The performance of the analog RMS chips like AD736 or AD8436 is not that great: The accuracy and BW is only good for not too small an amplitude and the frequency range is somewhat limited to the low side (e.g. already no longer really RMS below some 20-40 Hz, depending on the filter).
It does not need that much resolution (e.g. 12-14 bit ) to get comparable or better performance, with different weak points for both systems.
The ADC can be sampling, maybe a little better than those found in µCs, but also a SD ADCs. AFAIk the DMM chip-sets with integrated RMS use SD ADCs - so the same technique as the main ADC for DC, just faster.
The digital energy / power meter chips use something like 16+ bit 100 kHz range SD-ADCs, somewhat similar to the audio ADCs, though with better gain stability. They need some of the resolution to get good accuracy over a large current range (like still 1% accuracy with 0.1% of the FS current). Chances are some of the BW limiting is intentional because of the standards for power metering.
A more normal handheld meter should get way with less than 5 digits for AC and thus less ADC resolution.
The digital computation might as well be error free, but it is at the mercy of the high resolution sampling, which is no trivial task. We now have inexpensive integrated sampling ADCs with incredible performance.
Yes the newer IC ADCs are amazing in performance vs. cost, a direct tribute to the digital influenced CMOS scaling
Best,
The digital computation might as well be error free, but it is at the mercy of the high resolution sampling, which is no trivial task. We now have inexpensive integrated sampling ADCs with incredible performance.
it is available for a long time, but they have high power consumption which is not acceptable for handheld DMM which is powered from a battery.
The digital computation might as well be error free, but it is at the mercy of the high resolution sampling, which is no trivial task. We now have inexpensive integrated sampling ADCs with incredible performance.
it is available for a long time, but they have high power consumption which is not acceptable for handheld DMM which is powered from a battery.
Analog Devices has suitable ADCs now like the AD7988-1 which draw only a couple hundred microamps, so the power draw of the ADC itself would be insignificant. Of course that is not the sort of part which would be found in a device intended for the lowest cost market.
The chips for power metering use digital calculation, mainly for the true power, but many do also calculate the RMS values for the current and voltage. This market is somewhat price sensitive and not that small in numbers. However the chips indeed use a bit much power (e.g. 2-10 mA range) for bettery operation. Just a DMM function would be 1/2 or 1/3 of this with only 1 channel instead of 2 ADCs and power, current and voltage and maybe a few more results. Some of the new SAR chips are indeed rather low power and would be OK for battery operation.
Not sure how much worse a system using a µC internal 12 ADC would be. The computation part is not that bad and would not need a very fast µC (e.g. some 4 MHz clock for a ARM based µC could be enough). Chances are a µC with a slightly improved (e.g. 14 bit) ADC may be all it takes to get digital RMS with good performance. Even with only a 10 Bit ADC digital RMS is already at about 3 digit level and useful for many tasks.