Electronics > Metrology

Need Help with an LM399 Reference with Selectable Output Voltage

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Chance92:

--- Quote from: imo on August 14, 2022, 10:03:17 am ---I would add an output transistor (or better two transistors for the current limiting) at the output of the U5..
PS: see below an example - it assumes your opamp is rail to rail.. You may put that stage at the U4 as well (to de-load the U4).. The max output current aprox 30mA..

--- End quote ---

Thanks! I need to think about how much current I need.

PartialDischarge:

--- Quote from: Chance92 on August 14, 2022, 11:47:30 am ---I didn't know that. My knowledge on op-amps is very limited. Do you have any recommendations for op-amp?
I want to build the reference with maximum stability. I am planning to calibrate the reference afterwards and use those voltages as they are, so that I can avoid the instability introduced by the trimmers. What type of trimmers are you using?

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For the output opamp I can recommend an OPA189 for the low temp drift, low offset and low input bias, but there are many others.
I use multi-turn pots but my application is not very stringent, and anyway it is configured only to have a small adjusting range.

Chance92:

--- Quote from: Zero999 on August 14, 2022, 10:44:51 am ---R10 should be 11.1111 for 1mV out with 10V in. Use 12R and 150R in parallel, with the 12R being more critical.

The output impedance of the divider depends on the tapping point. R11 isn't doing anything. The bias currents of the AD706 are low enough not to matter.

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I was having trouble finding resistors below 1 ohm with TCR. Paralleling them is a good idea. I will try to find the suitable resistors.

Kleinstein:
The staility of the LM399 and ADR1399 is limited. So the AD706 would be good enough and no real need for 0.2 ppm/K or similar grade resiistors.
For the resistors one could consider a ready made resistor array (e.g. LT5400, maybe MORN ) to get a good ratio stability at moderate costs.
For the divider to very low values one can use a 2 stage divider to get a low voltage without needing 10 ohms or lower. So like 10 V to 1 V and 100 mV and than 100 mV to 10 mV as a an extra divider.  Ideally the divider would start from the 7 V and not the 10 V, so that the lower votlages avoid the 7 to 10 V gain step.

Other alternative OPs would be the OPA202, OPA207 and LTC2057, maybe MCP6V51.

The ADR1399 alterntive would like an extra RC for damping and the LM399 would be also be Ok with this.

Ideally there would be some extra measures to allow a capacitive load at the output - many OP-amps have a limited capacitor to drive capacitive loads.

Chance92:

--- Quote from: Kleinstein on August 14, 2022, 02:29:54 pm ---For the divider to very low values one can use a 2 stage divider to get a low voltage without needing 10 ohms or lower. So like 10 V to 1 V and 100 mV and than 100 mV to 10 mV as a an extra divider.  Ideally the divider would start from the 7 V and not the 10 V, so that the lower votlages avoid the 7 to 10 V gain step.

--- End quote ---

Thanks for your suggestions. May I ask how I should put the two dividers together exactly? Do I put a buffer between them or do I directly put the second divider in parallel with part of the first divider?


--- Quote from: Kleinstein on August 14, 2022, 02:29:54 pm ---The ADR1399 alterntive would like an extra RC for damping and the LM399 would be also be Ok with this.

--- End quote ---

Do you mean I should put a low-pass filter across the LM399? What cutoff frequency should I aim at?

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