[sdiy] Op-amps CA3140 v. TL074

[Ian Fritz]

At 10:42 AM 7/30/2010, David G. Dixon wrote:

>For CV buffers (S&H, etc), I typically use the LF44n (esp. the LF444, which
>is a relatively low-cost quad).  These are JFET input opamps with 10^12 ohm
>input impedance (compared to about 1.5 x 10^12 ohm for CA3140) and 50pA bias
>current (compared to 10pA for CA3140).  They are insensitive to static
>discharge.  However, they suffer from a low slew rate (only about 1V/us,
>compared to 9V/us for CA3140), which is why they are probably only suitable
>for CV buffering.

Thanks for that -- I don't remember looking at that chip before.  If you 
can live with those levels of bias current and slew rate then it looks like 
a good choice. It looks like the single unit has been discontinued, though.

On my final TGTSH board, which has a guard ring for the hi Z node, the 
output droop corresponds to a bias current (plus leakage) of 2 pA.  So you 
can't always go by the spec sheet, especially for these older chips -- the 
specs apparently do not get upgraded as fab technology improves.

   Ian