each
supply pin and a 10 µF capacitor at the supply source. The AD7608
can operate with the internal reference or an externally applied
reference. In this configuration, the AD7608 is configured to
operate with the internal reference. When using a single AD7608
device on the board, the REFIN/REFOUT pin should be
decoupled with a 10 µF capacitor. Refer to the Internal/External
Reference section when using an application with multiple
AD7608 devices. The REFCAPA and REFCAPB pins are
shorted together and decoupled with a 10 µF ceramic
capacitor.
A
A
When the AD7608 is placed in shutdown mode, the current
consumption is 11 µA maximum and power-up time is approximately 13 ms (external reference mode). In shutdown mode,
all circuitry is powered down. When the AD7608 is powered
up from shutdown mode, a RESET signal must be applied to
the AD7608 after the required power-up time has elapsed.
The VDRIVE supply is connected to the same supply as the processor. The VDRIVE voltage controls the voltage value of the
output logic signals. For layout, decoupling, and grounding
hints, see the Layout Guidelines section.
Table 7. Power-Down Mode Selection
STBY
Power-Down Mode
Standby
Shutdown
After supplies have been applied to the AD7608, apply a RESET
signal to the device to ensure it is configured for the correct
mode of operation.
E
A
0
0
POWER-DOWN MODES
There are two power-down modes available on the AD7608:
standby mode and shutdown mode. The STBY pin controls
whether the AD7608 is in normal mode or in one of the two
power-down modes.
E
A
ANALOG SUPPLY
VOLTAGE 5V1
1µF
REFIN/REFOUT
100nF
100nF
REGCAP2
AVCC
VDRIVE
REFCAPA
10µF
DB0 TO DB15
+
REFCAPB
REFGND
EIGHT ANALOG
INPUTS V1 TO V8
V1
V1GND
V2
V2GND
V3
V3GND
V4
V4GND
V5
V5GND
V6
V6GND
V7
V7GND
V8
V8GND
AD7608
CONVST A, B
CS
RD
BUSY
RESET
OS 2
OS 1
OS 0
REF SELECT
PARALLEL
INTERFACE
OVERSAMPLING
VDRIVE
PAR/SER SEL
RANGE
STBY
VDRIVE
AGND
1DECOUPLING SHOWN ON THE AV
CC PIN APPLIES TO EACH AVCC PIN (PIN 1, PIN 37, PIN 38, PIN 48).
DECOUPLING CAPACITOR CAN BE SHARED BETWEEN AV CC PIN 37 AND PIN 38.
2DECOUPLING SHOWN ON THE REGCAP PIN APPLIES TO EACH REGCAP PIN (PIN 36, PIN 39).
Figure 40. Typical Connection Diagram
Rev. A | Page 22 of 32
08938-038
+
MICROPROCESSOR/
MICROCONVERTER/
DSP
10µF
DIGITAL SUPPLY
VOLTAGE +2.3V TO +5V
RANGE
1
0
Data Sheet
AD7608
CONVERSION CONTROL
Simultaneously Sampling Two Sets of Channels
Simultaneous Sampling on All Analog Input Channels
The AD7608 also allows the analog input channels to be
sampled simultaneously in two sets. This can be used in powerline protection and measurement systems to compensate for
phase differences introduced by PT and CT transformers. In a
50 Hz system, this allows for up to 9° of phase compensation; and
in a 60 Hz system, it allows for up to 10° of phase compensation.
The AD7608 allows simultaneous sampling of all analog input
channels. All channels are sampled simultaneously when both
CONVST x pins (CONVST A, CONVST B) are tied together.
A single CONVST x signal is used to control both CONVST x
inputs. The rising edge of this common CONVST x signal
initiates simultaneous sampling on all analog input channels.
This is accomplished by pulsing the two CONVST x pins
independently and is possible only if oversampling is not in
use. CONVST A is used to initiate simultaneous sampling of
the first set of channels (V1 to V4) and CONVST B is used
to initiate simultaneous sampling on the second set of analog
input channels (V5 to V8), as illustrated in Figure 41. On the
rising edge of CONVST A, the track-and-hold amplifiers for
the first set of channels are placed into hold mode. On the
rising edge of CONVST B, the track-and-hold amplifiers for
the second set of channels are placed into hold mode. The conversion process begins once both rising edges of CONVST x
have occurred; therefore BUSY goes high on the rising edge of
the later CONVST x signal. In Table 3, Time t5 indicates the
maximum allowable time between CONVST x sampling points.
