OpAmp basics: unbalanced to balanced signal with dual OpAmp

RE: OpAmp basics: unbalanced to balanced signal with dual OpAmp

Jan Cumps — Fri, 02 Sep 2022 16:04:17 GMT

I leave it to fellow community members to put possibilities in the comments

Alight. They are asleep.

One alternative is to use a differential probe. This probe is isolated from ground.
It can measure the signal between two test points that aren't ground.

I've attached the two inputs of the differential probe to the output of OpAmp A and B. Those are the points that are also probed by oscilloscope CH 2 and 3.
In the capture you can see that the dark blue differential probe signal (oscilloscope CH 4) is virtually identical to the MATH signal (calculated difference between CH 2 and 3). But cleaner:

In the capture below, the original signal and the differential output are compared. The output has twice the amplitude of the input. In the capture they show with the same hight, because I set CH 1 to 500 mV/div and CH4 to 1 V/div.

RE: OpAmp basics: unbalanced to balanced signal with dual OpAmp

jc2048 — Thu, 01 Sep 2022 17:16:44 GMT

Hello Jan. Did you notice the voltage noise plot in the datasheet?

Above 10kHz, the noise starts going up (that's an unusual feature). That will add quite a lot of high-frequency noise to the outputs. If you assumed the noise stopped at the 1MHz limit of the graph (it doesn't), that still looks like at least 70uV or 80uV of noise into the load with the two op-amps (in practice it could be a couple of hundred uV). Not really a problem for you experimenting with the circuit, but it might be a bit of a concern in some applications, depending on what you were doing with it.

BTW be cautious of the spice model. Although they've characterised the noise, it looks like the high frequency end, above 10kHz, is somewhat different to the datasheet. In a simulation of your circuit, I get a total of 2mV of noise into the load. Let's hope it's the datasheet that's accurate and not the model.

A second thing to note is that these op amps can only drive 300pF, so watch out if you are intent on attaching long cables to the outputs.

RE: OpAmp basics: unbalanced to balanced signal with dual OpAmp

DAB — Tue, 30 Aug 2022 19:08:49 GMT

Nice back to basics post.

RE: OpAmp basics: unbalanced to balanced signal with dual OpAmp

shabaz — Sun, 28 Aug 2022 18:59:36 GMT

Nice tower of test equipment!

My sig-gen doesn't do balanced either, but I've been using a transformer, although it's then very frequency-dependent : (

There's a single-BJT option which is ok-ish, but again is frequency dependent and level dependent. It needs capacitors on inputs/outputs:

RE: OpAmp basics: unbalanced to balanced signal with dual OpAmp

genebren — Sun, 28 Aug 2022 18:57:29 GMT

Another informative blog! I really like the way that break down and clearly explain your experiments. That looks like a handy protoboard design.