If you want to measure AC (for feedback for example) similarly, probably the easiest to do is with a RMS to DC converter chip from AD. The LT parts from their portfolio are using delta-sigma modulation to do this conversion, and they are quite accurate once calibrated.
However if you want to be able to compare to the international standards you need to use a thermal converter. They are finicky, fragile and drift a lot but they are still the gold standard for AC.
I was thinking about the limitations of the square wave calibration. You could predict the error from the rolloff I believe and possibly figure out the bandwidth of the meter and potential HF accuracy using several frequencies. Ultimately you would need to compare back to a sine wave reference to be sure. I don't think its a dead end but the bandwidth correction will be essential for accuracy. I'm not sure how something like the HP3458 would react to that, the HP 3458 uses sampling in effect for AC measurement so some strange things could happen at certain frequencies.
The current best efforts for AC at midband (100 Hz to 10 KHz) are around 10 ppm or so.
HP made a sampling AC voltmeter that was good to 1 GHz around 50 years ago. They abandoned it for some reason, like no customers? They also made a Thermal RF power meter with higher accuracy (a version is still in production for $20K +) which does still have customers.
If you want to measure AC (for feedback for example) similarly, probably the easiest to do is with a RMS to DC converter chip from AD. The LT parts from their portfolio are using delta-sigma modulation to do this conversion, and they are quite accurate once calibrated.
If I wanted to measure RMS with as much accuracy as possible, then I would use a high resolution sampling converter and calculate it directly. Such a sampling converter could be integrated, or if I thought I could get better performance, I might use a discrete high precision sampler in front of a slower but more accurate converter.
The most significant limitation to accuracy would be the passband response of the stages up to and including the sampler. If the frequency content can be known, then correction could be applied after conversion.
HP made a sampling AC voltmeter that was good to 1 GHz around 50 years ago. They abandoned it for some reason, like no customers? They also made a Thermal RF power meter with higher accuracy (a version is still in production for $20K +) which does still have customers.
If I wanted to measure RMS with as much accuracy as possible, then I would use a high resolution sampling converter and calculate it directly. Such a sampling converter could be integrated, or if I thought I could get better performance, I might use a discrete high precision sampler in front of a slower but more accurate converter.
The most significant limitation to accuracy would be the passband response of the stages up to and including the sampler. If the frequency content can be known, then correction could be applied after conversion.
I think you underestimate the complexity of such systems.
Let's take the LTC1968. It will work to 500khz without too much trouble, costs 4 USD and it needs one capacitor (film preferably) and a slow 12-16 bit ADC to digitize it.
If you want to make a similar system with sampling, you have to make a DC ad AC accurate antialiasing filter,
Sample it with an ADC at least 1MSPS, but realistically, much higher, I would say 5MSPS. Then you have to make sure that your sampling frequency is not the multiple of the incoming signal, because then you sample it the same phase. Or you undersample it, and use an ADC with high input bandwidth. It will be a SAR ADC in either case, others will not have the sample rate.
Nevertheless, driving these is not trivial, needs a high bandwidth opamp, in the order of hundreds of MHz otherwise the input of the ADC will miss codes and all kinds of nastiness.
Then we arrive at the microcontroller.
When you all verified the system, wrote the code, and finished it up... Sounds like a lot of work doesn't it? Could be more accurate? Yes, sure, but be ready to those debugging sessions, when "I wish I would have a 16 bit scope to see what my ADC driver is doing, and why do I have INL errors in my output code".
HP made a sampling AC voltmeter that was good to 1 GHz around 50 years ago. They abandoned it for some reason, like no customers? They also made a Thermal RF power meter with higher accuracy (a version is still in production for $20K +) which does still have customers.
Racal-Dana had one also. I have a couple of Tektronix 7S11 samplers so could put something with similar or better performance together in about 10 minutes with what I have on hand. Tektronix even published an application note showing how to do it.
The advantage of sampling measurement over linear measurement (for lack of a better term) is that the frequency response of the sampler can be calculated from the sampling gate width which can be measured with an unleveled RF source. So if you know the frequency components of the measured signal, which will often be the case with an RF sampling voltmeter measurement, a simple lookup will correct for the passband response. Racal-Dana included the correction curve for their sampling RMS voltmeter in their manual.If I wanted to measure RMS with as much accuracy as possible, then I would use a high resolution sampling converter and calculate it directly. Such a sampling converter could be integrated, or if I thought I could get better performance, I might use a discrete high precision sampler in front of a slower but more accurate converter.
The most significant limitation to accuracy would be the passband response of the stages up to and including the sampler. If the frequency content can be known, then correction could be applied after conversion.
I think you underestimate the complexity of such systems.
Let's take the LTC1968. It will work to 500khz without too much trouble, costs 4 USD and it needs one capacitor (film preferably) and a slow 12-16 bit ADC to digitize it.
