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

Using the AD7328 8-Channel ADC in Single- Ended Applications
Source: ANALOG DEVICES

Introduction
Purpose
This training module discuss the design 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.
Outline
Overview of the AD7328 ADC
The AD7328 analog inputs configuration
Single-ended application using AD7328
Reference voltage source
Content
13 pages

Overview of the AD7328
The AD7328 can accept true bipolar analog input signals, ±10 V, ±5 V, ±2.5 V, and 0 V to +10 V unipolar signals.
The eight analog inputs can be configured as eight single-ended inputs, four true differential input pairs, four pseudo differential inputs, or seven pseudo differential inputs.
1 MSPS serial interface. SPI®-/QSPI™-/DSP-/MICROWIRE™-compatible interface.
Low power, 30 mW, at a maximum throughput rate of 1 MSPS.
Channel sequencer.
Analog Inputs

Typical Connection Diagram

Analog Inputs of the AD7328
Single-End Inputs
The AD7328 has a total of eight analog inputs when operating in single-ended mode.
Each analog input can be independently programmed to one of the four analog input ranges.
True Differential Mode
The AD7328 can have four true differential analog input pairs.
Differential signals have better noise immunity based on the device’s common-mode rejection and improvements in distortion performance.
Pseudo Differential Inputs
The AD7328 can have four pseudo differential pairs or seven pseudo differential inputs referenced to a common V IN− pin.
The V IN+ inputs are coupled to the signal source and must have an amplitude within the selected range.

Analog Input Structure
On the AD7328, the bipolar analog signal is sampled directly onto the sampling capacitor.
This gives the AD7328 high analog input impedance.
As the sampling frequency is reduced, the analog input impedance further increases.
As the analog input impedance increases, the current required to drive the analog input therefore decreases.
Equivalent Analog Input Circuit (Single-Ended) Equivalent Analog Input Circuit (Differential Mode) Z = 1/( f S × C S ) f S is the sampling frequency C S is the sampling capacitor value

Driver Amplifier Choice
The analog input of the AD7328 should be driven from a low impedance source.
Large source impedances must use an input buffer amplifier.
When no amplifier is used to drive the analog input, the source impedance should be limited to low values.
The driver amplifier must be able to settle for a full-scale step to a 13-bit level, 0.0122%, in less than the specified acquisition time of 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.

Single-Ended vs. Differential Applications
In single-ended applications , one wire carries a varying voltage that represents the signal, while another wire is connected to a reference voltage, usually ground.
In differential applications , two wires are routed in parallel, and sometimes twisted together so that both wires receive the same interference.

Single-Ended Configuration Verified Circuits are designed for ease of use and proven to work by ADI. The AD797 is configured as a buffer to drive the AD7328.

Single-Ended to Differential Conversion
The AD8620 is an ideal choice of op-amp which 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 operational amplifier
Noninverting signal Inverting signal Verified Circuits are designed for ease of use and proven to work by ADI.

Reference Voltage Source
On power-up, the external reference mode is the default condition.
The specified reference voltage range is from 2.5V to 3V.
When a 3 V reference is selected, the input ranges are ±12 V, ±6 V, ±3 V, and 0 V to +12 V.
For these ranges, the V DD and V SS supply must be equal to or greater than the maximum analog input range selected.
AD780

Converter Operation Figure 1 Figure 2 Figure 3 Figure 4

Additional Resource
For ordering the AD7328 and selected parts, please click the part list or
Call our sales hotline
For additional inquires contact our technical service hotline
For more product information go to
http://www.analog.com/en/content/CU_verified_circuits_CN0047/fca.html

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

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