Unused Op-Amp inputs ? - Electronic Circuits Projects Diagrams Free

SYE · 21st February 2007, 02:01 PM

Hi,
In packages with more than one op-amp, what is the best thing to do with the unused inputs, tie them together, tie them to GND or simply leave them floating

thanks.

eng1 · 21st February 2007, 02:22 PM

I usually connect the unused op amp as a voltage follower, biasing the non-inverting input with half the supply voltage (any voltage in the common mode range is fine) with a voltage divider.

dknguyen · 21st February 2007, 08:03 PM

http://pdfserv.maxim-ic.com/en/an/AN1957.pdf

SYE · 22nd February 2007, 10:33 AM

Thanks guys, that helps alot.

Optikon · 22nd February 2007, 05:27 PM

Quote:

Originally Posted by eng1

I usually connect the unused op amp as a voltage follower, biasing the non-inverting input with half the supply voltage (any voltage in the common mode range is fine) with a voltage divider.

Why waste resistors for a mid supply divider? Just wire as non inverting unity gain follower with input grounded and output O/C. Works fine.

Roff · 22nd February 2007, 05:51 PM

Quote:

Originally Posted by Optikon

Why waste resistors for a mid supply divider? Just wire as non inverting unity gain follower with input grounded and output O/C. Works fine.

As you said, that's fine if GND is within the common mode range. However, in the Maxim note, Vss may be GND, so the divider may be necessary.

Roff · 22nd February 2007, 05:56 PM

As an aside, Bob Pease has some interesting comments regarding the hazards of tying the noninverting input to hard GND (it's related to power supply sequencing).

ikalogic · 22nd February 2007, 09:56 PM

when designing pcb as small as possible, sometimes, i use 'unused' inputs to pass copper track right through them...

SYE · 23rd February 2007, 10:37 AM

Quote:

Originally Posted by ikalogic

when designing pcb as small as possible, sometimes, i use 'unused' inputs to pass copper track right through them...

I think this is probably the worst thing you could do for low noise applications, the Maxim app note explains things very nicely.

ikalogic · 23rd February 2007, 12:53 PM

Quote:

Originally Posted by SYE

I think this is probably the worst thing you could do for low noise applications, the Maxim app note explains things very nicely.

You'r right... but it depends on whether you need to save space or noize.. sometimes it would be better to short them to gnd, sometimes it could be better to use them like i do, it depends on the target..

Optikon · 23rd February 2007, 04:52 PM

Quote:

Originally Posted by Ron H

As you said, that's fine if GND is within the common mode range. However, in the Maxim note, Vss may be GND, so the divider may be necessary.

Humm.. yes I suppose. I usually tie the spares off with resistors on all the nodes anyhow in case I want to put it back into service while debugging hardware..

Hero999 · 24th February 2007, 01:19 PM

I normally connect one input to +V and the other to -V and the output to nothing.

Roff · 24th February 2007, 04:10 PM

Quote:

Originally Posted by Hero999

I normally connect one input to +V and the other to -V and the output to nothing.

Some op amps have parallel back-to-back diodes across the inputs, e.g AD706. A thorough search could turn up many more. this is not a good general practice. You should always check the datasheet before doing this. I think the diodes are most commonly found in amps with BJT inputs, where the Vbe breakdown voltage is less than the rail-to-rail voltage.

Hero999 · 24th February 2007, 08:23 PM

Good point.

Do you have any other ideas?

What's the best way to bias it so an op-amp uses very little power?

I suppose it depends on the type of op-amp, a rail-rail op-amp with a class A output stages is best biased either so the output is either +V or -V which is why I made such a suggestion.

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21st February 2007, 02:01 PM	(permalink)
SYE	Unused Op-Amp inputs ? Hi, In packages with more than one op-amp, what is the best thing to do with the unused inputs, tie them together, tie them to GND or simply leave them floating thanks.

21st February 2007, 02:22 PM	(permalink)
eng1	I usually connect the unused op amp as a voltage follower, biasing the non-inverting input with half the supply voltage (any voltage in the common mode range is fine) with a voltage divider.

21st February 2007, 08:03 PM	(permalink)
dknguyen	http://pdfserv.maxim-ic.com/en/an/AN1957.pdf

22nd February 2007, 10:33 AM	(permalink)
SYE	Thanks guys, that helps alot.

22nd February 2007, 05:56 PM	(permalink)
Roff	As an aside, Bob Pease has some interesting comments regarding the hazards of tying the noninverting input to hard GND (it's related to power supply sequencing). __________________ Ron (aka Rube)

22nd February 2007, 09:56 PM	(permalink)
ikalogic	when designing pcb as small as possible, sometimes, i use 'unused' inputs to pass copper track right through them... __________________ Ibrahim Kamal check my electronics and robotics page: http://www.ikalogic.com/

24th February 2007, 01:19 PM	(permalink)
Hero999	I normally connect one input to +V and the other to -V and the output to nothing. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez And http://www.silicontronics.com, same screen name as here.

24th February 2007, 08:23 PM	(permalink)
Hero999	Good point. Do you have any other ideas? What's the best way to bias it so an op-amp uses very little power? I suppose it depends on the type of op-amp, a rail-rail op-amp with a class A output stages is best biased either so the output is either +V or -V which is why I made such a suggestion. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez And http://www.silicontronics.com, same screen name as here. Last edited by Hero999; 24th February 2007 at 08:25 PM.