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I'm thinking about building a monitoring circuit for an existing residential power system. Currently there's a simple ammeter and digital voltmeter in use for some basic monitoring, but nothing fancy.

I'd like to augment this with something that supports some history and statistics. I've got some microcontrollers and LCDs lying about that I'd like to use for this, ideally. Here's a slightly simplified schematic for the system I'm thinking about trying building:

schematic

simulate this circuit – Schematic created using CircuitLab

Here's my concern: If I'm trying to track power use, then fundamentally, sampling with the A/D converter is giving me a box integration of the voltage difference across the shunt. Now, I can assuage my worries by using an A/D with a really fast sampling rate, but I don't know enough about the charge source input to be able to say how quickly the amperage will be changing. I suppose what I'm worried about is quick current spikes (<100μs) that regular sampling at 10ksps will miss.

What I'm considering is adding some filtering caps to the input, but since the amperage involved is quite high (>= 60A, and the shunt is rated up to 500A so higher peaks are definitely possible) I'm not sure that would be helpful. Am I just being very silly since A/D and amp error will contribute more to overall error than such peaks? Or would throwing a couple of low-ESR 100nF, 47μF, and 470μF caps actually make a significant difference? Is there anything else I should be concerned with? I'm quite the newbie when it comes to analog electronics, so I'm probably missing plenty of silly things. :)

Thanks!

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    \$\begingroup\$ Your filtering can happen before or after the amplifier. The LT1787 provides dedicated pins for filtering the input signal. You could replicate the four-resistor, one-capacitor filter, externally, but would need to match those resistors. Look at amplifiers specifically for current sensing, not general-purpose instrumentation amplifiers. \$\endgroup\$
    – user2943160
    Commented Jun 8, 2016 at 20:46
  • \$\begingroup\$ If you are planning on monitoring AC mains power it's going to be much more complicated than your question suggest that you think. It may be worth your while looking at openenergymonitor.org/emon where full designs, hardware and code are given for domestic power meters and analysers. You don't need to worry about < 100 μs or 100 ms spikes, for that matter, as your utility meter is unlikely to see them either. \$\endgroup\$
    – Transistor
    Commented Jun 8, 2016 at 22:19

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Putting sensible-sized caps across a 0.01 ohm resistor won't make much difference.

Instead add a low-pass filter ( a simple one would be a series resistor of much higher value and parallel capacitor) with an appropriate cutoff frequency, and sample much higher than the cutoff.

Ideally you want it to attenuate frequency components below half the sampling frequency to below the least significant bit.

Your circuit as shown will only work if the instrumentation amp gets a high enough supply voltage that 12V is within the common mode range of the amplifier. That range often does not include the positive rail.

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