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I would like to measure either power (running average, for example exponential average with a certain time constant) or energy (power integrated over time) with an analog circuit.

The setup I have is a pulse generator and the average output power and single-pulse energy are both important parameters. However, measuring current and voltage with an ADC and then calculating power digitally is unlikely to work well since the signal is fairly broadband, so it would have to be a heck of a fast ADC. I also would like to be able to measure single-pulse energy in a purely analog way for reliability reasons.

I think this must be a common problem in true-RMS power meters, but I have no idea how to approach it. Thanks for your ideas!

P.S. The question is not how to do a bench measurement (no doubt there are off the shelf tools for that) but how to build a circuit that does the measurement that can be used as part of the device I'm working on.

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  • \$\begingroup\$ Would quantizing it into a lower-bandwidth stream be acceptable? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Nov 23, 2014 at 3:02
  • \$\begingroup\$ @IgnacioVasquezAbrams: it would have to be high sample rate, the output pulses are a microsecond or two long. Using something like a 100msps adc is not possible for cost/space/power reasons. \$\endgroup\$
    – Alex I
    Commented Nov 23, 2014 at 3:09
  • \$\begingroup\$ "I think this must be a common problem in true-RMS power meters" -- these are often bandwidth-limited. And in kWh-meters you can do a lot of tricks such as sample for example at 49.9 Hz for 50Hz grids or 59.9 Hz for 60Hz grids, so that you get sligtly more than 1 sample per wave and the sampling point slowly "drifts" through the waveform. It works because power grid waveforms have a tendency of being very uniform for most of the time. \$\endgroup\$
    – PkP
    Commented Nov 24, 2014 at 19:12

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An analog 4-quadrant multiplier with (say) 10MHz bandwidth eg. AD734 could be used to calculate the instantaneous power. You could then low-pass anti-alias filter the resulting signal and digitize it.

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