ADA4940-1 single ended input

Good day,

I'm struggling to understand this FDA.

I plan to have a resistor divider which will output 4V at full scale. I'll then feed this output to an AD8421, which will give a gain of 2 and level shift the output down so as to center the signal at 0V. The signal will then have a range from -4 to 4V. I then want to feed this signal to the ADA4940-1 to drive the AD7768 ADC over it's full range.

The ADA4940-1 will be fed a Vocm from the AD7768 of 2.5V.

Here is the output of the Diff-Amp calculator :

Which seems perfect, but when I try to simulate I get the following:

Here is my simulation using the ltspice simulation model downloaded from the ADC driver tool. After downloading I edited it to a single ended input driven by the AD8421 with sinusoidal source.

ada4940 sim.zip

and here I have simulated it using the macromodel from your website:

with macromodel.zip

using the macromodel my results are completely incorrect and each output seems to be stuck at the rails.

Any help will really be appreciated. Thanks in advance



Uploaded correct image for diff amp calculator
[edited by: EngJunkie at 5:49 PM (GMT -4) on 23 Aug 2021]