I wrote (regarding replacing the op-amp in a Fluke 731A or B voltage standard):
> If you replace the op-amp with a modern precision part (I suggest the
> OPA277 -- others may try to talk you into an auto-zero op-amp, but I
> think that would be a mistake in this application),
Attila wrote:
Why would this be a mistake?
Let me count the ways....
First, the ground rules: My suggestion was to modify the 731 (we now
know the OP has a 731B) with as little invasion as possible. I
suggested (i) replacing the reference with an LM399 and changing the
gain-setting voltage divider (R2 or R3) to obtain 10v with the new
reference (only a small change would be necessary), (ii) if desired,
replacing the op-amp, and (iii) replacing all of the divider and
zener bias resistors with lower-tempco units (moot, now that we know
the unit is a 731B, which has low-tempco WW resistors). I do not
recall how much current the selected resistors, R5 and R6, deliver to
the existing reference; the LM399 seems to be most stable over the
long term when operated at 1.5 to 1.8mA, so R5/R6 should arguably be
changed (if necessary) to get the current into that range (about
2k). Any replacement resistors should have tempcos at least as good
as the original WW resistors, but anything better would be wasted on
a circuit that sits out in the open like the innards of the 731B.
My notion was to use an op-amp that will drop directly into the LM308
position, with only a few component changes on the circuit board. If
someone wanted to do something more elaborate, there may be other
possibilities, but even then I do not believe an AZ op-amp would be a
good choice for the reasons given below. [since Fluke used an LM308
in the TO-5 package, which is not a current package for either of the
op-amps I recommend, in reality a small adapter board would need to
be made (or the op-amp would need to be mounted on stilts -- ugh, PC
meets dead-bug)].
I have now recommended two candidate op-amps -- the OPA277 and the
LT1012. I have not run the numbers to see which is better in this
application, but I suspect the LT1012 is. The LT1012 was once made
in the TO-5 package -- if one can be found today, that would decide
the matter for me, for two reasons: (1) it fits with no surgery, and
(2) the hermetic package resists drift due to atmospheric humidity
and pressure. The LT1012 also has an overcompensation pin, like the
LM308, which is handy for making the particular circuit used in the
731B stable. So, I'm recommending the LT1012 in preference to the
OPA277, even if it is not in the hermetic package.
Finally, note that the LM308 is a respectable performer even today,
and with everything out in the open on the PC card it is quite
possible that other errors may completely swamp any improvement the
LT1012 might promise.
So let's start counting:
1. Maximum power supply voltage. The 731B uses an arrangement in
which it is powered by the regulated 10v supply. Precious few AZ
op-amps can handle a power supply of more than 5 or 6 volts, and the
ones that will are not the best choices for other reasons. Even the
reference output (not quite 7v, typically), which is connected to the
op-amp's noninverting input, is higher than the vast majority of AZ
op-amps' maximum supply voltages. So, even changing the topology of
the amplifier would not help.
2. Switching noise. All AZ amps suffer to a greater or
not-so-lesser degree from switching noise. Some of them allow the
designer do a halfway decent job of filtering the output -- but all
of them put all sorts of hash and garbage onto their supply
lines. As noted above, the supply is connected directly to the
regulated output in the 731B, so there is no good way to clean it
up. (Same for the AZ amp's output noise in the 731B circuit.)
3. Input noise current. AZ amplifiers have little or no 1/f noise,
because it is cancelled by the AZ action. However, they have
substantially more input bias (leakage) current than regular CMOS
op-amps and, consequently, a higher input noise current
density. This is a dirty little secret of AZ amplifiers -- most
manufacturers calculate the input noise current density as the shot
noise of the input current (if they even list a noise current spec --
many do not). However, the actual measured input noise current
density is frequently as much as 100x higher than this calculated
value. So, even ignoring switching noise, many AZ amplifiers are not
even as quiet in real-world circuits as run-of-the-mill JFET
op-amps. Since both of the op-amp input nodes are moderately
high-impedance, input current noise will be a factor in the noise analysis.
I'm sure I've forgotten a few, but those are some of the high points.
Best regards,
Charles
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