mains power measurement (again)

[djacobow]

In short: Get thee to a calibration lab.

Well, I was considering this the option of last resort. That's money and this is a hobby project.

If I were to go, I'd go with my DUCs. There are no trimpots on this board. All cal is done in software.

Though I have tried to be very careful with isolation, creepage/clearance, and I think it's probably about right, if not overkill for a 120V device, this is not a commercial product, and I don't need to meet UL standards.

Alternatively, you could look at hiring (or buying second hand) an AC calibrator.

That is an idea I can look into more. I mean, the hiring, not the buying.

[floobydust]

I noticed R9, R10 are seeing near mains voltage according to your simulation. The current-sense voltage divider is going to smoke?
Any high freq. noise bursts on mains overloads R7, R8, so they must be sized bigger than expected.

[coppice]

Based on your advice, Coppice, I just tried quick-n-dirty simulating the input filters based on the input impedances in the ATM90E26 datasheet (400k single-ended for the voltage inputs and 1k single-ended for the shunt input). Using a pretty significantly lagging load, I didn't really see much of a phase mismatch between the current in the shunt and the filtered voltage difference around it, nor did I see it between the voltage at the load and the voltage difference at the voltage inputs.

I could have set this up wrong, of course. I'm attaching the shot of the circuit and plots.

The two things critical to getting the phase shift of the filter to zero are the source impedances feeding into it, and how well the two halves of the differential filter balance. Try opposite extremes of tolerance on the capacitors.

How do you expect to see a fraction of a degree phase shift on those graphs?

[djacobow]

The two things critical to getting the phase shift of the filter to zero are the source impedances feeding into it, and how well the two halves of the differential filter balance. Try opposite extremes of tolerance on the capacitors.

Yeah, well, not much I can do about the source impedance. The source impedance of the AC line coming is probably on the order of a couple of ohms. I don't think that would affect the current measurement much. The source impedance of the voltage measurement is high, though,  because of the resistor divider.

How do you expect to see a fraction of a degree phase shift on those graphs?

How much does a fraction of a degree matter? pf = cos(phase_difference). So the maximum delta pf would be where the derivative (sin) is max, so pi/2, right? So, worst case error due to phase shift is when true phase is 45  degrees (and 135, 225, etc). So, say the phase angle is 45 degrees, but I measure 2 degrees off. That would get a pf of 0.731 where the true pf is 0.707, or 3.5% error. Not great, for sure, but something I'll have to think about tolerating given the calibration resources I have available to me.

EDIT: Whoops, that's not right. That's the point of worst case error in pf, not the point of worst case relative error. The worse case  relative, I guess, would occur at 90 degrees, where the true pf is 0 and a two degree error would yield a calculated pf of 0.035. Relative to 0, that's an infinity percent error. :-) Like if I had a 1kVAR load on there, I'd be saying there was 35W of real power, when in fact there should be none.

I'll look into the caps being at opposite ends of their tolerance bands as you suggest. In general, I took 5 minutes to make that sim, and it needs more thought.

[djacobow]

I noticed R9, R10 are seeing near mains voltage according to your simulation. The current-sense voltage divider is going to smoke?
Any high freq. noise bursts on mains overloads R7, R8, so they must be sized bigger than expected.

Well, in the real circuit R9 and R10 are the inputs to the differential ADC, and they are high impedance relative to "ground" so they don't really blow up. But I misinterpreted what the data sheet means by "single ended input impedance" and those resistors should not be grounded, but only connect to each other and where they connect, that connected to ground through a ridiculously high impedance like 100M. Result isn't much different.

Good catch on R7 and R8. Noise, and input protection are not something I have looked at.

I suspect that these filter networks are not a bad starting point, but without proper measurement equipment, I'm not sure I'll ever know.