0
\$\begingroup\$

I am using a couple of OPA2227UA op-amps for a analog circuit. Since these are dual-op amps, one of them ended up unconnected. (The +-In/Out B pins shown below):

enter image description here

The datasheet says what can I do for unconnected input offset pins (it says I can let them unconnected), but it does not say what should I do with input/output pins.

enter image description here

Should I connect those three pins somewhere, or let them unconnected? I am worried that the unconnected pins will result in oscillation and will interfere with its neighbor pins/traces.

\$\endgroup\$
3
  • 3
    \$\begingroup\$ TI has a note about this. \$\endgroup\$ – Unimportant Apr 5 at 10:24
  • \$\begingroup\$ Why do you talk about and show the datasheet for single opamps that have offset pins but your negative image box shows a completely different DUAL opamp with one opamp disconnected? \$\endgroup\$ – Audioguru Apr 5 at 15:28
  • \$\begingroup\$ Its on the same datasheet, (same product, slightly different pins). I know its not directly relative to the one I show, but I wanted to point out that this exists on the datasheet, and it was the only place to be found. \$\endgroup\$ – Christianidis Vasileios Apr 5 at 15:35
2
\$\begingroup\$

It's good practice to tie them somewhere.

If you can spare a couple of resistors, it's good to connect the non-inverting input to a defined voltage (like ground, or a rail) with a series resistor, and connect the output to the inverting input with a resistor, to make a voltage follower.

If you just connect the output to the -ve input, and the +ve to ground, then later you suddenly find a use for the amplifier, you may be stymied by the hard connections. You can remove resistors and use their pads as the connections to your new function.

If you're certain you're not going to want to use the other amp ever, then don't use the resistors. However, I've been electrical engineering for my living for 40 years, and I've never yet had a board enter production the same way it left design!

\$\endgroup\$
1
\$\begingroup\$

It really depends on what you want to achieve with it, and for example to achieve least power consumption it might be different for each op-amp.

In general, unused op-amp should be configured as unity gain buffers, for example in your case you have dual polarity supply, so you can just connect the positive input to ground and connect the output to negative input. It will keep the unused op-amp stable.

\$\endgroup\$
1
\$\begingroup\$

I'd connect them like this: -

enter image description here

Of course you could use a resistor between OUTB and -IN B in case you might want to use that op-amp at some future date. Adding the resistor means you have a more convenient way of making new connections to the op-amp channel B.

The OPA2227 is unity gain stable so connecting the unused op-amp like this is not a problem.

One more thing; the data sheet suggests using 0.1 μF decoupling capacitors for the op-amp. 10 μF should be OK if ceramic but may exhibit some problems given that this op-amp has an 8 MHz bandwidth if crappy 10 μF capacitors are used.

\$\endgroup\$
4
  • \$\begingroup\$ Since you mentioned unity gain stability: what to do with unstable amps? Tying them to GND to achieve a gain >1 would result in increased power draw. \$\endgroup\$ – tobalt Apr 5 at 10:31
  • \$\begingroup\$ @tobalt use two resistors to make sure the gain is above the minimum required to achieve stability. \$\endgroup\$ – Andy aka Apr 5 at 10:32
  • \$\begingroup\$ Yes that is what I meant. I hoped there would be a way that will not lead to the additional current. \$\endgroup\$ – tobalt Apr 5 at 10:34
  • \$\begingroup\$ If current consumption is a critical thing then I'd be using a single op-amp instead of a dual op-amp @tobalt \$\endgroup\$ – Andy aka Apr 5 at 10:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.