The AD7608 contains an on-chip oscillator that is used to
perform the conversions. The conversion time for all ADC
channels is tCONV. The BUSY signal indicates to the user when
conversions are in progress, so when the rising edge of CONVST x
is applied, BUSY goes logic high and transitions low at the end
of the entire conversion process. The falling edge of the BUSY
signal is used to place all eight track-and-hold amplifiers back
into track mode. The falling edge of BUSY also indicates that
the new data can now be read from the parallel bus (DB[15:0]),
or the DOUTA and DOUTB serial data lines.
There is no change to the data read process when using two
separate CONVST x signals.
Connect all unused analog input channels to AGND. The results
for any unused channels are still included in the data read because
all channels are always converted.
V1 TO V4 TRACK-AND-HOLD
ENTER HOLD
V5 TO V8 TRACK-AND-HOLD
ENTER HOLD
CONVST A
t5
CONVST B
AD7608 CONVERTS
ON ALL 8 CHANNELS
BUSY
tCONV
CS, RD
V1
V2
V8
08938-039
DATA: DB[15:0]
FRSTDATA
Figure 41. Simultaneous Sampling on Channel Sets Using Independent CONVST A/CONVST B Signals—Parallel Mode
Rev. A | Page 23 of 32
AD7608
Data Sheet
DIGITAL INTERFACE
The AD7608 provides two interface options: a parallel interface
and high speed serial interface. The required interface mode is
selected via the PAR/SER SEL pin.
The CS signal can be permanently tied low, and the RD
signal can be used to access the conversion results as shown
in Figure 4. A read operation of new data can take place after
the BUSY signal goes low (Figure 2), or alternatively a read
operation of data from the previous conversion process can
take place while BUSY is high (Figure 3).
E
E
A
A
The operation of the interface modes is discussed in the
following sections.
PARALLEL INTERFACE (PAR/SER SEL = 0)
A
E
A
A
A
E
A
E
A
A
E
E
A
A
A
E
A
A
A
A
A
The RD pin is used to read data from the output conversion
results register. Two RD pulses are required to read the full
18-bit conversion result from each channel. Applying a sequence
of 16 RD pulses to the AD7608 RD pin clocks the conversion
results out from each channel onto the 16-bit parallel output
bus in ascending order. The first RD falling edge after BUSY
goes low clocks out DB[17:2] of the V1 result, the next RD
falling edge updates the bus with DB[1:0] of V1 result. It takes
16 RD pulses to read the eight 18-bit conversion results from
the AD7608. On the AD7608, the 16th falling edge of RD clocks
out the DB[1:0] conversion result for Channel V8. When the
RD signal is logic low, it enables the data conversion result from
each channel to be transferred to the digital host (DSP, FPGA).
E
Data can be read from the AD7608 via the parallel data bus with
standard CS and RD signals. To read the data over the parallel
bus, the PAR/SER SEL pin should be tied low. The CS and RD
input signals are internally gated to enable the conversion result
onto the data bus. The data lines, DB15 to DB0, leave their high
impedance state when both CS and RD are logic low.
E
E
A
A
A
A
A
E
A
A
E
E
A
A
A
A
E
A
A
E
A
A
AD7608
INTERRUPT
E
BUSY 14
A
A
E
CS 13
RD/SCLK 12
DB[15:0] [33:24]
DIGITAL
HOST
[22:16]
08938-040
A
E
E
E
A
A
A
The rising edge of the CS input signal three-states the bus
and the falling edge of the CS input signal takes the bus out
of the high impedance state. CS is the control signal that
enables the data lines, it is the function that allows multiple
AD7608 devices to share the same parallel data bus.
A
E
A
E
A
A
A
E
A
A
A
Figure 42. AD7608 interface diagram—One AD7608 Using the Parallel Bus;
CS and RD Shorted Together
A
A
When there is only one AD7608 in a system/board and it does
not share the parallel bus, data can be read using just one control
signal from the digital host. The CS and RD signals can be tied
together as shown in Figure 5. In this case, the data bus comes
out of three-state on the falling edge of CS/RD. The combined
CS and RD signal allows the data to be clocked out of the AD7608
and to be read by the digital host. In this case, CS is used to
frame the data transfer of each data channel. In this case, 16 CS
pulses are required to read the eight channels of data.
E
A
E
A
A
A
E
A
E
A
E
A
A
A
E
A
A
A
E
A
A
A
Rev. A | Page 24 of 32
A
E
Data Sheet
AD7608
SERIAL INTERFACE (PAR/SER SEL = 1)
The SCLK input signal provides the clock source for the serial
read operation. CS goes low to access the data from the AD7608.
