Hi Ed. Thanks for the quick update. Had a quick look on eBay and the LMC chip varies in price from $2 to over $50AUD! Difficult to choose because of counterfeit chips.
I will wait until you finish your experiments before deciding. Lou On Tue, 7 Jul. 2020, 1:11 pm ed breya, <[email protected]> wrote: > 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 > > _______________________________________________ > volt-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/volt-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/volt-nuts_lists.febo.com and follow the instructions there.
