Quad, DIP op amp selection

[johnc124]

Matching source resistance to reduce noise is most effective in op amps which DON'T use bias current cancellation. This is because the input current noise in these op amps is mostly correlated. I think it is mainly produced by the shot noise in the tail current divided by the input transistor beta. Matching source resistances converts the correlated current noise into a common mode voltage which is then removed by the CMRR of the op amp.

However, in op amps that use bias current cancellation, this introduces 2 uncorrelated noise sources into the input bias current, and matching source impedances will only reduce the correlated noise.

Theoretically, the broadband current noise spectral density of an op amp should be In=sqrt(2*q*Ib) where Ib is the input bias current, and q is the charge of an electron. However, in bipolar input op amps which use Ib cancellation the current noise does not follow this rule due to the additional noise of the bias cancellation circuit. Worse yet, this noise is mostly uncorrelated, so it can't all be cancelled, but some of it can.

OK, update on this. I was wrong here. The bias cancellation noise IS correlated however the effectiveness of matching source resistance depends on the op amp. Essentially if the bias current cancellation circuitry does dominate the the input current noise, then yes, matching source resistance will reduce the total noise and this was an effect I observed on the OPA1612.

However, op amps without Ib cancellation are still a lower noise solution overall because they already have lower input current noise and the extra thermal noise contribution of the cancellation resistor is avoided.

So, long story short, I was confused on which component of the input current noise was correlated.

 

[Bandersnatch]

Sooo...unless I get a better idea I am going to replace the TL072's with OPA2134, the NJM2059 with NJM2060 and the duals with NJM4560. Current demand not too far over OE, better drive and noise performance on the NJMxx stuff.

Now, any big reason not to socket the PCB?
cheers,
Douglas

 

[sgrossklass]

So, long story short, I was confused on which component of the input current noise was correlated.

Phew. So I did remember that correctly after all. Thanks for the update.

Sooo...unless I get a better idea I am going to replace the TL072's with OPA2134, the NJM2059 with NJM2060 and the duals with NJM4560. Current demand not too far over OE, better drive and noise performance on the NJMxx stuff.

What's the 4560s for? Don't you already have 4580s in there? I would just leave those in.

Now, any big reason not to socket the PCB?

Same story as ever - thumbs up for service-friendliness, thumbs down for parasitics. It'll probably work out of the layout is reasonably tight to begin with (I spot a number of local rail decoupling caps on the schematic, and the parts used are only moderately fast, plus the generally highish impedances), but always reduces stability margins a bit. When repairing older gear with socketed chips, it also is fairly common practice to reseat those in order to restore proper contact.

 

[georgehifi]

Now, any big reason not to socket the PCB?
cheers,
Douglas

Same reason as sgrossklass says,
BUT I do use them, when I made a decision on what to use I solder the opamp into the socket, this also gives good heat sinking to them from the solid legs of the socket and air gap underneath if they're the hot running ones like AD825's and such.

Cheers George