AD8138: Diff-AMp Calculator issue

Document created by analog-archivist Employee on Feb 23, 2016Last modified by analog-archivist Employee on Feb 23, 2016
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I download the new version from website and have a try. Seems the value of Rf
is double of the value it should be.
For example, I designed buffer using AD8138, I set gain=1, Rg=200, Rf should be
200, too, but it showed to be 422. Please refer to the below picture? Could you
please help me to confirm if it is a bug or it caused by my incorrect usage?


The gain you enter in the amplifier symbol is the system gain. In other words,
Gain=(Vout-diff)/(Vs-diff). This system gain does not equal to the amplifier
gain when terminated.

In a properly terminated case, just like you have it, the gain from the Vs
inputs to the note called Vin is ½. So you need to compensate with increasing
the amplifier gain (Rf/Rg). That is why for  a system gain of 1, the ratio of
Rf/Rg will be slightly higher than 2 if the inputs are terminated.

If you unterminate, then the system gain equals the amplifier gain and
therefore the ratio of Rf/Rg will be equal to the system gain.

The numbers are correct.