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Equipment or setup for ratio measurements
slugrustle:
I often want to measure the ratio of two quantities, for example
* Input voltage and output voltage of an amplifier to measure gain
* Input current and output voltage of a current measurement chain to measure gain
* And so on
I usually hook up two DMMs, run trend charts to see when things have thermally stabilized, then set both meters to average and take a photo of both DMM screens once the average accumulates about 100 samples. Or after the digits stop moving, either way.
But I think this is somewhat inadequate for more precision work. I could improve the setup by triggering the measurements of both DMMs from the same source and writing an SCPI program to compute the ratio measurement for each simultaneous sample and average those computed values. And maybe I'll do that; it would be an improvement.
Is there a better method than that?
Is there a device that's made to make ratio measurements? I'm thinking like a 2-channel DMM where both channels can do voltage or current on different ranges, but it all hooks back to the same internal reference.
bdunham7:
Do you mean DC ratio of a stable signal? In that case, some DMMs have that as a built in feature, although AFAIK the samples aren't synchronized. And they only do it for voltage over a limited range, so you'd have to prescale/convert any really large (or perhaps small) voltages and any currents. Even with time-shifted sampling, if your signal is steady enough the fact that you are using one DMM with the same reference and input scaling gives you a pretty big advantage over 2 DMMs, at least for voltage ratios.
slugrustle:
Yeah, DC ratio of a stable signal. If the signal varies, it's usually due to thermally related shifts in a current sense shunt resistor or in my equipment. Things are usually pretty settled after 30 minutes.
Although below say 0.25V, I wish I had a more stable voltage source than my ITECH IT6123B. That thing is super nice in terms of being within 1mV across its range, typically, but at low voltages, even 100µV of drift matters.
Oh, perhaps I should have said: I thought to write this post while taking that measurement with 250mV and then 125mV input to hit a certain range on an amplifier. I could see the signal drifting during the long term average, hence my thought to synchronize the meters. Maybe I just need a more stable source for low voltages.
bdunham7:
--- Quote from: slugrustle on February 01, 2024, 05:07:18 am ---I could see the signal drifting during the long term average, hence my thought to synchronize the meters. Maybe I just need a more stable source for low voltages.
--- End quote ---
If the drift is slow, the short time interval between alternate samples would be very small. Your average and SD on the direct ratio measurement would give you a pretty good idea of the gain constant that you are looking for as well as the uncertainty of the measurement. The only drawback is I think most meters with a ratio function require the two signals to have a common.
slugrustle:
Yeah, agreed. I think drift between samples, even at 10 NPLC integration time, would be negligible.
I'll look around at meters with ratio measurements. But restrictions on ranges between the two measurements would probably be a show stopper, at least in some situations. I'm sometimes looking at amps with high DC gain.
And maybe synchronizing my two DMMs, or even using bus triggering with SCPI and not even synchronizing via a trigger mechanism, would be good enough.
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