Nikkei Electronics Asia -- April 2009
Solutions -- Web Articles
Eight-Channel Data Acquisition System Using Single ADC Driver

Apr 1, 2009 15:00 Nikkei Electronics Asia

Data acquisition systems are basically affected by speed, accuracy, power dissipation, package size, component cost, and various factors depending upon applications. This article illustrates how a single operational amplifier (op amp) can be used to drive an ADC in an 8-channel data acquisition system, while reducing the cost and size of the overall system.

The AD7329 is an 8-channel, 12-bit plus sign ADC featuring 1Msps. The ADC comprises an 8-channel multiplexer followed by a track-and-hold and successive-approximation ADC, a channel sequencer, a 2.5V reference, and an SPI-compatible interface. The analog input channels are routed through the multiplexer to the MUXOUT+ and MUXOUT- pins. The ADCIN+ and ADCIN- pins connect to the track-and-hold input switch and sampling capacitor. 

Flexible design of the ADC allows an op amp such as the AD797 to be placed between the MUXOUT+ and ADCIN+ pins, in which the op amp isolates the input source from the ADC's input structure to increase input impedance and reduce the current required to drive the ADC. This configuration also allows a single op amp to drive eight analog input channels at the maximum sampling frequency, leading to reduced component count, board area, and system cost.

Configuring the op amp for gain allows the ADC to accommodate signals in the mV range while maintaining high performance. Small signals are amplified by the AD797 with the amplified signal applied to the ADCIN+. To maximize performance, the gain is chosen such that the full-scale input signal uses the full dynamic range of the ADC.

by Jakub Szymczak, Analog Devices Inc

Full article:
http://techon.nikkeibp.co.jp/NEA/solutions/0904001.pdf

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