Unused Op-Amp inputs ?

[SYE]

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]

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]

https://www.electro-tech-online.com/custompdfs/2007/02/AN1957.pdf

 

[SYE]

Thanks guys, that helps alot.

 

[Optikon]

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]

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]

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

 

[ikalogic]

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

 

[SYE]

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]

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]

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]

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

 

[Roff]

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]

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.