Is it okay if I leave an opamp or comparator input open?

[MRW]

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!

 

[DJ Delorie]

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.

 

[MRW]

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!

 

[Eeyore]

I am curious if it is okay to leave an opamp or comparator inverting
or non-inverting input open while the device is powered.

It probably won't damage it but it certainly won't work in any meaningful way.

Graham

 

[John Larkin]

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

 

[MRW]

It probably won't damage it but it certainly won't work in any
meaningful way.

Graham

Thanks, Graham! I'd like to make it work in a meaningful way and not
damage it. I think I understand now.


--

 

[MRW]

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.

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:



or some other way to keep it happy when your input is disconnected.


John

Thanks, John!


--

 

[Manoj]

Thanks, John!

--- Hide quoted text -

- Show quoted text -

dear freinds,

there are many application notes to this effect from almost all big
manufacturers.
i think comparator pins can be left open, coz they are one way or the
other in saturation.
saturation state for op-amp is not good, one coz they take a lot of
current and second it will put in a lot of noise on the supplies.

the best idea is to keep op-amp in a known state and for that u should
tie both minus and positive pins such that u ahve a output close to
mid point of supply voltages applied. do read application notes of
national for this application.

with regards

manoj

 

[John Fields]

dear freinds,

there are many application notes to this effect from almost all big
manufacturers.
i think comparator pins can be left open, coz they are one way or the
other in saturation.

---
From:

http://www.national.com/mpf/LM/LM393.html#Datasheet

"All input pins of any unused comparators should be tied to
the negative supply."