The falling edge of CS takes the bus out of three-state and
clocks out the MSB of the 18-bit conversion result. This MSB
is valid on the first falling edge of the SCLK after the CS falling
edge. The subsequent 17 data bits are clocked out of the AD7608
on the SCLK rising edge. Data is valid on the SCLK falling edge.
Eighteen clock cycles must be provided to the AD7608 to access
each conversion result.
E
To read data back from the AD7608 over the serial interface,
the PAR/SER SEL pin should be tied high. The CS and SCLK
signals are used to transfer data from the AD7608. The AD7608
has two serial data output pins, DOUTA, and DOUTB. Data can be
read back from the AD7608 using one or both of these DOUT
lines. For the AD7608, conversion results from Channel V1 to
Channel V4 first appear on DOUTA while conversion results from
Channel V5 to Channel V8 first appear on DOUTB.
E
E
A
A
A
The CS falling edge takes the data output lines (DOUTA
and DOUTB) out of three-state and clocks out the MSB of
the conversion result. The rising edge of SCLK clocks all
subsequent data bits onto the serial data outputs, DOUTA
and DOUTB. The CS input can be held low for the entire
serial read, or it can be pulsed to frame each channel read
of 18 SCLK cycles.
E
A
A
A
E
A
A
The FRSTDATA output signal indicates when the first channel,
V1, is being read back. When the CS input is high, the FRSTDATA
output pin is in three-state. In serial mode, the falling edge of
CS takes FRSTDATA out of three-state and sets the FRSTDATA
pin high indicating that the result from V1 is available on the
DOUTA output data line. The FRSTDATA output returns to a
logic low following the 18th SCLK falling edge. If all channels
are read on DOUTB, the FRSTDATA output does not go high
when V1 is output on the serial data output pin. It only goes
high when V1 is available on DOUTA (and this is when V5 is
available on DOUTB).
E
A
E
A
A
A
Figure 43 shows a read of eight simultaneous conversion results
using two DOUT lines on the AD7608. In this case, a 72 SCLK
transfer is used to access data from the AD7608 and CS is held
low to frame the entire 72 SCLK cycles. Data can also be clocked
out using just one DOUT line, in which case DOUTA is recommended
to access all conversion data as the channel data is output in
ascending order. For the AD7608 to access all eight conversion
results on one DOUT line, a total of 144 SCLK cycles are required.
These 144 SCLK cycles can be framed by one CS signal or each
group of 18 SCLK cycles can be individually framed by the CS
signal. The disadvantage of using just one DOUT line is that the
throughput rate is reduced if reading after conversion. The
unused DOUT line should be left unconnected in serial mode.
For the AD7608, if DOUTB is used as a single DOUT line, the
channel results will output in the following order: V5, V6, V7,
V8, V1, V2, V3, V4; however, the FRSTDATA indicator returns
low once V5 is read on DOUTB.
E
A
E
A
E
A
A
E
A
A
A
E
A
A
Figure 6 shows the timing diagram for reading one channel of
data, framed by the CS signal, from the AD7608 in serial mode.
READING DURING CONVERSION
Data can be read from the AD7608 while BUSY is high and
conversions are in progress. This has little effect on the
performance of the converter and allows a faster throughput
rate to be achieved. A parallel or serial read may be performed
during conversions and when oversampling may or may not
be in use. Figure 3 shows the timing diagram for reading while
BUSY is high in parallel or serial mode. Reading during conversions allows the full throughput rate to be achieved when using
the serial interface with a VDRIVE of 3.3 V to 5.25 V.
Data can be read from the AD7608 at any time other than on
the falling edge of BUSY because this is when the output data
registers get updated with the new conversion data. Time t6, as
outlined in Table 3, should be observed in this condition.
E
A
CS
72
SCLK
DOUTA
V1
V2
V3
V4
DOUTB
V5
V6
V7
V8
Figure 43. AD7608 Serial Interface with two DOUT Lines
Rev. A | Page 25 of 32
08938-041
A
A
AD7608
Data Sheet
Figure 44 shows that the conversion time extends as the oversampling rate is increased, and the BUSY signal lengthens for the
different oversampling rates. For example, a sampling frequency
of 10 kSPS yields a cycle time of 100 µs. Figure 44 shows OS × 2
and OS × 4; for a 10 kSPS example, there is adequate cycle time to
further increase the oversampling rate and yield greater improvements in SNR performance. In an application where the initial
sampling or throughput rate is at 200 kSPS, for example, and
oversampling is turned on, the throughput rate must be reduced
to accommodate the longer conversion time and to allow for the
read. To achieve the fastest throughput rate possible when oversampling is turned on, the read can be performed during the
BUSY high time. The falling edge of BUSY is used to update the
output data registers with the new conversion data; therefore, the
reading of conversion data should not occur on this edge.
