Diff-Amp Calculator has a problem with ADA4937?

Hi barryvo,

With 3.3V single supply, the ADA4937 input CM voltage is from 0.3V to 1.2V per datasheet page 20 copied below:

"For 5 V single-supply operation, the input common-mode range at the summing nodes of the amplifier is 0.3 V to 3.0 V, and 0.3 V to 1.2 V with a 3.3 V supply."

I believe there is an issue with the Diff-Amp Calculator where it won't allow any Vocm entry less than 1.2V with 3.3V single supply operation. I'll raise this issue to folks in charge and update you later.

For the time being, you could partially get around this by may be using a VEE of say -1.5V to avoid this input clipping issue, as shown below:

I'll get back to you soon.

Regards,

Thanks, Hooman. But why should the OUTPUT common mode voltage cause the INPUT to clip? Further, If I'm using an AC-coupled input, the input common mode voltage will go to 1.65V (half the supply) which is already outside the specified common mode rage. This should never work! 

I've simulated this in LTSpice, and although the inputs do, indeed, go to 1.65V, everything SEEMS to look ok and there doesn't appear to be any clipping.

Hi barryvo,

It turns out the Diff-Amp Calculator limit of 1.2V minimum on Vocm is accurate:

The issue is that with AC coupled 1 scheme, you are at the edge of the input CM voltage range of 0.3V to 1.2V.

One way to get around this is to use AC couple 2 where the AC coupling cap is moved towards the generator. Then, if you use Vocm = 2.1V, the device input pins will be at 0.34V which is within range, as shown below:

Regards,

Hooman