Circuit Note
CN-0047
Devices Connected/Referenced
Circuit Designs Using Analog Devices Products
Apply these product pairings quickly and with confidence.
For more information and/or support call 1-800-AnalogD
(1-800-262-5643) or visit www.analog.com/circuit.
AD7328
Bipolar Input, 8-Channel, 12-Bit
Plus Sign ADC
AD8620
Precision, Wide BS JFET Dual Op Amp
AD780
Ultrahigh Precision Voltage Reference
Using the AD7328 8-Channel ADC in Applications with Single-Ended IndustrialLevel Signals
The AD7328 is designed on the iCMOS® (industrial CMOS)
process. This is a process combining high voltage silicon with
submicron CMOS and complementary bipolar technologies.
The AD7328 can accept true bipolar analog input signals. The
AD7328 has four software-selectable input ranges: ±10 V, ±5 V,
±2.5 V, and 0 V to +10 V. Each analog input channel can be
independently programmed to one of the four input ranges.
The analog input channels on the AD7328 can be programmed
to be single ended, true differential, or pseudo differential.
CIRCUIT FUNCTION AND BENEFITS
This circuit is designed to optimize the performance of the
AD7328 bipolar input, 8-channel, 12-bit plus sign ADC. The
AD7328 can operate at a throughput rate of 1 MSPS. The
selected operational amplifier (op amp) and reference voltage
source ensure that maximum AD7328 performance is achieved
with industrial-level, single-ended signal sources by providing a
low impedance driver with adequate settling time and an
accurate reference supply.
1.5kΩ
+12V
1.5kΩ
VIN
±10V
½ AD8620
VIN+ V
DD VCC
1.5kΩ
VDIFF
1.5kΩ
AD7328*
VIN–
1.5kΩ
VSS REFIN
DGND
1.5kΩ
+5V +12V
AGND
±10V
±
–12V
10V
½ AD8620
1µF
+2.5V
AD780
+12V
–12V
08535-001
10kΩ
0.1µF
*ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 1. Single-Ended-to-Differential Input
(Simplified Schematic: Decoupling and All Connections Not Shown)
Rev. A
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CN-0047
Circuit Note
CIRCUIT DESCRIPTION
In applications where harmonic distortion and signal-to-noise
ratio are critical specifications, the analog input of the AD7328
should be driven from a low impedance source. Large source
impedances significantly affect the ac performance of the ADC
and can necessitate the use of an input buffer amplifier. When
no amplifier is used to drive the analog input, the source impedance should be limited to low values. Due to the programmable
nature of the analog inputs on the AD7328, the choice of op
amp used to drive the inputs is a function of the particular
application and depends on the input configuration and the
analog input voltage ranges selected.
Differential operation requires that the VIN+ and VIN− be
simultaneously driven with two signals of equal amplitude that
are 180° out of phase. Because not all applications have a signal
preconditioned for differential operation, there is often a need
to perform a single-ended-to-differential conversion. This
single-ended-to-differential conversion can be performed using
an op-amp pair illustrated in Figure 1.
The circuit accepts a bipolar ±10 V single-ended signal. The dual
AD8620 amplifiers are connected in a cross-coupled configuration to produce 20 V p-p signals at VIN+ and VIN− that are
180° out of phase. Therefore, the differential input signal to the
AD7328, VDIFF, is 40 V p-p. The cross-coupled configuration
provides for excel-lent phase match between the two outputs.
The AD7328 has a total of eight single-ended analog input
channels. Figure 2 shows a typical connection diagram when
operating the ADC in single-ended mode, where the AD797
is used to buffer the signal before applying it to the ADC
analog inputs.
The analog input channels on the AD7328 can be independently programmed to accept one of four input ranges. The
AD7328 can accept input signals of ±4 x VREFIN, ±2 x VREFIN,
±VREFIN, and 0 to 4 x VREFIN.
The AD8620 is an ideal choice of op amp that can be used to
provide a single-ended-to-differential driver for AD7328. The
AD8620 is a precision, low input bias current, wide bandwidth
JFET dual operational amplifier.
The AD7328 allows for an external reference voltage to be applied
to the REFIN/REFOUT pin. The specified voltage input range
on the reference voltage is from 2.5 V to 3 V. Using 3 V reference
voltages instead of 2.5 V allows the AD7328 to accept larger
input signals. In Figure 1 and Figure 2, the AD780 is used as an
external reference source. The AD780 is a 2.5 V/3 V ultrahigh
precision voltage reference, which allows flexibility in the
voltage range selected.
The circuit configuration illustrated in Figure 1 shows how an
AD8620 op amp can be used to convert a single-ended signal to
a differential signal that can be applied to the AD7328 analog
inputs. The signals at points VIN+ and VIN− must have equal
amplitude and be 180° out of phase.
To achieve the specified performance, the circuit must be constructed on a multilayer PC board with a large area ground plane.
Proper layout, grounding, and decoupling techniques must be
used to achieve optimum performance (see MT-031 Tutorial,
MT-101 Tutorial, and the AD7328 evaluation board layout).
+15V
VIN+
VDD VCC
AD7328*
VSS
*ADDITIONAL PINS OMITTED FOR CLARITY.
0.1µF
+2.5V
AD780
+15V
08535-002
1µF
–15V
REFIN
DGND
AD797
AGND
AGND
5V
Figure 2. Single-Ended Input Operation
(Simplified Schematic: Decoupling and All Connections Not Shown)
Rev. A | Page 2 of 3
Circuit Note
CN-0047
COMMON VARIATIONS
Data Sheets and Evaluation Boards
Suitable voltage references for the AD7328 include the REF192,
AD1582, ADR03, ADR381, ADR391, and ADR421. The
AD8022 dual high-speed, low-noise op-amp can also be used in
high-frequency applications where a dual op-amp is desired. In
high-performance systems, a pair of AD8021s, a single-channel
variant of the AD8022 can also be used in place of the AD8022.
For lower frequency, single-ended applications, op-amps such
as AD797 (single) and AD8610 (single), AD8620 (dual),
AD8599 (dual), and ADA4941-1 (differential), are suitable
alternatives.
AD7328 Data Sheet
LEARN MORE
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND." Analog Devices.
AD7328 Evaluation Board
AD8620 Data Sheet
AD780 Data Sheet
REVISION HISTORY
2/10—Rev. 0 to Rev. A
Updated Format ................................................................. Universal
Changes to Circuit Function and Benefits..................................... 1
Changes to Circuit Description....................................................... 2
10/08—Revision 0: Initial Version
MT-036 Tutorial, Op Amp Output Phase-Reversal and Input
Over-Voltage Protection. Analog Devices.
MT-074 Tutorial, Differential Drivers for Precision ADCs.
Analog Devices.
MT-075 Tutorial, Differential Drivers for High Speed ADCs
Overview. Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
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©2008-2010 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN08535-0-2/10(A)
Rev. A | Page 3 of 3