Author Topic: DIY ACV/DCV calibrator with AD5791  (Read 3872 times)

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Offline RikV

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Re: DIY ACV/DCV calibrator with AD5791
« Reply #25 on: May 31, 2021, 04:16:38 pm »
Thanks for the response, Micke. I'll keep you informed of my advances. LTZ1000 will have to wait (a long time) but I have a 10V source at hand based on 2x LM399. Tha should work for the beginning.
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Offline mawyatt

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Re: DIY ACV/DCV calibrator with AD5791
« Reply #26 on: May 31, 2021, 07:07:56 pm »

Your design is at another level in performance, nice work!! We did a lower performance calibrator based around the LM399 a few years ago, mostly for a learning experience rather than a precision reference. The output was amplified up to 10 volts, then a 6 equal value resistor 10X divider (3 series, 3 parallel, so 3R to R/3 resistive divider) used to get 1V buffered by a unity gain chopper amp, then this output brought down to 0.1V using another 6 resistor divider and chopper amp.

Just 3D printed a case, and also did a 3D case for a LTZ1000 & LM399 combined version but this doesn't have a 1V or 0.1V output just zener voltages and 10 volts outputs. Don't have any results yet (busy with other projects), been "aging" the LM399s and now just started on the LTZ1000 "aging", so far seems to be settling in.

Anyway, the 6 resistor divider seems to initially work OK with inexpensive 0.1% 15ppm resistors. Will see how these hold up over time once we begin taking data with the KS34465A which is our best DMM.

Had you considered using a chopped precision voltage (ideal squarewave) as an AC reference? We did this in another design test concept awhile back and used a FF to assure 50% duty cycle, should give same readings for Average DC and AC and ideally be 1/2 the reference.

Research is like a treasure hunt, you don't know where to look or what you'll find!
~Wyatt Labs by Mike~
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Offline Micke

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Re: DIY ACV/DCV calibrator with AD5791
« Reply #27 on: June 01, 2021, 10:42:29 am »
Thanks mawyatt!  :)

Yes aging LM399 and LTZ1000 indeed takes some time!
When I built the first LTZ1000 references I had access to a calibrated 3458A, did measurements every day during workdays, did a "ACAL DCV" before each measurement and logged the measurements in a spreadsheet, voltage dropped every day, but after like 2000h it seems to have settled quite good. I still have them powered 24/7, started March 2017! But unfortunately no 3458A now...

Regarding AD5791, did dome measurements with AD5791A+AD581LH with a just one month old Keysight 34470A, below 1V or so the performance is not just that good, so am planning to add extra resistor divider and chopper amp. I attach measurement as xls and a PDF. Might be my AD5791B+LTZ1000 is behaving better, will do similar measurements and compare!

Have not yet thought of your idea of chopped precision DC voltage, but good idea to be able to get higher frequency! My limit right now is 2kHz and with jitter as bonus...  :o

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