[Since87]

Originally posted by Groth

In some amp pdf I was reading (which one? :shrug, they talked about noise propagation in cascaded amplifiers. It convinced me to try some gain in the difference stage for version .3.

I'd thought about adding gain at the difference amp, but then I wouldn't be able to use the resistor networks I can get 'cheaply'. Having a first stage output that is of known accuracy without any tweaking would cut down on the number of pots needed to get an accurate output. As you mention though, there are potential advantages WRT noise in putting gain in the first stage.

Originally posted by Groth

I'm also planning to roll back the first stage filter to it's former 1 Mhz corner - I don't want to throw out what may be signal when summed with the other channels.

There's a problem here, in that your A/D is only sampling at what? Around 10 Hz maybe? Nyquist's Theorem states that your sampled result is only going to be valid for signal components whose frequency is below half the sampling rate. Signal components of higher frequency are going to be aliased and effectively result in noise in your measurement. I can't think of anything to be gained by preserving high frequency components unless, a much higher speed A/D is going to be used.

Also, with a 1 MHz corner frequency, matching the capacitance value of the filter capacitors becomes critical in preserving the high frequency components with any accuracy.

When you increase the gain of the difference amp, you lose bandwidth too, which will cause problems. You can reduce this problem by using an OPA228 instead of an OPA227. (The OPA228 is only stable with a gain of 5 or higher.)

I'm just not seeing any practical benefits to higher bandwidth, and I'm seeing a lot of difficulties to overcome.

Originally posted by Groth

Don't s'pose you could borrow that oscilloscope again, and take a peek at the switching noise?

Sorry, I'll leave the risk of damaging a motherboard to others.

If you look at the datasheet for the regulator on pHaestus' mobo, Figure 1 shows the basic triangle wave of the current through the switching inductor. I know from experience that there will be substantial spikes that ring at high freqency at the peaks of those waveforms, but for some reason they don't show up on that scope image. (I suspect clamp on current probes were used to acquire those signals, and the current probes didn't have the bandwidth to show what was going on at high frequency.)