Minimising current in unused op amp

Hi All,

Having found dual op amp packages around a fifth the price of
comparable single packages from online distributors, I think i'll go
for the double.
(BTW, I'm using supply of +16, and ground).

I don't want the unused channel drawing current unnecessarily though,
or bouncing the output around.. - If I tie the non-inverting input to
+3.3V, and have a high resistance feedback resistor to the inverting
input (with no other input to that pin), would that be as good as
anything? I figure the output won't have to 'try' very hard to modify
the voltage of an input pin with no other voltages on it..

Am I right?

Thanks..

 

[Chris]

On Jul 4, 3:20 am, angus.oli...@gmail.com wrote:

Hi All,

Having found dual op amp packages around a fifth the price of
comparable single packages from online distributors, I think i'll go
for the double.
(BTW, I'm using supply of +16, and ground).

I don't want the unused channel drawing current unnecessarily though,
or bouncing the output around.. - If I tie the non-inverting input to
+3.3V, and have a high resistance feedback resistor to the inverting
input (with no other input to that pin), would that be as good as
anything? I figure the output won't have to 'try' very hard to modify
the voltage of an input pin with no other voltages on it..

Am I right?

Thanks..

As long as you've chosen a single supply op amp (like the LM358) with
input range extending to the negative rail, just grounding the non-
inverting input on the unused op amp/voltage follower should be OK.
But even if you've got an op amp like the LM1458, 3.3V should be
fine. If you've got an LM358, GND will be lower current, because it
doesn't have the noise of another voltage input.

The "high resistance feedback resistor" won't help. At worst (for
high resistance values), it can encourage oscillations, which will
dramaticaly increase quiescent current (charging and discharging the
internal compensation cap). Just connect the inverting input directly
to the output to make a voltage follower, and save the price of the
resistor.

By the way, make sure you change the circuit layout for the new op amp
pinout.

Cheers
Chris

 

[John Larkin]

On Fri, 4 Jul 2008 01:20:49 -0700 (PDT), angus.oliver@gmail.com wrote:

Hi All,

Having found dual op amp packages around a fifth the price of
comparable single packages from online distributors, I think i'll go
for the double.
(BTW, I'm using supply of +16, and ground).

I don't want the unused channel drawing current unnecessarily though,
or bouncing the output around.. - If I tie the non-inverting input to
+3.3V, and have a high resistance feedback resistor to the inverting
input (with no other input to that pin), would that be as good as
anything? I figure the output won't have to 'try' very hard to modify
the voltage of an input pin with no other voltages on it..

Am I right?

Thanks..

Yup, keep it closed-loop, away from the rails. You don't need the
resistor, just connect the output to the IN- input. 3.3 volts on the
IN+ input should be fine for any opamp.

Some, maybe most, opamps draw a lot of extra current if railed, so
don't tie the ni input to V+ or V-/ground. Some will even mess up the
other amp sections if one section is run open-loop.

If you don't have a suitable dummy voltage handy, and the signal isn't
really fast, you can use the extra opamp section as a follower of the
section that you do use; that keeps it happy, too.

John

 

Thanks all of you - I appreciate it..

I've pretty much settled on a LM2904, with:
” Input common-mode voltage range includes
negative rail
” Differential input voltage range equal to the
power supply voltage
” Large output voltage swing 0V to (V
CC+ -1.5V)

Taking into account all your comments, I think I'll turn it into a(n
unnecessary) voltage follower (my op amp is only to scale a voltage
for an A/D conversion).
I'll have the output of the used amp as input into the non-inverting
input, and tie the output of this second amp back to the inverting
input for a unity gain voltage follower with "No extra current draw
involved" (Phil).

Thanks everyone,

Cheers,

Gus