DIGITAL FILTER
The AD7608 contains an optional digital first-order sinc filter
that should be used in applications where slower throughput
rates are used or where higher signal-to-noise ratio or dynamic
range is desirable. The oversampling ratio of the digital filter is
controlled using the oversampling pins, OS [2:0] (see Table 8).
OS 2 is the MSB control bit, and OS 0 is the LSB control bit.
Table 8 provides the oversampling bit decoding to select the
different oversample rates. The OS pins are latched on the falling
edge of BUSY. This sets the oversampling rate for the next
conversion (see Figure 45). In addition to the oversampling
function, the output result is decimated to 18-bit resolution.
If the OS pins are set to select an OS ratio of 8, the next
CONVST x rising edge takes the first sample for each channel,
and the remaining seven samples for all channels are taken with
an internally generated sampling signal. These samples are then
averaged to yield an improvement in SNR performance. Table 8
shows typical SNR performance for both the ±10 V and the
±5 V range. As Table 8 indicates, there is an improvement in
SNR as the OS ratio increases. As the OS ratio increases, the
3 dB frequency is reduced, and the allowed sampling frequency
is also reduced. In an application where the required sampling
frequency is 10 kSPS, an OS ratio of up to 16 can be used. In
this case, the application sees an improvement in SNR, but the
input 3 dB bandwidth is limited to ~6 kHz.
tCYCLE
tCONV
CONVST A,
CONVST B
19µs
9µs
4µs
OS = 0 OS = 2 OS = 4
BUSY
t4
t4
t4
CS
The CONVST A and CONVST B pins must be tied/driven
together when oversampling is turned on. When the oversampling function is turned on, the BUSY high time for the
conversion process extends. The actual BUSY high time
depends on the oversampling rate selected: the higher
the oversampling rate, the longer the BUSY high, or total
conversion time (see Table 3).
08938-043
RD
DATA:
DB[15:0]
Figure 44. No Oversampling, Oversampling × 2, and Oversampling × 4 While
Using Read After Conversion
CONVST A,
CONVST B
CONVERSION N
OVERSAMPLE RATE
LATCHED FOR CONVERSION N + 1
CONVERSION N + 1
BUSY
tOS_HOLD
08938-042
tOS_SETUP
OS x
Figure 45. OS Pin Timing
Table 8. Oversample Bit Decoding
OS [2:0]
000
001
010
011
100
101
110
111
1
OS Ratio
No OS
2
4
8
16
32
64
Invalid
SNR ±5 V
Range (dB)1
90.5
92.5
94.45
96.5
99.1
101.7
103
SNR ±10 V
Range (dB)1
91.2
93.4
95.7
98
100.4
102.8
103.5
3 dB BW ±5 V
Range (kHz)
15
15
13.7
10.3
6
3
1.5
SNR values taken with a full scale 100 Hz input signal.
Rev. A | Page 26 of 32
3 dB BW ±10 V
Range (kHz)
22
22
18.5
11.9
6
3
1.5
Maximum Throughput
CONVST x Frequency (kHz)
200
100
50
25
12.5
6.25
3.125
Data Sheet
AD7608
NO OVERSAMPLING
1377
1170 1208
1200
1001
2176
2000
708
1500
1000
648
0
411
400
–2
–1
0
CODE
146
82
66
–9 –8 –7 –6 –5 –4 –3 –2 –1 0 1
CODE
2
3
4
5
21 5
6
7
0
1
2
2
3
4
8 9
Figure 46. Histogram of Codes—No OS (18 Codes)
2000
OVERSAMPLING BY 2
1759
1800
1600
1524
1397
1400
1200
1065
OVERSAMPLING BY 16
3947
4000
NUMBER OF OCCURENCES
0
–3
44
4500
188
3
78
4
Figure 49. Histogram of Codes—OS × 8 (9 Codes)
328
3 35
457
–4
588
600
1756
3500
3000
2703
2500
2000
1500
1081
1000
385
500
1000
69
902
7
0
–2
800
–1
0
1
2
3
CODE
538
600
498
08938-148
800
2500
500
852
200
NUMBER OF OCCURENCES
3027
1000
08938-044
NUMBER OF OCCURENCES
1400
OVERSAMPLING BY 8
3000
08938-047
1600
3500
NUMBER OF OCCURENCES
Figure 46 to Figure 52 illustrates the effect of oversampling on
the code spread in a dc histogram plot. As the oversample rate
is increased, the spread of codes is reduced. (In Figure 46 to
Figure 52, AVCC = VDRIVE = 5 V and the sampling rate was scaled
with OS ratio.)
