Is Single to Differential Conversion Really Needed?

[gigavolt]

I have a differential ADC lying around, but want to use it to digitize a single-ended signal. According to its reference design, TI recommends sticking a differential output op amp in front and using that to drive the ADC. My question is if this is really necessary or if I can just connect in_negative to ground and the (always positive) signal to in_positive?

Also, I did hear that ADC inputs can be unfriendly. Maybe that's a good reason to use the driver? However, the input signal is itself coming from an op amp.

The diagram is on page 33 of the following PDF.

https://www.ti.com/lit/ds/symlink/ads8867.pdf

edit: removed garbage from the link

[jwet]

In most cases the common mode voltage of diff input ADC's is limited, but the particular device that you've chosen has an unusual R-R common mode input range.  You should be able to do what you want.  A few considerations- you're giving up 6 db or 1 bit of resolution right off the bat since your input is unipolar- 15 bits is still pretty good.  The input is a 55 pF cap behind an analog switch- that's a rough load.  Look at the DS around for recommendations- it looks like something like 100 ohms on the op-amp output and then a 1000 pF cap to ground, the slug of current for the input will come from that cap- the sereis R will keep your amp from oscillating.  It sounds like you're just experimenting- it will certainly work if you take a little care with the single ended drive to keep your driver happy.

[gigavolt]

Thanks! That's good to know.

I'm adding an antialiasing filter right before the ADC too, so hopefully that cap can help out with the filling up the ADC's input.