If you want to make a similar system with sampling, you have to make a DC ad AC accurate antialiasing filter,
Nope, one of the chief advantages of the sampling RMS measurement is that no antialiasing is required at all.
But what I was suggesting is either precision sampling followed by slow precision RMS conversion, or the direct computational approach where a high frequency high resolution sampling ADC is used for low frequency RMS conversion. I was told about 20 years ago that some multimeters had started using the later method in lieu of analog computation but have not been able to verify that.
The Racal-Dana and HP RF voltmeters mentioned above use the former method, and I am suggesting extending this idea to low frequency measurement.QuoteSample it with an ADC at least 1MSPS, but realistically, much higher, I would say 5MSPS. Then you have to make sure that your sampling frequency is not the multiple of the incoming signal, because then you sample it the same phase. Or you undersample it, and use an ADC with high input bandwidth. It will be a SAR ADC in either case, others will not have the sample rate.
The bandwidth is determined by the sampling gate width, which produces a non-linear passband response that can be easily calculated.
I suggested a simple system relying on a sampling ADC with high input bandwidth used at much lower frequencies, but an alternative is to implement a precision external sampler with a slower ADC. I messed around with the later many many years ago but not with this application in mind and I mostly learned what does not work for precision sampling.QuoteNevertheless, driving these is not trivial, needs a high bandwidth opamp, in the order of hundreds of MHz otherwise the input of the ADC will miss codes and all kinds of nastiness.
The chief advantage of the sampling method is that within the input signal range, no processing is required before sampling, removing those error contributions, which is the major reason to use it. I can design and fabricate a sampler or sampling system, but constructing a high accuracy thermal RMS converter would be much more difficult, at least for me.QuoteThen we arrive at the microcontroller.
When you all verified the system, wrote the code, and finished it up... Sounds like a lot of work doesn't it? Could be more accurate? Yes, sure, but be ready to those debugging sessions, when "I wish I would have a 16 bit scope to see what my ADC driver is doing, and why do I have INL errors in my output code".
Calibration requires an unleveled RF source to measure the sampling gate width. If you are desperate, processing after sampling can be done with any low frequency RMS measuring tool. I could set something up which makes RMS measurements to GHz frequencies in about 10 minutes with what I have on hand, but it would not be real suitable for the low frequency high resolution measurements we are discussing, although it would work.
OK, an RF sampling ADC. I'm not familiar with those, so actually don't know how well they would work, so I take your word for it.
I can tell you that this wouldn't work with a SAR ADC.
Those LTC196x chips actually are not thermal based, they are using some sigma-delta conversion, and they are super simple to use. But I guess, each to their own.
I have been told that some modern multimeters do exactly this with a fast high resolution sampling SAR ADC operating at a relatively low sample rate and RMS conversion done on the digital side, but I have not confirmed it directly.
Not sure exactly how the DMM6500 performs RMS, but it agrees very well with the KS34465A which also uses a digital computational method. Maybe the Keysight and Keithley methods are similar. We found the old 34401A using the analog method also agree well with the DMM6500 which is our latest DMM only a few weeks old. Superb instrument BTW!!
I guess one could assume that both methods should give acceptable results if the Crest Factor and Waveform Fundamental Frequency are low enough. That was the basis for using the squarewave as a comparison waveform, since it's relatively easy to get a precise known peak value.
Best,
I have been told that some modern multimeters do exactly this with a fast high resolution sampling SAR ADC operating at a relatively low sample rate and RMS conversion done on the digital side, but I have not confirmed it directly.
The relatively popular DMM chip set HY3131 (e.g. used in one of Daves DMMs) have the option to use digital RMS with a relatively fast internal SD ADC.
The BW is still a bit limited in this case, and an external RMS chip may still be used. Both ways have there advantages.
The LTC196x would use switched capacitors like other SD ADC chips and many modern SAR ADC chips. So the driving needs can be similar.
For the RMS measurement a defined bandwidth can be an advantage. Higher BW is not allways better as this also means more noise. Especially with a calibrator the signal is relatively well behaved with a known frequency and low crest factor. The higher harmonics should not contribute too much. So things are a bit easier than with a normal DMM input. If filtering is needed some of this could be before the output and no extra anti aliasing needed at the ADC.
For the RMS calculation alising can be good and include frequencies higher than Fs without much extra effort. So the limit is not from the sampling rate, but the sampling part of the ADC and the input amplifier.
Perhaps start with an 8k resistive load that reflects 8 ohm back to the amplifier (eg. for the M1120, the 1.25W tap presents 8kohm for an 8 ohm speaker load, with a 31.6:1 turns ratio). You could sweep through the frequency range to see what voltage response you get, but at low voltage (as an 8k load would consume some power for higher voltages). You may still get a reasonably flat response for higher resistance loads.
It however does not really test possible ringing mich higher than the amplifiers BW. It can be enough for the test, but it may miss the upper frequency end, which is often the point where ringing happens.