Circuit Note | Using the AD7328 8-Channel ADC in Single Ended Applications

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Using the AD7328 8-Channel ADC in Single Ended Applications

(CN0047)

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Circuit Types:	ADC Circuit/Driver
Optimized For:	High Performance
Applications:	Instrumentation, Process Control

Circuit Function and Benefits

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.

Figure 1: Single Ended to Differerential Input

Circuit Description

In applications where the 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 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 (Dual).

The circuit configuration illustrated in Figure 1 shows 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 V- have 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.

Figure 2: Single Ended Operation Mode

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.

Contributed October, 2008

Circuit Function and Benefits
Circuit Description
Common Variations

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