Circuit Note
CN-0148
Circuit Designs Using Analog Devices Products
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Devices Connected/Referenced
AD7606
8-Channel DAS with 16-Bit, Bipolar,
Simultaneous Sampling ADC
AD7606-6
6-Channel DAS with 16-Bit, Bipolar,
Simultaneous Sampling ADC
AD7606-4
4-Channel DAS with 16-Bit, Bipolar,
Simultaneous Sampling ADC
ADR421
Precision, Low Noise XFET® Voltage
Reference
Layout Considerations for an Expandable Multichannel Simultaneous Sampling
Data Acquisition System (DAS) Based on the AD7606 16-Bit, 8-Channel DAS
Rev. 0
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CIRCUIT FUNCTION AND BENEFITS
In power line measurement and protection systems, there is a
requirement to simultaneously sample large numbers of current
and voltage channels of multiphase power distribution and
transmission networks. In these applications, the channel count
can vary from as few as six channels to greater than 64 channels.
The AD7606 8-channel data acquisition system (DAS) with
16-bit bipolar simultaneously sampling SAR ADCs with on-
chip overvoltage protection greatly simplifies signal condi-
tioning circuitry and reduces the overall parts count, board
real estate, and cost of the measurement and protection board.
Even with its high level of integration, each AD7606 requires
only nine low value ceramic decoupling capacitors.
In measurement and protection systems, simultaneous
sampling capability is needed to maintain the phase informa-
tion between the current and voltage channels on multiphase
power line networks. The wide dynamic range capability of the
AD7606 makes it ideal for capturing both under voltage/current
and over voltage/current conditions. The input voltage range is
pin-programmable for either ±5 V or ±10 V.
This circuit note describes details of the recommended PC
board layout for applications using multiple AD7606 devices.
The layout is optimized for channel-to-channel matching and
part-to-part matching and will help reduce the complexity of
calibration routines in high channel count systems. The circuit
provides the ability to use the AD7606 2.5 V internal reference
when channel-to-channel matching is important or an external
ADR421 precision high accuracy (B grade: ±1 mV max), low
drift (B grade: 3 ppm/°C max), low noise (1.75 µV p-p, typical,
0.1 Hz to 10 Hz) reference for high channel applications that
require excellent absolute accuracy. The low noise and the stability
and accuracy characteristics of the ADR421 make it ideal for
high precision conversion applications. The combination of the
two devices yields a level of integration, channel density, and
accuracy that is unsurpassed in the industry.
CIRCUIT DESCRIPTION
The AD7606 is an integrated, 8-channel data acquisition system
with input amplifiers; overvoltage protection; second-order
analog antialiasing filters; analog multiplexer; 16-bit, 200 kSPS
SAR ADC; and a digital filter—all included on-chip. The circuit
shown in Figure 1 consists of two AD7606 devices configurable
with the ability to use either the internal 2.5 V reference or an
external 2.5 V ADR421 reference. When the REF SELECT pin
is connected to a logic high, the internal reference is selected.
When the REF SELECT pin is connected to a logic low, the
external reference is selected.
The power supply requirements are as follows: AV
CC
= 5 V,
V
DRIVE
= 2.3 V to 5 V (depending on external logic interface
requirements).
This circuit note describes the layout and performance
of an evaluation board that contains two AD7606’s,
making a 16-channel data acquisition system. Complete
16-channel DAS PC board documentation is available at
www.analog.com/CN0148_PCB_Documentation.
Symmetrical layout around the analog input channels and
device decoupling is important for good channel-to-channel
matching and part-to part matching. Data is shown to support
the matching performance obtainable with the 16-channel ADC
shown in Figure 1.
Dual AD7606 Board Layout for 16-Channel DAS
In a system that contains multiple AD7606 devices, a
symmetrical layout between the AD7606 devices is
important to ensure good device-to-device performance
matching. Figure 2 shows a layout with two AD7606 devices.
CN-0148 Circuit Note
Rev. 0 | Page 2 of 6
ANALOG SUPPLY
VOLTAGE 5V
1
DECOUPLING SHOWN ON THE AV
CC
PIN APPLIES TO EACH AV
CC
PIN (PIN 1, PIN 37, PIN 38, PIN 48).
DECOUPLING CAPACITOR CAN BE SHARED BETWEEN AV
CC
PIN 37 AND PIN 38.
2
DECOUPLING SHOWN ON THE REGCAP PIN APPLIES TO EACH REGCAP PIN (PIN 36, PIN 39).
DIGITAL SUPPLY
VOLTAGE 2.5V
V
DRIVE
AV
CC
AGND
V
DRIVE
+
REFIN/REFOUT
DB0 TO DB15
CONVST A, CONVST B
CS
RD
BUSY
RESET
AD7606
1µF
10µF
100nF
EIGHT ANALOG
INPUTS V1 TO V8
PARALLEL
INTERFACE
REGCAP
2
+
10µF
REFCAPA
REFCAPB
OS 2
OS 1
OS 0
100nF
V1
PAR/SER SEL
STBY
REF SELECT
RANGE
V2
V3
V4
V5
V6
V7
V8
REFGND
V1GND
V2GND
V3GND
V4GND
V5GND
V6GND
V7GND
V8GND
V
DRIVE
V
DRIVE
AV
CC
AV
CC
1
AGND
V
DRIVE
+
REFIN/REFOUT
DB0 TO DB15
CONVST A, CONVST B
CS
RD
BUSY
RESET
AD7606
1µF
10µF
100nF
EIGHT ANALOG
INPUTS V1 TO V8
PARALLEL
INTERFACE
REGCAP
2
+
10µF
REFCAPA
REFCAPB
OS 2
OS 1
OS 0
100nF
V1
PAR/SER SEL
STBY
REF SELECT
RANGE
V2
V3
V4
V5
V6
V7
V8
REFGND
V1GND
V2GND
V3GND
V4GND
V5GND
V6GND
V7GND
V8GND
V
DRIVE
V
DRIVE
AV
CC
AV
CC
1
FPGA
DB[15:0]
DB[15:0]
DB[15:0]
OS 2
OS 1
OS 0
OS 2
OS 1
OS 0
CS
RD
CS1
RD
OS 2
OS 1
OS 0
CS1
CONVST
CONVST
CONVST
CS
RD
BUSY
U1
U2
BUSY
ADR421
AV
CC
+
10µF
100nF
+VIN
VOUT
REFIN/REFOUT
REFIN/REFOUT
AGND
100nF
REFIN/REFOUT
RESET
RESET
RESET
96 WAY BOARD CONNECTOR
V
DRIVE
SEE TEXT
V
DRIVE
SEE TEXT
08906-001
CONVERTER EVALUATION &
DEVELOPMENT BOARD
EVAL-CED1Z
POWER SUPPLY
CIRCUITRY
Figure 1. 16-Channel, 16-Bit Data Acquisition System Using Two AD7606 8-Channel DAS (Simplified Schematic, All Connections Not Shown. See Text for Specific
Connections Between Devices Corresponding to Channel-to-Channel and Part-to-Part Matching Tests)