Download Using the AD7328 8-Channel ADC in Single Ended Applications

Circuit Note
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Using the AD7328 8-Channel ADC in Single- Ended Applications
Devices connected in this Circuit Note:
AD7328 Bipolar Input, 8-Channel, 12-Bit Plus Sign ADC
AD8620 Precision, wide BW JFET dual Op Amp
Circuit Function and Benefits
AD780
Ultrahigh Precision Voltage Reference
The circuits shown are designed to optimize the
performance of the AD7328. The selected operational
amplifier (op-amp) and reference voltage source ensure
that the maximum AD7328 performance is achieved, by
providing a low impedance driver with adequate settling
time and an accurate reference supply.
Contributed November 7, 2008
Circuit Description
In applications where the harmonic distortion and signalto-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
Figure 1 Single-Ended to Differential Input
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.
V+
5V
AGND
VIN+
AD797
VDD VCC
A D 73 28 *
VSS
V–
1µF
REF
0.1µF
REFERENCE
SOURCE
*ADDITIONAL PINS OMITTED FOR CLARITY.
04852-026
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 singleended-to-differential conversion. This single-ended-todifferential conversion can be performed using an opamp pair illustrated in Figure 1. The AD8620 is an ideal
choice of op-amp which can be used to provide a singleended-to-differential driver for AD7328. The AD8620 is a
precision, low input bias current, wide bandwidth JFET
operational amplifier (Dual).
The circuit configuration illustrated in Figure 1 shows
Figure 2 Single-Ended Operation Mode
how an AD8620 op-amp can be used to convert a single
ended into a differential signal which can be applied to the AD7328 analog inputs. The signals at points V+ and Vhave equal amplitude and are 180° out of phase.
The AD7328 has a total of 8 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.
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Circuit Note
CN0047
Using the AD7328 8-Channel ADC in Single- Ended Applications
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 VREF, ±2 x VREF, ±VREF and 0 to 4 x VREF.
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. On both figures the AD780 is used as an external reference
source. The AD780 is a 2.5 V/3 V ultra-high precision voltage reference, which enables flexibility in the voltage
range selected.
Common Variations
Suitable reference sources 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 (single-ended to differential), are
suitable alternatives.
Additional Documentation
Datasheets for the mentioned devices AD7328, AD8620, AD780, can be found at following locations:
http://www.analog.com/en/AD7328/productsearch.html
http://www.analog.com/en/AD8620/productsearch.html
http://www.analog.com/en/AD780/productsearch.html
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Verified Circuit Designs Using Analog Devices Signal Processing Devices
Visit our design center at http://www.analog.com/en/design-center/index.html and our web
resources at http://www.analog.com or call 1-800-AnalogD (1-800-262-5643) for further support.
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