What is AD629 behaviour if max. CM voltage exceeded?

Document created by analog-archivist Employee on Feb 23, 2016
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What is AD629 behaviour if max. CM voltage exceeded? I have tested some samples
of the AD629 and found that operating at the upper voltage extremes the output
no longer became predictable.

Effectively I am working with 270V+ at the current measuring resistor ( on a
20KHz PWM H-Bridge) and when the common mode voltage is above 270V then the
output goes to maximum +15V.  Does ADI have a recommended way to increase the
common voltage range to say 400V?


The maximum operating common mode voltage is = +/-270Volts…if you look at the
datasheet of the AD629, you will note that this high voltage is achieved by the
use of an internal resistor network. The internal resistor network of the AD629
divides the input common mode voltage by a factor of 20, so that if you have a
common mode voltage of +270V, you will have  a common mode voltage of
+13.5Volts at the op-amp input terminals. These terminals can swing to within a
volt of the rails, therefore a max diff voltage of +/-13Volts, will be also
devided down to +/- 0.65Volts, with the result that each input terminals will
swing 13.5 Volts ( common mode ) +/- 0.325 Volts.

If however you go above these ratings you will reach a point whre the input
terminals of the op-amp have a voltage applied which is too close to the rails,
and so you will hit saturation point, which will cause the output to hit the
We do not recommend using the device outside of the specifications.