12-02-2014, 12:05 PM
Measuring AC is surprisingly difficult. Measuring "true RMS" is even harder 
For example, just compare the DC and AC specification of a good DVM - let's take the Fluke 87V: 0.05% vs 0.7%
The USP of the heater method is that it works over a very wide bandwidth. Very important to know the BW of your test gear (a fairly pedestrian 20kHz for the 87V - the 187 does 100kHz).
As far as I'm aware, you can't build an RMS rectifier with just a single op-amp; the best you could come up with would be "peak or mean sensing, RMS calibrated", meaning that the output is simply scaled to give the right RMS answer from whatever detection method you employ. As such, it becomes inaccurate with anything that isn't a sine wave. To do genuine RMS detection, you need an IC containing very complex analogue circuity - usually employing analogue multipliers and other such gems to do the root-mean-square "calculation".
They are like miniature analogue computers! For years, the AD536 was commonly used in precision voltmeters - indeed, I note that it is still in production. They were even used in audio - some compressor/expanders used them to measure the amplitude of the signal, and it was claimed that RMS detection gave better results subjectively than peak or mean sensing. Maybe, maybe not, but using one of these ICs was a lot easier than building an op-amp based rectifier, if you didn't mind the much higher BOM cost. But I digress... Have a look at the datasheet - it's fascinating stuff:
www.analog.com/static/imported-files/data_sheets/AD536A.pdf
Back to multimeters, they do still seem to do the RMS conversion in the analogue domain - perhaps surprisingly in this day and age. So if you see "True RMS" flagged on the front panel, you will find a dedicated chip in there doing just this - you can verify this by looking for a specification for "crest factor". In my experience of Fluke and similar models, they do AC-couple the converter. The higher-end models - such as the 187, 189, 287, 289 - do have AC+DC modes, where they display both quantities separately, or add them together for you. Very nice, occasionally very useful. I also have that facility on an old Tektronix DMM914, and the Fluke 45 (a basic bench meter).
So in general, it seems that most DMMs AC-couple the rectifier - true-RMS or not - and I don't think that's a massive problem. Should you really need to do the measurement on a machine that doesn't have the AC+DC mode, there's nothing stopping you taking both a DC and an AC measurement, and manually summing them.
It's worth saying that a multimeter connected to Lawrence's pulse would give readings on both AC and DC - the accuracy and utility would depend on both the signal and the instrument, but it proves beyond all reasonable doubt that we have a mixture of AC and DC signals here
For example, just compare the DC and AC specification of a good DVM - let's take the Fluke 87V: 0.05% vs 0.7%
The USP of the heater method is that it works over a very wide bandwidth. Very important to know the BW of your test gear (a fairly pedestrian 20kHz for the 87V - the 187 does 100kHz).
As far as I'm aware, you can't build an RMS rectifier with just a single op-amp; the best you could come up with would be "peak or mean sensing, RMS calibrated", meaning that the output is simply scaled to give the right RMS answer from whatever detection method you employ. As such, it becomes inaccurate with anything that isn't a sine wave. To do genuine RMS detection, you need an IC containing very complex analogue circuity - usually employing analogue multipliers and other such gems to do the root-mean-square "calculation".
They are like miniature analogue computers! For years, the AD536 was commonly used in precision voltmeters - indeed, I note that it is still in production. They were even used in audio - some compressor/expanders used them to measure the amplitude of the signal, and it was claimed that RMS detection gave better results subjectively than peak or mean sensing. Maybe, maybe not, but using one of these ICs was a lot easier than building an op-amp based rectifier, if you didn't mind the much higher BOM cost. But I digress... Have a look at the datasheet - it's fascinating stuff:
www.analog.com/static/imported-files/data_sheets/AD536A.pdf
Back to multimeters, they do still seem to do the RMS conversion in the analogue domain - perhaps surprisingly in this day and age. So if you see "True RMS" flagged on the front panel, you will find a dedicated chip in there doing just this - you can verify this by looking for a specification for "crest factor". In my experience of Fluke and similar models, they do AC-couple the converter. The higher-end models - such as the 187, 189, 287, 289 - do have AC+DC modes, where they display both quantities separately, or add them together for you. Very nice, occasionally very useful. I also have that facility on an old Tektronix DMM914, and the Fluke 45 (a basic bench meter).
So in general, it seems that most DMMs AC-couple the rectifier - true-RMS or not - and I don't think that's a massive problem. Should you really need to do the measurement on a machine that doesn't have the AC+DC mode, there's nothing stopping you taking both a DC and an AC measurement, and manually summing them.
It's worth saying that a multimeter connected to Lawrence's pulse would give readings on both AC and DC - the accuracy and utility would depend on both the signal and the instrument, but it proves beyond all reasonable doubt that we have a mixture of AC and DC signals here
