AD628: High level ADC interface

Q 

I would like to get more detailed application information for the high level
adc interface described in Design Ideas issue of October 30, 2003, (see
attached). In particular, how to set gain/attenuation, if even larger input
signals possible, what other ADCs can be used?

 

A 

There are several ways we can go about setting the gain. Before we proceed, we
need to constrain the design problem. The circuit shown in the article
illustrates conditioning a large voltage and driving a differential ADC. If you
do not need to drive a differential ADC, you simplify the circuit (and reduce
cost).

The AD628 performs the attentuation. You can add gain/attenuation using the
second stage of the AD628. Setting the gain is shown on page 16 of the Rev. G
data sheet. That section shows you how to attenuate as well.

If you need to measure even larger input voltages, then you can try building a
discrete difference amplifier with matched resistors and an op amp. We have
several articles that discuss how to build a discrete difference amp.(finding
the needle in a haystack -Analogue dialogue 34-1 (2000))

Another method to measure high voltages is to use an AD629. It has very well
matched resistors offering up to 86dB of CMRR (hard to do with discrete
resistors). It has a gain of 1. So this option would only work if your
differential signal is within the supplies and the common-mode voltage needs
attenuation. If you use the AD629 and a low voltage ADC, you will have to
attenuate the output to interface with an ADC. You can follow the AD629 with an
AD8275 which will allow you to interface with a 5V SAR ADC.