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Should spare op-amp connections be left floating or tied to ground?

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Marmotta:
I'm working on a circuit that uses a 2 channel op-amp, but as the part is out of stock at the moment, I was going to amend it using a 4 channel op-amp, with two channels going unused, but am unsure what to do with the unused pins. The part is a TI OPA1678 and I can't see any specific mentions either way

https://www.ti.com/lit/ds/symlink/opa1678.pdf?HQS=dis-dk-null-digikeymode-dsf-pf-null-wwe&ts=1652355647548&ref_url=https%253A%252F%252Fwww.ti.com%252Fgeneral%252Fdocs%252Fsuppproductinfo.tsp%253FdistId%253D10%2526gotoUrl%253Dhttps%253A%252F%252Fwww.ti.com%252Flit%252Fgpn%252Fopa1678

ledtester:
TI's advice:

https://e2e.ti.com/blogs_/archives/b/thesignal/posts/the-unused-op-amp-what-to-do

Also:

https://www.ti.com/lit/pdf/sboa204

Basic advice is to use a voltage follower configuration with the non-inverting input tied to an intermediate voltage within the input common-mode range.

floobydust:
OPA1678 common-mode voltage is spec'd at +0.5V above V- to -2V below V+.
So that looks like in the middleish and I would use a resistive divider or shared Vref connection.

free_electron:
tie the inputs in such a way that the opamp output is set mid-point.
- no floating , that's asking for an oscillator
- no tying both inputs to same level , that is asking for an oscillator. the input offset voltage will create misery.
- no opamp in unity gain . some opamps do not like this and they will ... you guessed it : oscillate
- beware of current feedback opamps !. never tie output to inputs ! there needs to be a resistor.
- no hard tying to vcc / ground. that saturates the output drivers and increases power consumption. you may even exceed differential input levels ( not all opamps can have their inputs go against the rails ! )

Zero999:

--- Quote from: Marmotta on May 17, 2022, 11:08:04 pm ---I'm working on a circuit that uses a 2 channel op-amp, but as the part is out of stock at the moment, I was going to amend it using a 4 channel op-amp, with two channels going unused, but am unsure what to do with the unused pins. The part is a TI OPA1678 and I can't see any specific mentions either way

https://www.ti.com/lit/ds/symlink/opa1678.pdf?HQS=dis-dk-null-digikeymode-dsf-pf-null-wwe&ts=1652355647548&ref_url=https%253A%252F%252Fwww.ti.com%252Fgeneral%252Fdocs%252Fsuppproductinfo.tsp%253FdistId%253D10%2526gotoUrl%253Dhttps%253A%252F%252Fwww.ti.com%252Flit%252Fgpn%252Fopa1678

--- End quote ---
The OPA167x is unity gain stable, so it can be configured for unity gain, with the input set to a fixed voltage, within the common mode range.

Please post a schematic.

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