Electronics > Repair
HP 3456A voltage offset
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Kleinstein:
The leakage from flux residue usually does not change that much with temperature (may even go down from a humidity effect). It is still possibly to have leakagef from residue. AFAIK some of the critical nodes use extra teflon isolation posts, not just the PCB diretly.

Some spike during the AZ switching is normal (specially with non zero voltage measured), as the compensation circuit for the switching charge can't be perfect with nonlinear capacitance of the FETs.

Q105 should not be less of an issue (at least not cause a negative offset) as it is connected also for the low side reading and thus with compensation of the input resistance.

U307 is originally a LF351. There should be better matches than an OP07, that could well be too slow. So more like TL071, AD711, LF411 or the LF351 as in the original. I don't see how a fast OP-amp there should cause much porblems. It is more that too slow an amplifier could cause instability / ringing. A faster amplifier may need better decoupling  (e.g. maybe add an extra capacitor directly on top of the chip). However the +30 V supply may not like capacitive loading - so there could be an issue in the design. The TLE2071 has the oddity that the supply current goes up a low when saturated in one direction - this could be an issue more than the higher speed.

Maybe also check if the +5 V is clean near the amplifier - it is used as a auxiliary reference and interference there could end up at the output.
stevopedia:
Yes, most of the sensitive nodes are on Teflon insulators.

It isn't the autozero itself that's causing the spikes, it's the ADC switching between the S+4 and S-4 slopes during run-up (which happens very frequently with a 0 V input). I'll grab some scope captures when I can. Those spike are much more apparent with the input amp in the x10 and especially x100 gain configs.

I'll freely admit that my understanding of things like circuit poles and zeroes aren't nearly as well-developed as I'd like. Why would a slower part oscillate where a faster one wouldn't? All things being equal I'd figure more gain at frequency would tend to reduce stability. Would a slower part necessarily have less phase margin at frequency?
Kleinstein:
U307 works with local feedback for a gain of ~25 and thus quite some gain. The main loop for the amplifier has separate compensation (set by C303 and R324-R327 depending on the gain). A slow amplifier for U307 would additional phase shift and could destabilize to amplifier. The LF351 is already not that fast for the rather high gain.
A problem with a fast amplifier for U307 could be that the +30 V is just from an OP-amp output and no good decoupling (and also not easy to add) at this supply.

It is rather unusual to get effects from the S+4 / S-4 switching at the amplifier. Normally the amplifier should not see much, as the signal goes to the virtual ground at the integrator and only to tiny residual signal there (may be a few 10 mV) and some 100 K to the amplfier. So interference would more be via the supplies. Especially the +5 V may effect the amplifier output - so maybe check decoupling caps at the +5 V (at the ADC, maybe near the amplifier). An effect from the +5 V should still be about the same for all the gain settings - so likely not the culprit.

Having some signal from the ref. switching at the amplifier or supplies may lead to nonlinear errors.
stevopedia:
I tried replacing Q110 with a new PN4392 and... it had no effect.

I think Q101 can be ruled out because there is no change in the displayed offset when the analog filter is on or off. Q112 was changed previously, so unless the new part is unacceptably leaky it can be ruled out. That leaves Q105 and Q116. I haven't checked them yet.

I tried some more investigation with warm air and everything I heated resulted in the offset very quickly moving in the positive direction--in fact it went comfortably positive (5-10 μV) before slowly falling back to -5 μV after heat was removed.

@Kleinstein, your reply reminded me to get the scope captures--I'll do that now.
stevopedia:
Here are the captures. They were all made with the Volts input shorted, and the instrument in the 100 mV DC range with autozero on. The only difference is that some were made with the analog filter off and some were made with the filter on, as the file names indicate.

To reiterate, the only time this appears to affect the measurement is when the 3456A is set to 100 cycles integration and autozero is on and the analog filter is on. Change any of those and the problem goes away.

And this is all secondary to the offset anyway :P
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