Hi @cfortner, overall this looks like a solid setup. But there is a bit of a wrench lurking inside the setup that isn’t surmountable, but needs to be understood since you are working towards a universal setup for all topologies.
Let’s start with SPICE to understand the external divider. In the picture below, I’m using a voltage controlled current source to generate a balanced signal. This is TI’s preferred way of nudging SPICE to do what is needed. I wish I could find the TI engineer’s name that did the blog post on this, but I cannot. But in short, it’s straightforward. We take a voltage source with a peak voltage of 1.41V. And then we use two voltage controlled voltage sources to generate two sines of half amplitude AND opposite polarity (VCVS2 has a gain of MINUS 500m)
For the simulation, we’ll drive at 100Vrms (141Vp) and the attenuator should give us a factor of 100 attenuation. When we run the transient simulation, we can clearly see the stimulus at 141Vp:
Now, the divided signals are too hard to see, so let’s just plot those. Here we can see the differential signal is precisely as expected (divided by 100) with a peak of 1.41V. And we can see each leg of the divider is half of that and out of phase by 180 degrees. This makes perfect sense, and feeding this signal into the QA403 balanced inputs would agree.
OK, now let’s short one side of the divider as you have in your schematic.
And now run the simulation again:
And we can see the gold trace (VM1) is what we’d expect at 0.5Vrms. And that’s well within the range of the QA403 balanced input. HOWEVER, look at each leg of the balanced input. One leg is swinging about +/-35.6Vp, and the other leg is swinging +/- 34.77Vp. And so, the gold difference signal (VM1) is correct. But this requires the QA403 inputs to handle massive swings. 36Vp=25.5Vrms=28 dBV. And it can, but you need to be at a much higher full scale input level, which hurts your noise. It’s much better to let the attenuator do the heavy lifting outside of the QA403.
So, the summary here is this: If you want to measure a balanced amplifier that is actually driving push pull, then your setup (minus the DUT ground on the output) is correct. In that case, you’d want to establish the ground on the input of the DUT.
If you want to measure a single-ended amp (one where the + is driven and the - is ground), then you’d want to short/bypass the lower R3 resistor. In that case, the - output should read as a short to the input ground of the amp.
And finally, be aware that the BNC shells on your scope are usually at ground. That is, you can touch your DVM in continuity mode to the shell of the scope and to the ground prong in the wall and read continuity. This means if you clip your scope ground lead to the - of a push-pull amp, some bad things can happen. The class D amps will usually cope with the faults. But other topologies may not.
Can be tricky stuff, so sorry if too verbose!