Electronics > Metrology

DIY Precision AC-RMS to DC Transfer Standard

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I have been working on a simple precision DC to Sinewave converter - so if you enter 5V DC in, you get a very precise and very stable 5V AC out. Enter 10mV DC in and you get 10mV AC out.

But then I started to think - How do you verify it is accurate?

I have a Fluke 540B that can thermally compare the heat produced in a 100 ohm resistor by AC to the heat produces by DC and it has a 0.01% accuracy. The great thing about this solution is you only need to have an accurate DC standard, and you can use it for AC calibration as well.


Can common parts get similar results?

Thinking about it, I went for a 100 ohm 402 SMD load resistor glued to diode (a C-B junction of a SMD NPN transistor). The use of a diode as a temperature sensor allows two options - forward biased where you get about -2mV/C coefficient. This is good as we need to know the resistor temperature. My initial target is 100 degC so that I have a safe margin. The diode could also be used in reverse mode using the diode leakage current.

The wires are 0.1mm enamelled copper - soldering wasn't much fun. Finding very fine wire is easy - old phone earpieces, small speakers, etc. Mine came from a dud 40mm speaker from a transistor radio.

The initial results surprised me. Got the 100 Deg C target temperature at 3V - that is 12 times worse then the Fluke, but the settling time looked useable. I was getting the diode voltage going up and down by 50mV every few seconds. Then I realised that this was just air movement in a room. Stuck it in a jar and it became very stable - no fluctuations at all.

Based on my very quick first tests, 0.01% should be achievable. The sensitivity means that 0.01% difference between the AC and DC corresponds to a 20uV DC voltage change.  I am not concerned with the high power requirements of my sensor as we have something that Fluke did not have in the 1960's - we can easily get 100MHz+ opamps that should have a flat enough frequency response to get DC to 1kHz accuracies of 0.01% easily.

Wow very neat

Mickle T.:
Nice work!

There is another way to make a stable and predictable AC voltage standard via the calculable AC voltage reference.

The Solutions : Calculable AC Voltage Reference https://www.ncsli.org/c/f/p13/REG_2013.PRE.1147.1876.ppt
Digitally Generated AC Reference Source http://www.transmille.net/Presentations/AC%20Reference/Digital%20AC%20Source.pdf
Guildline Model 7410 AC Voltage Reference http://www.guildline.com/Datasheet/Guildline7410Datasheet.pdf
Sine Wave Generation Techniques http://my.ece.ucsb.edu/York/Bobsclass/2C/Tutorials/App%20notes/an-263.pdf

This is my first try: simplified version of the 50 Hz - 1 MHz fixed voltages AC calibrator.

Looks very interesting
thanks for sharing

Very interesting, I know in a LT app note they use a dual matched thermistor. That was for higher frequencies, many Mhz.
 Subscribing to this topic.


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