What to do with unused opamp pins?

[AndreasM]

Hello,
What should I do with the input pins of an unused opamp on a dual or quad IC ?
I had some thoughts on it:
-let them open
-connect inv. and non-inv. input
-both inputs to ground
-voltage follower config. and non inv. input to ground
- it doesn´t matter
-looking for a circuit with even number of opamps.

What will be the proper way ?

 

[Mark Johnson]

Here's what I do. I build unity gain voltage followers (Out connected to VIN-) with the unused opamp(s). Then I connect their VIN+ terminals to the output of a not-unused opamp in the same package.

Advantages: (1) easy to route on the PCB, you simply short together adjacent pins on the opamp package. (2) it's a "legal" configuration for all opamp types, even those that aren't RRIO, even those with protection diodes between VIN+ and VIN- (like 5532). (3) The unity gain buffers happen to accidentally provide completely isolated probe point(s) that let you observe the output signal without probing it directly.

 

[JMFahey]

Cool idea

The standard procedure is to connect output to -IN so you have unity gain and +IN to ground if split supply or Vref if single supply or biased to some other voltage.

Of clourse, using them as Test Point buffers is a novel and useful idea

 

[AndreasM]

Thank you for the answers. Provided that the opamp. is unity gain stable it´s a interesting idea.
I agree to the 3 advantages you mentioned ,on the other hand I also see some drawbacks.
You get extra bias current to the output connected to Vin+ and extra current noise from the unused opamp. Due to the low output impedance of the used one it might be negligible.

 

[Mark Johnson]

Thanks for your comment. I did not know that decompensated quad opamps existed until today. But they do! LT1127 and LT1674 are quads which are not unity gain stable. A delightful discovery.

 

[jan.didden]

Here's what I do. I build unity gain voltage followers (Out connected to VIN-) with the unused opamp(s). Then I connect their VIN+ terminals to the output of a not-unused opamp in the same package.

Advantages: (1) easy to route on the PCB, you simply short together adjacent pins on the opamp package. (2) it's a "legal" configuration for all opamp types, even those that aren't RRIO, even those with protection diodes between VIN+ and VIN- (like 5532). (3) The unity gain buffers happen to accidentally provide completely isolated probe point(s) that let you observe the output signal without probing it directly.

That's a very good suggestion Mark!

Jan

 

[AndreasM]

Thanks for your comment. I did not know that decompensated quad opamps existed until today. But they do! LT1127 and LT1674 are quads which are not unity gain stable. A delightful discovery.

Me too, it was just more a theoretical consideration. So, do you think I should not worry about additional bias current or noise? It´s a tempting thing to have a isolated probe point.

 

[johnc124]

Me too, it was just more a theoretical consideration. So, do you think I should not worry about additional bias current or noise? It´s a tempting thing to have a isolated probe point.

Considering that the output of an op amp in a closed loop configuration is a very low impedance, the additional bias current and current noise shouldn't matter at all.

I really like this clever trick!

If I had my way there would be tons of decompensated op amps on the market since they are very useful. But people get scared when something isn't unity gain stable and they also feel that they HAVE to put that cap across the feedback resistor...

 

[AndreasM]

Considering that the output of an op amp in a closed loop configuration is a very low impedance, the additional bias current and current noise shouldn't matter at all.

I really like this clever trick!

If I had my way there would be tons of decompensated op amps on the market since they are very useful. But people get scared when something isn't unity gain stable and they also feel that they HAVE to put that cap across the feedback resistor...

Thank you for your conclusive comment, I will try out this recommended configuration.