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I am building a simple inverting operational amplifier circuit where I am using a chip with dual amplifier. I am only using one of the two amplifiers. What should I do to the unused amplifier's input pins? Should I leave them floating or is there a good way to sort of terminate them.

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It depends a bit on the op-amp. If it's a bipolar op-amp with high input bias current in a non-precision circuit (referring to the other op-amp) it may be safe to leave the inputs open. Think LM324 in some non-critical circuit.

One common practice is to connect the op-amp as a voltage follower and tie the non-inverting input to some voltage within the input and output common-mode ranges. Since the unused op-amp is operating normally, all the datasheet guarantees apply, so this could be considered a best practice approach.

You can also tie the inputs to potentials such that the output (left open) is railed high or low (there may be some small power consumption advantage).

I don't suggest connecting the inputs to the same potential since the output of some units might drift around and slightly affect the other op-amp. Similarly, leaving inputs open on an amplifier with low input bias current (those with bias current cancellation or JFET/CMOS input op-amps) can cause pickup that will show up in power consumption and can show up as crosstalk in the amplifier that is being used.

Edit: As the actual amplifier OPA2614 appears to be a non-unity-gain stable very high frequency type, it would likely be best to either make a gain-of-five amplifier or apply a small DC voltage of a few hundred mV on the inputs and leave the output railed.

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  • \$\begingroup\$ Yes, I am using the bipolar op-amp with ultralow input bias current. \$\endgroup\$
    – dr3patel
    Jun 21 '15 at 15:58
  • \$\begingroup\$ @dr3patel bipolar and ultra low bias current? what part? maybe it has some fets for the input pair \$\endgroup\$ Jun 21 '15 at 17:59
  • \$\begingroup\$ ti.com/lit/ds/sbos305d/sbos305d.pdf \$\endgroup\$
    – dr3patel
    Jun 21 '15 at 18:45
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    \$\begingroup\$ @dr3patel Arg. That's very high input bias current: -6uA typical, and quite high input noise current with an amplifier that is really high GBW (290MHz). The 2614 isn't even unity-gain stable, so the voltage follower would be a bad idea. I'd say tie the inputs with 100mV < Vin < 300mV across them and leave the output railed. \$\endgroup\$ Jun 21 '15 at 18:59
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    \$\begingroup\$ @SpehroPefhany: yes leaving the output open (railed) works fine for both in simulation and breadboard prototype. \$\endgroup\$
    – dr3patel
    Jun 22 '15 at 20:24

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