[time-nuts] Replacement A9 boards for the HP 5065A

Charles Steinmetz csteinmetz at yandex.com
Sat Feb 24 23:44:16 EST 2018

Returning to the topic of what op-amp to use:

Bert asked if the LT1793 is a good choice.  I suggested the LT1012 as 
superior in the specific parameters that will provide best performance 
in the HP circuit 
Attila suggested the LTC6240HV.

I continue to believe that the LT1012 is the best choice (or, 
alternatively, the similar OP97 -- but the LT1012 would be my first choice).

As Attila noted, the 6240 is limited to +/- 5v supplies.  I do not know 
if the output voltage of the A9 ass'y needs to be able to swing beyond 
+/- 5v, but I would not be inclined to change this without a lot of 
careful thought (and adding another op-amp does not excite me).  But 
really, that is not necessary.  Even if the 6240 could supply outputs up 
to +/- 15v, the LT1012 would still be a better choice.

Given the relatively low resistances at the op-amp inputs (10k ohms), 
the ultra-low input "bias" (leakage) current of the 6240 is simply 
unnecessary.  Any offset due to the input currents (within the general 
range of any of these op-amps) is insignificant compared to the op-amp's 
offset voltage.  Thus, offset voltage, offset voltage tempco, and offset 
voltage long-tem drift are the critical parameters (as Poul-Henning 
pointed out).  And here, the 1012 is clearly the best of the three.  In 
addition to having the lowest input offset spec, the 1012 has guaranteed 
maximum specifications for these important parameters.  The 6240 (for 
good reason) is *not even rated* for long-term stability (drift). 
(Long-term offset stability is a particular weakness of CMOS op-amps.)

Finally, as I noted before, the 1012 has an overcompensation pin that 
may be useful to improve the damping and the wideband noise of the 
integrator.  (This would, of course, require a small amount of design 
work to place overcompensation pole(s) and possibly zero(es) to best 
advantage, but may provide significantly improved performance.)

Of all the op-amps I know and use, the LT1012 would be my first choice 
for the A9 integrator board.

Attila also advocated using the "much cheaper and easier to solder TPS7A49
[and] TPS7A3001" voltage regulators, as opposed to the LT3042 and a 
negative-regulator-to-be-named-later that I mentioned.  I concur with 
his suggestion.

Best regards,


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