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From: "ATT Broadband News"
Subject: Re: Unused op amp inputs
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NNTP-Posting-Date: Fri, 20 Dec 2002 00:05:17 GMT
Organization: AT&T Broadband
Date: Fri, 20 Dec 2002 00:05:17 GMT
It is not always safe to ground the +input and tie the -input to the output.
If the op-amp is a current feedback amplifier then tying the -input to the
output will cause the amplifier to oscillate increasing the supply current
considerably. What you should do with a current feedback amplifier is tie
the output to the -input through a resistor. Typically the data sheet will
suggest a typical resistor for the feedback. That should be sufficient to
You also should tie the +input to a voltage within the common mode range of
the op-amp. If you are using an amplifier referred to as a single supply
op-amp then this input common mode range will usually include one or both of
the supply rails. But many op-amps require the inputs closer to the midpoint
of the supply pins, i.e. ground in a dual supply configuration or Vsupply/2
in a single supply configuration. Tying it to a input voltage outside the
common mode range of the part will often cause saturation and this can
result in high supply currents. The common mode voltage range of the part
is usually specified in the data sheet.
The situation is even more complicated for voltage feedback op-amps that are
not unity gain stable. These may be stable at gains of 2, 5, 10, or even 25
or greater but will oscillate in a unity gain configuration causing large
supply currents. With these you cannot tie the output to the -input or it
will oscillate. You cannot even use a single resistor like you can with the
current feedback amplifier. You must use a pair of resistors Rf and Rg that
put the amplifier into the minimum gain specified in the data sheet.
There are many other subtleties that may result in some configurations
having less supply current than others but you must keep in mind the above
for all unused op-amps. I hope this helps.
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