Electronics > Metrology

using an ICL7650S to buffer the output of an LM399

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    i'm wanting to set up an LM399 as an 'artifact' reference, ie, where the output is buffered only, and not scaled up to 10v. to do this i have a couple or three choices:

1. a regular precision opamp (such as an LT1001) with the output trimmed using pins 1 and 8, to achieve 1uV offset at 23 degrees C, and hope for minimal drift with temperature changes. how well does this work out in practice?

2. an ICL7650S chopper opamp, in a 14-pin DIP package. the 14-pin package comes with a 250Hz output on pin 12, which can be checked to ensure the part is genuine - the parts i've ordered ar us$7 for 10 pieces, so just a tad suspect! i already have some 8-pin DIP versions, but these have no 250Hz output to check.

3. an LTC1050 or similar chopper opamp, and hope that the offset falls within the typical range (+/-0.5uV). but these are relatively expensive, and i don't have any to hand.

the current for the LM399 zener will be derived using a second cheap op-amp.

how good/bad does the ICL7650S choice sound? how dodgy does the idea of just trimming an LT1001 sound, and will the drift scuttle the idea?

rob   :-)


ICL7650 and LTC1050 (which is rather similar) can only deliver 1-2 mA.
For buffering references I would use LTC2057 or ADA4522 with some filtering against EMI.
See for example here:

(later I added also a transient zener to the output)

but of course you can also use a LT1001 as Op-Amp if you regard the offset as part of the zener voltage.
Especially if you have not so much temperature change in your lab as I have in mine (18-34 deg C over the year)

with best regards


hi Andreas,
    i have read your thread on the AD587 travel standard a few times over the years, in many regards it is pretty close to what i have in mind.

regarding ICL7650S/LTC1050, even if only able to source a couple of mA, this should still be sufficient to drive into a 4k ohm or higher load; one of the reasons for using a buffer is so that whatever you connect the artifact's output to can't divert away any of the LM399's zener current. opinions seem to vary upon how important a precise current going into pin 1 of the LM399 actually is, but i am inclined to err on the safe side.

would adding a JFET to the output of the opamp (but within the feedback loop) offer any disadvantage? this would also eliminate the potential of any internal heating of the opamp die caused by the output load.

rob   :-)

The dynamic impedance of the 399 is around 1ohm, of the 1399 0.011ohm to 0.04ohm.
To divert a "significant current" off the 399 the input impedance of the opamp has to be "comparable" with that of 399..
But usually it is at least 10^7 times higher for any type, imho.


--- Quote from: iMo on April 08, 2024, 02:07:30 pm ---The dynamic impedance of the 399 is around 1ohm, of the 1399 0.011ohm to 0.04ohm.
To divert a "significant current" off the 399 the input impedance of [...]
--- End quote ---

oops, my mistake. when raising the issue of diverting current away from the LM399, i was talking about the case of an unbuffered LM399, with a load consuming a significant fraction of 1mA then applied to the output (between pins 1 and 2):

rob   :-)


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