AVR-Chat

On Thu, Dec 13, 2007 at 10:31:38AM -0700, Cat C wrote:
> 
> Why never use gain of less than 1?  Stability?  Can't that be sorted
> out and still use gain of less than 1?

With a gain of 1 your feedback current equals your input current. With
gain of less than 1 the current through the feedback resistor is greater
than your input resistor and the opamp is not stable. The opamp is
driving more of its own input than its getting from your source.

The stable solution is to use the opamp as a buffer to drive a divider.

Was also suggested one can use opamps to subtract an offset from the
input signal. If one is interested in monitoring a 12V battery with a 3V
AVR one might subtract 9V from the input then divide by 2 so that 12V
sits squarely in the middle of the 0 - 3V A/D range which becomes an
effective 9V to 15V "expanded scale" voltmeter.

-- 
David Kelly N4HHE, dkelly@HiWAAY.net
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Whom computers would destroy, they must first drive mad.