Figure 50. Histogram of Codes—OS × 16 (6 Codes)
400
1
57
15 54
–8 –7 –6 –5 –4 –3 –2 –1 0
CODE
1
2
3
4
5
OVERSAMPLING BY 32
9
6
Figure 47. Histogram Of Codes—OS × 2 (14 Codes)
2500
2224
1913
2000
1551
3000
2000
1460
1301
0
–2
684
199
4
40
–5
–4
64
–3
–2
–1
0
CODE
1
2
3
–1
0
CODE
1
2
Figure 51. Histogram of Codes—OS × 32 (5 Codes)
427
500
17
11
4
14
5
Figure 48. Histogram of Codes—OS × 4 (11 Codes)
Rev. A | Page 27 of 32
08938-149
1072
1000
0
4000
1000
1500
08938-046
NUMBER OF OCCURENCES
OVERSAMPLING BY 4
5403
5000
NUMBER OF OCCURENCES
0
165
08938-045
0
6000
208
200
AD7608
Data Sheet
7000
OVERSAMPLING BY 64
0
6489
AVCC = 5V
VDRIVE = 5V
TA = 25°C
±10V RANGE
OS BY 4
–10
–20
5000
ATTENUATION (dB)
NUMBER OF OCCURENCES
6000
4000
3000
2000
1238
–30
–40
–50
–60
–70
–80
1000
465
1
–20
10M
–30
–40
–50
–60
–70
–80
–90
–100
100
1k
10k
100k
1M
10M
FREQUENCY (Hz)
Figure 55. Digital Filter Response for OS × 8
–30
0
AVCC = 5V
VDRIVE = 5V
TA = 25°C
±10V RANGE
OS BY 16
–40
–10
–50
–20
–70
–80
10k
100k
FREQUENCY (Hz)
1M
10M
Figure 53. Digital Filter OS × 2
–30
–40
–50
–60
–70
–80
–90
–100
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 56. Digital Filter Response for OS × 16
Rev. A | Page 28 of 32
10M
08938-154
1k
ATTENUATION (dB)
–60
–90
100
1M
AVCC = 5V
VDRIVE = 5V
TA = 25°C
±10V RANGE
OS BY 8
–10
AVCC = 5V
VDRIVE = 5V
TA = 25°C
±10V RANGE
OS BY 2
08938-151
ATTENUATION (dB)
0
ATTENUATION (dB)
Figure 53 to Figure 58 show the digital filter frequency profiles
for oversampling by 2 to oversampling by 64. The combination
of the analog antialiasing filter and the oversampling digital
filter can be used to eliminate or reduce the complexity of the
design of the filter before the AD7608. The digital filtering
combines steep roll-off and linear phase response.
–20
100k
Figure 54. Digital Filter Response for OS × 4
When the oversampling mode is selected, this has the effect
of adding a digital filter function after the ADC. The different
oversampling rates and the CONVST x sampling frequency
produces different digital filter frequency profiles.
–10
10k
FREQUENCY (Hz)
Figure 52. Histogram of Codes—OS × 64 (3 Codes)
0
1k
08938-153
0
CODE
08938-150
–1
–100
100
08938-152
–90
0
Data Sheet
AD7608
0
–20
–20
ATTENUATION (dB)
–30
–40
–50
–60
–70
–80
–30
–40
–50
–60
–70
–80
–90
1k
10k
100k
1M
FREQUENCY (Hz)
10M
Figure 57. Digital Filter Response for OS × 32
–100
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 58. Digital Filter Response for OS × 64
Rev. A | Page 29 of 32
10M
08938-156
–90
–100
100
AVCC = 5V
VDRIVE = 5V
TA = 25°C
±10V RANGE
OS BY 64
–10
08938-155
ATTENUATION (dB)
0
AVCC = 5V
VDRIVE = 5V
TA = 25°C
±10V RANGE
OS BY 32
–10
AD7608
Data Sheet
LAYOUT GUIDELINES
The printed circuit board that houses the AD7608 should be
designed so that the analog and digital sections are separated
and confined to different areas of the board.
