Why wouldn't you use a Kill A Watt? It must surely be cheaper than anything you can make yourself?
"It depends".
You could have the micro on the mains side too, and just isolate the comms to/from that. There are also SPI and I2C isolators. Incidentally the device you linked does SPI and RS232, not I2C.
If you just want a simple solution: Grab something like the ADUM5211 for RS232 or ADUM5411 for SPI. They provide 30mA of isolated power (plenty for a energy metering AFE) + signalling, all in one chip.
On the CTs vs shunt resistors:
For small currents (< 5A I guess) a shunt resistor works well. They dissipate some heat, so dissipation and temperature coefficient can be an issue.
For large currents (>20A I guess) a CT is used to reduce the current and then put it through a shunt. The current in the shunt is low so dissipation is low.
There’s a range in the middle where things are open for debate :-)
For utility energy measurement, shunts are extensively used up to 200A. The popular energy measurement chips have reasonably high gain ADC inputs, so a 100 micro-ohm shunt for 200A, or a 200 micro-ohm shunt for 100A works well when directly connected to them. The energy loss with those resistances is pretty low. Most energy meter specs define an upper bound for insertion loss, and shunt based solutions easily meet those specs.
You could power it with an isolated high frequency inverter, or a small power transformer, or even with a series capacitor directly from the AC line if its current demands are low enough.
I only need to measure up to a US household circuit, so 20A. I'm using a 1 mOhm shunt (actually, two 2 mOhm in parallel) and did not notice any obvious warming even pulling 13A (1500W space heater) through it. Admittedly, I used the very unscientific measurement of putting my face very, very close to the shunts (without touching) to see if I could feel heat coming off. But at 20A, that's only 200 mW per resistor, and they were 2010 resistors rated for 1 W.
A 2 terminal 1 milli-ohm SMD shunt like that can work very well, as long as you are careful about the PCB layout. If you allow any copper to sit in the measured path, its 0.4%/C temperature coefficient can really hurt your accuracy over a reasonable temperature range. This http://www.analog.com/en/analog-dialogue/articles/optimize-high-current-sensing-accuracy.html has some pretty good info about getting reasonable accuracy from a 2 terminal shunt. Of course, 4 terminal shunts can do an even better job, but at a much higher price.
Short writeup of my meter project: https://toolsofourtools.org/archives/725
In your writeup you don't seem to understand why an energy calibration is needed in addition to the voltage and current calibrations. Its because you need to adjust the phase between the voltage and current samples. Around unity power factor this has almost no effect, but at poor power factors even a small fraction of a degree of phase error starts to affect the accuracy of the power measurements. Typically, a calibration is done with a 60 degree phase shift between the voltage and current waveforms exciting the meter under test. The meter under test is then compared with a reference meter measuring the same voltage and current signals (typically through the energy pulses from the meters, rather than the power measurement), and the phase correction is tweaked to make them match.
Don't think you have a phase shift issue? Try looking at your design again. I say this because a number of people will say "I'm using a shunt. There is no significant phase shift.". However, by the time the voltage and current signals reach the measurement chip there is normally enough phase shift to need tweaking.
In your writeup you don't seem to understand why an energy calibration is needed in addition to the voltage and current calibrations. Its because you need to adjust the phase between the voltage and current samples. Around unity power factor this has almost no effect, but at poor power factors even a small fraction of a degree of phase error starts to affect the accuracy of the power measurements. Typically, a calibration is done with a 60 degree phase shift between the voltage and current waveforms exciting the meter under test. The meter under test is then compared with a reference meter measuring the same voltage and current signals (typically through the energy pulses from the meters, rather than the power measurement), and the phase correction is tweaked to make them match.
Don't think you have a phase shift issue? Try looking at your design again. I say this because a number of people will say "I'm using a shunt. There is no significant phase shift.". However, by the time the voltage and current signals reach the measurement chip there is normally enough phase shift to need tweaking.Yeah, you are right, i didn't understand it, and I am surprised to hear that there is a phase shift between the shunt and the ADC inputs. But I believe you.
Unfortunately, though I can scrounge up passable resistive loads, I don't know of a way to generate calibrated 60 degree or other shift. I mean, I could just make a little RC or LC network, but to get 60 degrees, we're talking a lot of L or C, particularly if the R is low enough that we're going well into the meter's range. That is, I'd get a much better calibration with a 3A current than at 30mA current.
Do you have any suggestions for cheap / clever ways to get the energy cal done without spending the big bucks on the proper test gear?
Professionally these calibrations are done with expensive test benches that contain both a controllable signal source and a high accuracy reference meter. Professionally you don't want to be consuming many kilowatts as you test a meter. The test benches generate separate voltage and current signals, so you can avoid the consumption of power in a load. However, consuming substantial power for a short period is no problem as an experimenter. You just need a load with a horrible power factor, and an accurate meter to use as a reference.
A lot of the kill-a-watt type meters don't do proper power measurements, and give horrible results at poor power factors. However, you seem to have a genuine kill-a-watt. When I have checked a couple of those, the results have been pretty accurate, but I don't know if that is the case with all their models.
So, connect your meter and the kill-a-watt to a load with a horrible power factor, and see how their readings compare. You have self-powered your isolated area with a cap drop supply, so its pulling a little power from the input. The kill--a-watt is also self powered. Whichever you put first in the chain is going to measure the self consumption of the second in the chain. This consumption may not be that high, but it can be enough to distort the measurement results when you are trying to achieve well below 1% error. You could try swapping their orders in the chain, as see the effect on the readings.
The Atmel/Microchip ATM90E26 is a nice device. It started life as an IDT product, and worked really well in customer testing. However, nobody seemed to sign up to use it in volume, and the sold the business was sold to Atmel. Atmel seemed to want it mostly to use the IP in their integrated metering ICs, but they still sell the standalone devices they inherited. Atmel has reduced the spec a bit (e.g. the temp coefficient), but it should still be a nice performer. With a good layout, and a good shunt, it can achieve 0.1% over a wide current range.
If you want to see the main source of those phase shift errors, look at the RC filters they put in front of the ADCs. How well do you think the filter in the voltage channel matches the one in the current channel, especially considering the very different source impedances?