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What do you do with unused opamps in a quad package?

M

MRW

Jan 1, 1970
0
Hi all. I remember posting a question before regarding leaving opamp
inputs open. From what I read, the inputs should be connected to ground
or some voltage source to have some meaningful output.

So, if I used an IC with 4 opamps and only utilized 2 of them, does
that mean that I have to ground the inputs of the other two and just
leave the output open? Or do I connect the unused opamp output to a
grounded resistor?

.... or do I connect the unused opamp inputs together to insure that
both inputs have the same voltage? Or do I just configure the unused
opamps as voltage followers?

My gut feeling is that I should connect the inputs of the unused opamps
together to make sure that they have the same voltage. I think that if
I don't, then noise might turn the output on and off and maybe "eat" up
on the voltage source.

--
 
D

DJ Delorie

Jan 1, 1970
0
As I posted earlier, the right way to tie off unused op amps is to
connect IN- to OUT, and tie IN+ to a resistor pair - one to V+, one to
V+ - so that IN+ is about halfway. You need only one resistor pair
for all the unused opamps, and they can be really big resistors.

Doing this guarantees that all the inputs are in spec. If you just
tie the two inputs together, any input offset in the op amp will be
amplified to the output, which may put the output out of spec.
 
C

Chris Jones

Jan 1, 1970
0
MRW said:
Hi all. I remember posting a question before regarding leaving opamp
inputs open. From what I read, the inputs should be connected to ground
or some voltage source to have some meaningful output.

So, if I used an IC with 4 opamps and only utilized 2 of them, does
that mean that I have to ground the inputs of the other two and just
leave the output open? Or do I connect the unused opamp output to a
grounded resistor?

... or do I connect the unused opamp inputs together to insure that
both inputs have the same voltage? Or do I just configure the unused
opamps as voltage followers?

My gut feeling is that I should connect the inputs of the unused opamps
together to make sure that they have the same voltage. I think that if
I don't, then noise might turn the output on and off and maybe "eat" up
on the voltage source.

--

I connect them as unity gain followers (connect the inverting input to the
output), and then make them buffer some convenient voltage that is within
their common-mode range, e.g. connect the non-inverting input to some
resistive divider that I already have in the circuit, say at mid-rail or
something.

The above procedure is probably not necessary with most op-amps, but it
doesn't cost anything extra, and it prevents any of the current sources in
the op-amp from going into saturation that could conceivably upset the
other op-amps in the package by pulling down shared bias rails or injecting
substrate currents (if the opamp design were not very clever).

Chris
 
M

MRW

Jan 1, 1970
0
DJ said:
As I posted earlier, the right way to tie off unused op amps is to
connect IN- to OUT, and tie IN+ to a resistor pair - one to V+, one to
V+ - so that IN+ is about halfway. You need only one resistor pair
for all the unused opamps, and they can be really big resistors.

Doing this guarantees that all the inputs are in spec. If you just
tie the two inputs together, any input offset in the op amp will be
amplified to the output, which may put the output out of spec.

Awesome, thanks, DJ!


--
 
M

MRW

Jan 1, 1970
0
Chris said:
I connect them as unity gain followers (connect the inverting input
to the output), and then make them buffer some convenient voltage
that is within their common-mode range, e.g. connect the
non-inverting input to some resistive divider that I already have in
the circuit, say at mid-rail or something.

The above procedure is probably not necessary with most op-amps, but
it doesn't cost anything extra, and it prevents any of the current
sources in the op-amp from going into saturation that could
conceivably upset the other op-amps in the package by pulling down
shared bias rails or injecting substrate currents (if the opamp
design were not very clever).

Chris

Thanks, Chris! The text is clearer to me.


--
 
C

cpemma

Jan 1, 1970
0
MRW said:
Hi all. I remember posting a question before regarding leaving opamp
inputs open...
Google AN1957 for a Maxim Application Note on the subject; it explains the
pitfalls of *not* doing it as DJ Delorie & Chris outlined
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
Onan amplifier the problen is not acute. but yes tie the inputs to gnd. the output probably will saturate one way or the other but they will not oscillate for example. oscllitation on the internal chip can be coupled to others.on CMOS chips it is mandatory to tie pins most if not all to a potential they are hi inpedance devices and will cause logic problems. tied the outputs to potential is not recomended not unless you pay attention to the logic output otherwise heating becomes a problem.
 
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