# Differences in +/-outs in ADA4932

Question asked by HugoTUe on Sep 8, 2011
Latest reply on Sep 12, 2011 by charlyelkhoury

Hi all,

I am curently simulating a circuit which requires a single-ended to differential conversion, for which I have chosen the ADA4932. My basic data are:

Vin=3.6Vpp

Vout<2Vpp

Vcmout=1.5V

Rsource=0

To make it more clear: I have a buffer (simple op. amp.) sending a +/-1.8V signal through a coaxial cable. The cable's characteristic impedance Z0 is 50ohm. On the output side I need to have both signals (+out and -out) moving within the +1V to +2V range. Therefore, I use a gain of 0.5 in the circuit, plus a Vcmout of 1.5V. That would mean having a 0.9Vpp in each output, which is fine since I want to have some safety margins. Supply voltage for the ADA4932 is +/5V

Now, here are the problems:

I have seen under different conditions (whether it is single ended or differential, terminated or no), that there is a sort of mismatch between the +out and the -out. What I actually see is that one of them seems to have a larger gain than the other. Though the actual difference between signals results in correct peak-to-peak values, each one individually do not seem correct.

I have seen this effect changing based on the order of magnitude of the gain and feedback resistors; the lower the resistors, the higher this difference is, which makes me think there might be a current-related problem. I have also followed all the recommended steps for the termination circuit, simulated with and without source resistance, with and without a transmission line, etc. So far, everything points to the AD4932 itself.

What I would like to know, first of all, is whether this is an effect that appears due to simply the simulation model, or whether this has been seen to occur in real-life circuits as well. Unfortunately I do not have the resources/time to test myself with an evaluation board.

I would appreciate any help you can provide me in this matter.

Thanks,

Hugo