[sdiy] mosfet/cmos opamp

[Harry Bissell]

OK THAT explains why I saw no difference. I use much larger
sampling caps so the droop was acceptable (unnoticible) in both
cases. Likewise I didn't need a guard ring...

good to know...

H^) harry


----- Original Message -----
From: Ian Fritz <ijfritz at comcast.net>
To: Harry Bissell <harrybissell at wowway.com>
Cc: dan snazelle <subjectivity at hotmail.com>, synth-diy DIY <synth-diy at dropmix.xs4all.nl>
Sent: Thu, 28 Oct 2010 12:23:50 -0400 (EDT)
Subject: Re: [sdiy] mosfet/cmos opamp

At 06:50 AM 10/28/2010, Harry Bissell wrote:
>Do you find the CA3140 to be much better  the TL07x (or 08x) series
>in a sample hold application. I have used both but have not noticed that
>much difference.

My notes indicate that I got .4 mV/sec drift corresponding to a leakage 
current of 2 pA.  This was considerably better than what I got with an 
OPA134 (spec'ed at 5 pA input bias current).  The OPA602 (1 pA spec) worked 
pretty well, but it costs over $5.  I didn't measure with a TL07x, but they 
are spec'ed at 65 pA typical, 200 pA max.

My circuit uses a 30 usec sampling pulse to allow sampling rates well up 
into the audio range, so getting the droop down is pretty critical.  A 
circuit with a guard ring is required to get the best performance. On the 
SBB the measured leakage was a couple of pA larger.  :-)

Ian 


-- 
Harry Bissell & Nora Abdullah 4eva