Forrest wrote:
I looked up the schematic for the 5065A, couldn't find
the OPA111 anywhere
The LT1793 in particular is marketed as a drop-in replacement for the
OPA111, although some of the specs seem worse.
TI points people toward the OPA124 and OPA627 parts. In particular the
OPA627 is interesting in that it's available in the same TO-99 package
See attached schematic of the newer A9 assembly.
The parameters of the OPA111 that could be relevant in the A9 integrator
circuit are (i) offset voltage, (ii) input bias (leakage) current, (iii)
noise, and (iv) the hermetic package.
Looking at the schematic, both op-amp inputs see an input resistance of
10k. Because of this low resistance, the opamp would have to have an
input bias current in the low nanoamp range before the offset due to the
bias current would equal the offset due to the input offset voltage.
So, we can conclude that ultra-low bias current is *not* the reason HP
chose the OPA111.
While the OPA111 input noise is reasonably low as FET-input op-amps go
-- in the signal band -- it is bordering on outrageously high at
baseband (60-80 nV/sqrtHz at 10Hz), and it is this baseband noise that
is important for an integrator. So, it appears that ultra-low noise is
also *not* the reason HP chose the OPA111. So we are left with offset
voltage (<250uV) and the hermetic packaging as the qualities that may be
important for a replacement op-amp (assuming its input bias current is
less than, say, 1nA).
Unfortunately, the isolation from environmental factors (humidity and
barometric pressure) provided by the hermetic package probably *IS* the
primary reason HP chose the OPA111 in the first place (there are lots of
precision op-amps with better offset performance than the 111 and bias
currents <1nA). Also unfortunately, all of the other precision op-amps
in the hermetic TO-99 package have the same problem as the OPA111 --
they cost US$50 to US$100. (This is because of the packaging.) So, you
might as well just get the OPA111.
If you want to try a plastic-packaged replacement part, knowing that it
may be unstable over variations in humidity and barometric pressure, the
OPA124 and LT1793 appear to be the best choices. The 124 has the same
poor baseband noise performance as the 111 and the same (untrimmed)
offset voltage spec, while the 1793 has much improved baseband noise but
a somewhat relaxed (untrimmed) offset voltage spec. Since the offset
will be trimmed, I think I'd choose the 1793 in the hope that the lower
baseband noise would improve the overall stability of the 5065A.
Other candidates are the LT1012 and OP97 (both bipolar). Both have much
better (untrimmed) offset voltage specs and lower baseband noise than
the 111. However, both also have offset trim circuits referenced to the
positive supply, while the 111's is referenced to the negative supply,
and they also use one different offset trim pin than the 111 (pin 8
rather than pin 5).
Finally, you would need to either build or buy little adapter boards, or
make an ugly kludge, to fit any of the plastic parts onto the A9 PC board.
Note that the OPA627 has two things against it in this application,
notwithstanding its availability in the hermetic package. First, its
offset trim circuit is referenced to the positive supply, while the
111's is referenced to the negative supply. And second, the 627 is a
wideband amplifier, which is no advantage in the integrator circuit and
could be a problem if the A9 layout, which was not designed with a
wideband op-amp in mind, causes it to oscillate.
There are no ideal choices, if one considers the price of the OPA111 a
problem.
Best regards,
Charles
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