Electronics > Metrology

LM399 based 10 V reference

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dietert1:
Looking at this image i noticed a wiring error. The second PWM DAC (ripple cancellation) got wired to flip flop pin 10 instead of pin 8. Pin 8 remained unconnected. In effect ripple cancellation was implemented but remained inactive. Some days ago i corrected that mistake. PWM output shifted by -2 +/- 0.5 ppm and noise is lower now. Right now the HP 3456A used for logging delivered more than 18000 consecutive readings with no other results than 10.00000 and 10.00001 V. Need to improve logging for better resolution.

Some months ago i got a batch of 10x LM399AH and made two more 5x LM399 modules like the one shown above. When i started fitting them into an enclosure i got the idea to change the circuit into 5x 14 V instead of 10x 7 V. Then the PWM DAC divides the 14 V average down to 10 V. The DAC will no longer be in a control loop, but just feed an output buffer. And the reference will have programmable output voltage proportional to PWM ratio. I already got the ten heaters and the 5x 14 V pairs working. Power consumption of the ten heaters is about 1.8 W but there will be more insulation. This concept can be very cost efficient, since a TEC controller isn't needed.

Regards, Dieter

Kleinstein:
Starting from some 14 V (2 reference in series) and go down with a PWM divider makes some sense, especially with a variable PWM ratio, to also get voltages different from 10 V.  A similar configuration is used with some of the fluke calibrators.
Driving the 14 V reference may need an auxiliary voltage a little higher than 15 V, so likely more than 15 V supply needed.

With multiple LM399 in parallel it may be worth having a way to measure the individual reference too, to identify more drifty units and also the higher / lower noise ones. The noise can be quite different between units and a single noisy unit would effect the average quite a bit. Ideally a more noisy unit would get a larger resistor to contribut (R ~ En²) to get the lowest noise for the average.

dietert1:
Yes, the supply is completely different now. The heaters are running from 19 V. The same voltage is used to drive the dual n-channel mosfet for the DAC output stage. And there is a 1.2x amplifier to derive 17 V from the 14 V reference voltage. This amplifier isn't very critical, deviations get attenuated by about a factor 3000 (2 * 0,5 Ohm / 3 KOhm).
Then there is a buffer between reference voltage and PWM output stage. So, together with output buffer of the PWM filter this time there are two opamps involved with their errors. The output buffer may need bootstrapping for perfect linearity, don't know yet.

Regards, Dieter

TiN:
Was the ref powered all the time with no interruptions? Drift can't be estimated easily with just few points for not permanently on zeners.

dietert1:
Yes, after there was some interest in this PWM reference i kept it logging continuously. Will show results later.

Regards, Dieter

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