Electronics > Metrology

DIY Precision AC-RMS to DC Transfer Standard

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amspire:
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.

http://bama.edebris.com/manuals/fluke/540b/

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.

alanambrose:
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.

quarks:
Looks very interesting
thanks for sharing

Vgkid:
Very interesting, I know in a LT app note they use a dual matched thermistor. That was for higher frequencies, many Mhz.
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