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Thoughts on voltage reference board?

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c4757p:
I've been working on building replacements for some of my old, bulky equipment (mostly just for fun), and the current contestant on Who Wants to Be Replaced is my Power Designs 2005 precision power supply. It's specified at 0.1% ± 1mV with five digit precision to 20.000V and 10ppm/°C; I'd like to at least match that, adding computer control, multiple outputs (one high precision and two lower precision, all operating off the same reference, part of my recent efforts related to automated testing) and better constant-current accuracy.

Here's what I have for the voltage reference board. Does anybody have any suggestions or corrections before I make it? Some of the parts are pretty expensive (free samples, hell yes - but I've only got one of each), so I don't want to screw it up. I'd especially rather not have to desolder the reference chip once it's been soldered on.

The following parts (at least - depending on what I can fit, I haven't done the PCB yet) will be under a small metal can with a heater, which is then surrounded by insulation, for thermal stability. (Gilding the lily a bit, most of this has good specs over a full typical ambient temperature range.) It will be kept at 50°C by an external PIC running a P-I algorithm.

- U2 (LM317, powers the reference) and its feedback resistors
- TL431 (powers U3 - missing a ref des... oh well)
- U1 (ADR4550 voltage reference, 2ppm/°C).
- Output voltage divider and trimmer.
- U3 (MCP6V06 output chopper amp).
- Of course, U4 (LM35 temperature sensor), and Q3 and Q4, the heaters.

Keep in mind, I've chosen some of the parts simply because I have them, so I know some of it isn't the most ideal design. Still, if my calculations are correct, I should be able to very easily achieve 1-3ppm/°C and hopefully better than that.

Edit: Removed the old schematic to not waste too much of Dave's server space. See below for updated.

c4757p:
I may toss the temperature controller PIC on the board too, outside the can. I didn't want anything digital on it, but with the grounded metal can surrounding the oven area, it's pretty much a separate PCB-within-a-PCB, and that would make testing easier since I can get it up to fully regulated temperature by itself without having the main board attached.

GK:
I'm pretty sure the TL431 is intollerant of capacitive loads (oscillates with a parallel capacitor).

c4757p:
It's intolerant of capacitive loads between about 10nF and 5uF (depending on voltage and current), but loads much larger than that are OK.

Updated to include PIC.

Of particular interest to me is the trimmer network for the reference. I borrowed that arrangement from the TI datasheet for LM399. They don't address why it's like that, but as far as I can tell, it's to compensate for the shit tempco of the trim pot. While the total resistance of the pot may drift, the proportion should not, and the dependence here on the total pot resistance is small. (I may decrease it further to 10k to decrease the dependence even more, but I want to keep the output current from the ADR4550 low.) Is this correct?

Edit: Removed the old schematic to not waste too much of Dave's server space. See below for updated.

quantumvolt:

--- Quote from: c4757p on July 23, 2013, 12:54:33 am ---...
Of particular interest to me is the trimmer network for the reference. I borrowed that arrangement from the TI datasheet for LM399. They don't address why it's like that, but as far as I can tell, it's to compensate for the shit tempco of the trim pot. While the total resistance of the pot may drift, the proportion should not, and the dependence here on the total pot resistance is small. (I may decrease it further to 10k to decrease the dependence even more, but I want to keep the output current from the ADR4550 low.) Is this correct?

--- End quote ---

Yes. The op amp sees 5 k from the two 10 k resistors, but more than 68 k from the trim. So the effect is small. If you don't like the full swing of the trim, you can change it for two (possibly different low tempco) resistors and a smaller trim.

In general, imo your design cannot be flawed. All you do is hooking up the parts in ways that have been done over and over again. No fancy analog regulating or feedback circuitry. You might have to change/add/remove small bits if you want to optimize the circuit. But that is normal.

If the 7.5 V is solely an internal supply I don't see the point in protecting it unless you will plug in modules with possibly charged capacitors. Also the 10 Ohm low pass filter with the small 100 uF looks special imo.

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