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Dave,

Home Run!

"Personally rather than all the messing around testing 100s LM317s, I would have designed a power supply a voltage reference chip used in 6.5 digit multimeters - the LM399 "

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It deftly demonstrates you have never repaired a bench grade DMM. Nor have you read the LM399 and LM317 datasheets. For that matter, you've not bothered with the topic context either. As it is building a PSU for an LM399.

Where do *you* get stable power for high precision long term (multi-year) measurements? I invite you to enlighten us as to how you address the +/-5 % mains tolerances and their ensuing effects. Rotating convertors are fabulous, but rather expensive to operate.

The LM317s are to regulate the mains power to an LM399, one or more CTI OSC5A3B02 OXCOs and the instrumentation amplifier voltage followers. Among other things. I need to reduce the 1e-1 grid variation and *all other* external unpredictable perturbations to well under 1e-9 to 1e-12. Per year!

That requires at least 2 LM317s (e.g. following the preregulator arrangement in the datasheet). I'm likely to do 3 or 4. Dual LM317 circuit from 27 Vp transformer nominal to 15 Vdc with large (100 Farad) capacitor bank to provide load stability followed by 10 V output from 3rd LM317 for the LM399 and the voltage followers with a similar 10 WVDC capacitor bank. Similar feed and topology for the 5 V supply for the OXCO and ATMega. Very heavy filtering and isolation throughout.

By careful selection from data measured over a range of voltages and temperatures, I should be able to make up pairs with as close to a zero tempco as can be done. I'll let an MCU deal with the last few bits.

The tempco matching may be an argument for dual LM317s for each output voltage. I spent much of the day contemplating using a Peltier junction to create a pair of reversible tempco test tunnels.

Have Fun!
Reg