Buffer op amp for AD8421 REF pin

Document created by analog-archivist Employee on Feb 23, 2016
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I have a design using the AD8421 instrumentation amplifier with +20V and -5V
supplies, the sense inputs have a common mode voltage between 0 and 15V and
differential voltage of up to 140mV.  The reference is provided by a resistor
divider providing 108mV into a MAX9617 op amp with unity gain as suggested in
the datasheet.

In my application I can see the 108mV reference input to the MAX9617 is always
constant, but the output reference signal going  into the AD8421 settles at a
different value after power up depending on the differential input voltage to
the AD8421 (the higher the differential voltage, the lower the reference). 
After power up the output reference voltage remains at the same level
regardless of any changes to the inputs of the AD8421.  The reference voltage
should not be affected by the sense voltages in this way, can you tell me what
might be causing this?


This seems to be a question for Maxim. But my guess is that you are powering
your buffer with a single supply, which could mean that the output stage is
running out of drive, and therefore, the current flowing out of the reference
pin (which depends on the differential and common mode voltages) could be
pulling the opamp to a different voltage. My suggestion is to

a) Use an ADI OpAmp
b) Do not use a zero drift OpAmp
c) Use dual supplies

Some of the zero drift op amps have start up issues. So when you power on, the
current from the REF pin somehow causes the auto-zero to enter an unwanted
state, therefore the output would be wrong. The easiest thing to try would be
various resistors in the feedback of the MAX op amp. Maybe a reasonable value
would limit the current at start-up and avoid the issue. Or you should try a
different op amp like ADA4805-1. In the AD8421 data sheet, we suggest using
OP1177 as a reference buffer. Our next gen OP1177 is the ADA4077-1. It operates
with supplies of +/-2.5V to +/-15V and has low offset and drift.