What should I do with an un-used amplifier in a multiple amplifier package? I have quad op amp and I'm only using 3 of the amplifiers, what should I do with the last amplifier?
Good question, the best way to "terminate" an un used amplifier is to configure the amplifier in a unity gain (gain = +1) configuration and tie the noninverting input to mid supply. So when using bipolar supplies this would be ground. If a single supply is used, you want to tie it to the middle of that supply, this could be done with a simple resistor divider or some other source.
What should I do with an un-used amplifier in a multiple amplifier package? Part 2
Another method, which i often do regarding, how best utilise unused op-amp. is ... Rather than tying-off unused inputs.
Why not make use of them by means of duplication i.e.doubling up your design and route your duplicated o/p signal out, for example, for testing/debugging and monitoring purposes. This technique is useful where signals (i.e. buried pcb tracks) that are difficult to access or are sensitive to loading.
By doubling up, i mean replicating your op amp #1 function/parameters to op-amp #2 having the same i/p signal.
By adhering to layout/practise/guidelines and caveats, this will avoid loading and minimise losses. The only downside is having these extra passive and active components, ... often are of 0603 (resistor/capacitor) or smaller packages, which are negligible comparing to (PCB) space saving wrt testability/debugging, production/monitoring and time saving.
I would add that it's not critical that the input to the voltage follower go to exactly midsupply. It does have to be connected to a voltage source that is within the stated Input Voltage Range (IVR) in the spec table. For example, on +/-15V, you could connect to ground, or if due to pc board routing constraints, it is more convenient to tie it to a reference bus, such as 2.5V, this would also be acceptable.
Good point, as long as you connect the input and to a supply that is within the Input Common Mode Range Voltage (ICMVR) you will be ok. And as you stated you can tie the input to ground for equal and opposite supplies, which also happens to be mid-supply
If for example, for a dual supply (+/- Vs), multiple amplifier package Op-Amps. With the un-used amplifier inputs both (e.g.) connected to ground. Due to CMRR the output (presumable will now be saturated). If so, wouldn't this result lead to a higher power consumption when e.g. compare to normal usage. Appreciate your advise to this.
I didn't mean to imply that both inputs should be tied together anywhere. I have seen both inputs tied to V+,
or both to V-, or both to ground. All of these configurations lead to trouble. The only valid way to "disable"
the unused section is to hook it up as a follower and connect the noninverting input to a voltage that is within
the Input Voltage Range (IVR) as stated in the spec table. You want to avoid slamming the output to either
rail, especially with rail-to-rail output op amps.
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