Is it okay if I leave an opamp or comparator input open?
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Posted by MRW on July 24, 2007, 10:18 pm
Please log in for more thread options I am curious if it is okay to leave an opamp or comparator inverting or non-inverting input open while the device is powered. Thanks! | |||||||||||||
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Posted by DJ Delorie on July 24, 2007, 10:47 pm
Please log in for more thread options Op amps have such a high gain that leaving an input open will probably drive the output into saturation, which isn't ideal. Better to configure unused op amps as unity-gain (tie in- to out) and use a pair of resistors to set the input voltage to half the supply voltage. That keeps all the pins in spec and happy. | |||||||||||||
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Posted by MRW on July 25, 2007, 12:20 am
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Okay. I think I get it. Thanks! Hmm.. I do remember reading somewhere that the opamp input terminal has to have a path back to ground to make it happy.. or something like that. I don't quite remember, but I guess that would make sense if we tie the inputs to a voltage source or ground. Would a circuit like this work? GND | | V - | GND | |\| VIN2 -- ----|-\ | >-VO
VIN3-|+/
|/| VCC (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) VIN3 in the diagram is a fixed voltage source, while VIN2 is either there or not (like plugged in or plugged out). Thanks! | |||||||||||||
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Posted by John Larkin on July 25, 2007, 11:03 am
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>> Op amps have such a high gain that leaving an input open will probably
>> drive the output into saturation, which isn't ideal. Better to >> configure unused op amps as unity-gain (tie in- to out) and use a pair >> of resistors to set the input voltage to half the supply voltage. >> That keeps all the pins in spec and happy. >
>Okay. I think I get it. Thanks! Hmm.. I do remember reading somewhere >that the opamp input terminal has to have a path back to ground to >make it happy.. or something like that. I don't quite remember, but I >guess that would make sense if we tie the inputs to a voltage source >or ground. > >Would a circuit like this work? > > > > GND > | > | > V > - > | GND > | |\| > VIN2 -- ----|-\ > | >-VO > VIN3-|+/ > |/| > VCC > >(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) > > >VIN3 in the diagram is a fixed voltage source, while VIN2 is either >there or not (like plugged in or plugged out). Thanks! > Some opamps will draw a lot of supply power if their outputs rail, as they will if they are not operating closed-loop. And in some dual or quad-section opamps, running one section open-loop will mess up other sections. In general, keep all opamps running closed-loop, like this: > +--------------+
> | | > | GND | > | |\| | > +-----|-\ | > | >-----+----VO > VIN3-|+/ > |/| > VCC or > vin2---+------1M------+
> | | > | GND | > | |\| | > +-----|-\ | > | >-----+----VO > VIN3-|+/ > |/| > VCC or some other way to keep it happy when your input is disconnected. John | |||||||||||||
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Posted by MRW on July 25, 2007, 10:34 pm
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John Larkin wrote: >
> >> Op amps have such a high gain that leaving an input open will
> probably >> drive the output into saturation, which isn't ideal.
> Better to >> configure unused op amps as unity-gain (tie in- to out) > and use a pair >> of resistors to set the input voltage to half the > supply voltage. >> That keeps all the pins in spec and happy. > > > > Okay. I think I get it. Thanks! Hmm.. I do remember reading > > somewhere that the opamp input terminal has to have a path back to > > ground to make it happy.. or something like that. I don't quite > > remember, but I guess that would make sense if we tie the inputs to > > a voltage source or ground. > > > > Would a circuit like this work? > > > > > > > > GND > > | > > | > > V > > - > > | GND > > | |\| > > VIN2 -- ----|-\ > > | >-VO > > VIN3-|+/ > > |/| > > VCC > > > > (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) > > > > > > VIN3 in the diagram is a fixed voltage source, while VIN2 is either > > there or not (like plugged in or plugged out). Thanks! > > >
> Some opamps will draw a lot of supply power if their outputs rail, as > they will if they are not operating closed-loop. And in some dual or > quad-section opamps, running one section open-loop will mess up other > sections. In general, keep all opamps running closed-loop, like this: > > > > +--------------+
> > | | > > | GND | > > | |\| | > > +-----|-\ | > > | >-----+----VO > > VIN3-|+/ > > |/| > > VCC >
> or > > > vin2---+------1M------+
> > | | > > | GND | > > | |\| | > > +-----|-\ | > > | >-----+----VO > > VIN3-|+/ > > |/| > > VCC >
> or some other way to keep it happy when your input is disconnected. > > > John Thanks, John! -- | |||||||||||||
> drive the output into saturation, which isn't ideal. Better to
> configure unused op amps as unity-gain (tie in- to out) and use a pair
> of resistors to set the input voltage to half the supply voltage.
> That keeps all the pins in spec and happy.