Maker Pro
Maker Pro

Floating Op-amp inputs?

M

MikeN

Jan 1, 1970
0
Hi

If I use say 2 devices in a multiple op-amp package - say a TL084,
should the unused inputs be left floating, or tied to one or other of
the supply rails.

Thanks

MikeN
 
T

Tim Wescott

Jan 1, 1970
0
MikeN said:
Hi

If I use say 2 devices in a multiple op-amp package - say a TL084,
should the unused inputs be left floating, or tied to one or other of
the supply rails.

Thanks

MikeN

My preference is to wire them as voltage followers. The + input could
be run to ground, or to something inside the common-mode voltage range
if you're running single-ended and want to be fancy.
 
M

Mac

Jan 1, 1970
0
Hi

If I use say 2 devices in a multiple op-amp package - say a TL084,
should the unused inputs be left floating, or tied to one or other of
the supply rails.

Thanks

MikeN

I would not leave them floating or tie them to a supply rail. Well, if
you are using positive and negative supplies, you can probably ground
the positive terminal.

Anyway, In general, I would take the output back to the inverting input,
and wire the non-inverting terminal to some voltage between the two supply
rails (read the datasheet for the part to see where the valid input range
is).

Of course, not all op-amps are stable at a gain of 1, so this might not
work for all op-amps.

For some op-amps, you might be able to connect the terminals to opposite
rails, but this is not a good general practice. For example, some op-amps
have diodes in place to limit the differential input voltage. These
op-amps will draw a lot of current from the supply rails if you exceed the
maximum differential input voltage!

I would NEVER connect the inverting and non-inverting terminals to each
other directly, unless it is some kind of special op-amp where the offset
voltage is guaranteed to be clearly one side or the other of zero.

There really is no substitute for reading the datasheet carefully. ;-)

--Mac
 
Top