Electronics > Metrology

Prema 6048 teardown

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With just U10 replaced with an OPA189, for instance, the instrument responds correctly to commands and inputs. I'll let it warm up, null it, and see if I get more consistent behavior from it.

There is a slight chance that the integrator gets unstable with a different, much faster OP-amp. Here it is not just the GBW, but also the phase reseve under the given conditions that can make a difference. So the OPA189 may not be suitable.  With some of the rail to rail output stages it can help to have an additional RC load (e.g. 50 ohm and 100 pF in series) at the output of the OP-amp as this can slightly reduce the gain of the amplifier and improve stability.  Even if not actually oscillating a lot of ringing may be worse than the bit slow reaction of the OP77.

Candidate OPs to test could be ADA4077 (only 3.9 MHz but still much faster than the OP77), OPA205 (a similar part from Ti), OPA182 (5 MHz brother to the OPA189) and the OPA145 (5.5 MHz) and OPA140/141/OPA1641 (some 11 MHz).


--- Quote from: Kleinstein on December 31, 2023, 05:51:52 pm ---OPA140/141/OPA1641 (some 11 MHz).

--- End quote ---

I also have OPA140s at hand - given Dieter1's success with those - so I could try to use that for U9.

Before I try the OPA140 for U9, I thought I'd run more tests with the OPA189 for U10 and the original OP77 for U9.

I'm doing successive nulling operations and then measuring the FX to see if I observe any noticeable jumps in value from one nulling to another. After 5 of these or so, it seems the measurements are pretty consistent. I probably still need to run this more to fully stabilize temperature-wise, but the last few readings have been within less than .02-.03ppm from each other (averaging slight variations of the last count from one integration to another). In other words, it seems that from one nulling to another the readings seem to be very consistent (essentially identical).

Does this sound like a relevant test? Aside from a more invested INL plot which would be more than what I can really do today.

Next step accomplished - it turns out my sample unit agrees with using the OPA140 for U9... Yeah!

To summarize, my unit's now using:

* U9: OPA140
* U10: OPA189
And remains stable and responsive.

I'll run some nulling cycles and check for signs of similarly consistent readings as I've seen with the OP77/OPA189 combo.


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