If the AD7608 is in a system where multiple devices require
analog-to-digital ground connections, the connection should
still be made at only one point: a star ground point should be
established as close as possible to the AD7608. Good connections
should be made to the ground plane. Avoid sharing one connection for multiple ground pins. Individual vias or multiple vias to
the ground plane should be used for each ground pin.
Avoid running digital lines under the devices because doing so
couples noise onto the die. Allow the analog ground plane to
run under the AD7608 to avoid noise coupling. Fast switching
signals like CONVST A, CONVST B, or clocks should be shielded
with digital ground to avoid radiating noise to other sections of
the board, and they should never run near analog signal paths.
Avoid crossover of digital and analog signals. Run traces on
layers in close proximity on the board at right angles to each
other to reduce the effect of feedthrough through the board.
08938-051
Use at least one ground plane. It can be common or split between
the digital and analog sections. In the case of the split plane, the
digital and analog ground planes should be joined in only one
place, preferably as close as possible to the AD7608.
Figure 59. Top Layer Decoupling REFIN/REFOUT, REFCAPA, REFCAPB, and
REGCAP Pins
Good decoupling is also important to lower the supply impedance
presented to the AD7608 and to reduce the magnitude of the
supply spikes. The decoupling capacitors should be placed close
to (ideally right up against) these pins and their corresponding
ground pins. Place the decoupling capacitors for the REFIN/
REFOUT pin and the REFCAPA and REFCAPB pins as close
as possible to their respective AD7608 pins and where possible
they should be placed on the same side of the board as the AD7608
device. Figure 59 shows the recommended decoupling on the
top layer of the AD7608 board. Figure 60 shows bottom layer
decoupling. Bottom layer decoupling is for the four AVCC pins
and the VDRIVE pin.
Rev. A | Page 30 of 32
08938-052
The power supply lines to the AVCC and VDRIVE pins on the
AD7608 should use as large a trace as possible to provide
low impedance paths and reduce the effect of glitches on the
power supply lines. Where possible, use supply planes. Good
connections should be made between the AD7608 supply pins
and the power tracks on the board. Use a single via or multiple
vias for each supply pin.
Figure 60. Bottom Layer Decoupling
Data Sheet
AD7608
To ensure good device-to-device performance matching, in a
system that contains multiple AD7608 devices, a symmetrical
layout between the AD7608 devices is important.
Figure 61 shows a layout with two devices. The AVCC supply
plane runs to the right of both devices. The VDRIVE supply
track runs to the left of the two devices. The reference chip
is positioned between both the two devices and the reference
voltage track runs north to Pin 42 of U1 and south to Pin 42
to U2. A solid ground plane is used.
AVCC
U2
These symmetrical layout principles can be applied to a system
that contains more than two AD7608 devices. The AD7608
devices can be placed in a north-south direction with the reference
voltage located midway between the AD7608 devices with the
reference track running in the north-south direction similar to
Figure 61.
08938-053
U1
Figure 61. Layout for Multiple AD7608 Devices—Top Layer and
Supply Plane Layer
Rev. A | Page 31 of 32
AD7608
Data Sheet
OUTLINE DIMENSIONS
0.75
0.60
0.45
12.20
12.00 SQ
11.80
1.60
MAX
64
49
1
48
PIN 1
10.20
10.00 SQ
9.80
TOP VIEW
(PINS DOWN)
0.15
0.05
SEATING
PLANE
0.20
0.09
7°
3.5°
0°
16
0.08
COPLANARITY
VIEW A
33
32
17
VIEW A
0.50
BSC
LEAD PITCH
0.27
0.22
0.17
ROTATED 90° CCW
COMPLIANT TO JEDEC STANDARDS MS-026-BCD
051706-A
1.45
1.40
1.35
Figure 62. 64-Lead Low Profile Quad Flat Package [LQFP]
(ST-64-2)
Dimensions shown in millimeters
ORDERING GUIDE
Model 1
AD7608BSTZ
AD7608BSTZ-RL
EVAL-AD7608EDZ
CED1Z
1
Temperature Range
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
Package Description
64-Lead Low Profile Quad Flat Package [LQFP]
64-Lead Low Profile Quad Flat Package [LQFP]
Evaluation Board for the AD7608
Converter Evaluation Development
Z = RoHS Compliant Part.
©2011-2012 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D08938-0-1/12(A)
Rev. A | Page 32 of 32
Package Option
ST-64-2
ST-64-2