Hi Lou.
I'll be putting up more info as the project progresses. This is kind of
an experiment with how some of the modern CMOS opamps are specified, how
they actually behave, and how to get the best performance from them.
I've been investigating these over the years, and it's quite a
complicated story. The bottom line so far is that some fairly mundane
parts are capable of phenomenal performance, once you know what's
inside. The key issues are the package and pinout versus application,
the input protection design, and of course temperature.
The 417 upgrade is based on an LMC6001B, which is about a twenty or more
year old design, that was promoted as an "electrometer-grade" opamp with
25 fA maximum bias current (for the premium "A" version). I got a bunch
of free sample, lesser performance "B" (100 fA) ones many years ago, and
decided to start there. The pinout is amenable to use in the inverting
mode, which is right for the 417, to replace the entire amplifier
circuit. From the specs, I expected it to be on par with the original
5886 tube, but found that it had only a few fA input bias at room temp,
and was even better (1-2 fA) after washing the input section. It's now
settles to somewhere below 1 fA as far as I can tell, after burning in
for a few days. This is with symmetric supplies +/-6.2V from 1N821 TC
Zeners, and a voltage follower buffer amp to minimize load-dependent
(feedback divider, meter movement, and possible external load up to 1
mA) self-heating of the LMC6001. The non-inerting input is only a few
k-ohms to common, which protects against leakage from the adjacent
negative supply pin. The pin 1 on this part is a no-connect, so has very
low leakage issues against the high-Z inverting input. So, the inverting
input is shielded by a floating pin (which could be grounded, except
that would increase the capacitance) on one side, and on the other side
by the non-nverting input that can only be up to a few mV away.
The opamp approach is also much more stable than the 5886 type, in terms
of offset voltage and drift - it's rock solid in comparison.
One problem with replacing the original circuits with opamps, is the
huge open-loop voltage gain. The original circuits had maybe 10,000
tops, while almost any modern opamps have ten to a hundred times as
much, and some even more. This makes the compensation and stabilization
trickier, especially considering that the original amplifiers often have
various compensation loops within their discrete stages, and also range
dependent. Using a single opamp gain block needs a different setup, and
that's the biggest problem right now.
Ed
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