[sdiy] mosfet/cmos opamp

[Ian Fritz]

At 10:21 AM 10/28/2010, David G. Dixon wrote:

>I typically use LF44x (IBC 10pA typ, 100pA max) for S&H buffers, and have
>been very pleased with the results.  These have similar input current specs
>to CA3140 (IBC 10pA typ, 50pA max) and significantly better specs than TL07x
>(IBC 65pA typ, 200pA max), but really any of them will do for S&H unless you
>hold the sample for minutes.

Something I've never understood.  Yes, it says 10 pA for the CA3140 (@ 15 
V).  But it also says 1.5 TOhm input impedance. What is the explanation for 
this huge discrepancy?  But whatever the spec sheet says, I measure under 2 pA.

Whether "any of them will do for S&H" depends on how large a cap you 
use.  You are using 100 nF.  I am using 5 nF, in order to be able to charge 
it in 30 usec. So I disagree with your conclusions by a factor of 20.  :-)

Ian