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Original question:

OPA2188: opa2188 use

OPA2188: what to do with unused pin?

Evan Wang
Expert 6440 points
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

  • 0
    Guru 276960 points

    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.)

  • 0
    Guru 140200 points

    Hi Evan,

    we have already discussed this issue here:

    Kai

  • 0 in reply to kai klaas69
    TI__Expert 6440 points

    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?

  • 0 in reply to Evan Wang
    Guru 140200 points

    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

  • 0 in reply to kai klaas69
    TI__Expert 6440 points

    Hi All,

    Thank you for your help. 

    Best Regards

    Evan

  • 0 in reply to Evan Wang
    Guru 140200 points

    You are welcome :-)

    Kai