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# OPA2188: what to do with unused pin?

Part Number: OPA2188

Hi all,

The customer is asking about the unused pins of OPA2188.

if we only use rail B, should we left the Pin 1/2/3 floating or it has to be connected to GND?

or there will be some issue and we should pay attention to?

thank you for your help.

Best Regards

Evan

• Inputs should never float.

The simplest circuit is a voltage follower, i.e., connect OUT A to –IN A, and +IN A to any voltage inside the common-mode voltage range (VCM). (V– is allowed, but a voltage higher than the negative rail might result in a slightly lower supply current.)

• Hi Evan,

we have already discussed this issue here:

Kai

• Hi all,

Thank you for your time.

I would like to confirm some points.

if I connect the "+INA" to "V-"(as below), why the supply current would be lower?

Also, what if I connect the "V-" to Ground, can I still connect the  "+INA" to "V-" if rail A is not used?

• Hi,

if I connect the "+INA" to "V-"(as below), why the supply current would be lower?

We haven't said that but just the opposite :-)

It's not only the input voltage which counts here, but also the output voltage. You should not force the output stage of a voltage follower to completely swing to V+ or V- (also called the supply rails). As the output can never fully swing to the rails, the output stage will fully turn-on in the attempt to accomplish this. And this can result in an increase of supply current.

So in order to properly bias an unused OPAmp, wire it as a voltage follower (output to -input) and connect the +input to a potential which

1. lies within the allowed common mode input voltage range

and which at the same time

2. allows the output to swing within the linear output voltage range.

So for the OPA2188 I would connect the +input of unused OPAmp (wired as voltage follower) to a potential somewhere in the range ((V-) + 100mV) ... ((V+) -1.5V).

Kai

• Hi All,

Thank you for your help.

Best Regards

Evan

• You are welcome :-)

Kai