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From: Winfield Hill
Subject: Re: OPA627 - Is this proper?
Date: 30 Oct 2002 11:18:13 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.00
Daniel Haude wrote...
> Says the 1994 Burr-Brown data book (I found one):
> "The case connection [...] may also be driven at guard
> potential to minimize any leakage which might occur
> from the input pins to the case. The case is not
> internally connected to -Vs."
> And the 2000 TI data sheet:
> "The case [...] is internally connected to the negative
> power supply as it is with most common op amps."
> A pretty blatant contradiction. Probably they moved the manufacturing
> to a different fab when TI took over BB. The cases of my units are
> still stamped with the "BB" logo though.
Aha! And if you go back further... Part first introduced in their
vol 33 databook, circa 1989. Two page data sheet, no detail on case
and pin 8 connections. A full 14-page data sheet appears in the
supplement, volume 33b, dated 1990. Shows pin 8 connected to the
case for the TO-99 types, says as follows:
The case connection (pin 8 of TO-99 metal package only) should be
connected to an AC ground for lowest possible pickup of external
fields. While DC ground would be the most likely choice, pin 8
could also be connected to either power supply. (The case is not
internally connected to the negative power supply as it is with
most common op amps.) For lowest possible input bias current, the
case may be driven as a guard - see Input Bias Current section.
In that section they say, as is repeated in your 1994 edition,
The case connection [...] may also be driven at guard potential
to minimize any leakage which might occur from the input pins
to the case. The case is not internally connected to -Vs."
It appears that it's dangerous to take advantage of an unusual
feature and count on it being available for the life of the part.
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