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What to do with unused op-amp I/O?

DrG

Active Member
So, here is the deal. I am working with this CO sensor https://www.spec-sensors.com/wp-content/uploads/2016/10/ULPSM-CO-968-001.pdf. This configuration of the sensor includes their potentiostat. You can also buy the sensor alone. This configuration was a good way to go. I could probably struggle through building my own, but it is complicated https://www.bio-logic.net/assets/app notes/Application note 4.pdf and the design parameters and board layout seem to be some pretty important considerations. For me, it is worth the extra $$ to let them design that part.

The sensor has voltage outputs and they can be read by an mpu using adc. It’s not that that straightforward, but that is the basic idea. See the data sheet for the full story.

OK, that’s the back story.

There are three outputs of specific interest. Two of them outright state that they are high impedance and require a buffer. The third no such statement or anything else concern that aspect of the output and it has caused me to think.
.
I have been using an MCP6022P (3V3 and GND) http://ww1.microchip.com/downloads/en/devicedoc/20001685e.pdf opamp (dual) in a unity gain configuration with two of the outputs. So far, so good. I am considering using a unity gain buffer on that third output. In that case, I would use the quad version, MCP6024.

This would leave me with one unused op amp and my question is, what do you do with the I/O for that unused op amp?

So, I looked into what is out there. Here are three guidance documents.

Maxim https://www.maximintegrated.com/en/design/technical-documents/app-notes/1/1957.html

Analog Devices https://www.analog.com/en/analog-dialogue/raqs/raq-issue-46.html#

TI http://www.ti.com/lit/an/sboa204a/sboa204a.pdf

This is way more complicated than I had thought. From the TI source: Unused op amps in multi-channel devices must be configured properly to avoid possible device degradation, extra power consumption and noise.

Extra power consumption, within reason, is not a big concern. The other two, however, are a big concern.

I was thinking that I would favor what is shown in the Maxim source as Figure 1, using something like 10K resistors.

I am asking because this is, again, way more complicated than I thought and wondered what other, more experienced, users might think.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
I'd just ground the positive input and connect the negative to the output.
(Possibly fractionally higher power consumption; no other downside).
 

DrG

Active Member
Can you just configure the fourth opamp as a voltage follower and let it follow one of the other outputs?

Mike.
I don't know. It makes sense I guess, you just chain two together - what could be wrong with that? I don't know. In the old days, when you had a unused PIC input port, you tied it high or low and then read a ton of people who said one or the other was wrong....it was so simple back then :)
 

DrG

Active Member
I'd just ground the positive input and connect the negative to the output.
(Possibly fractionally higher power consumption; no other downside).
Yeah, see that is what I would do. But then I thought nooooooo go read about how this is supposed to be done and why.

Take a look here https://www.maximintegrated.com/en/design/technical-documents/app-notes/1/1957.html under
Examples of An Improperly Terminated Operational Amplifier
The one for D (left) in Figure 3 with th explanation: D. These two circuits are the most common mistake made when terminating an uncommitted op amp. They are equally bad because the op amp's output will be railed low, causing the op amp to consume more supply current.

Now, as I said, if it just consuming a little more power, well, I can live with that, but what is there is exactly why I thought I would go to Figure 1. But I still feel like I am guessing or playing follow-the-leader.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Fig 1 in the maxim note essentially you have a dual supply op-amp that's outputting 0V. It says you can ground the non-inverting terminal.
If you had -3 and 3 V supplies, 2 resistors give you essentially 0V.

What about powering up when the supplies are or different.

Any resistor in series with the non-inverting input is probably good. making the OP-amp to be able to be commited in the future by cutting traces and adding resistors can also be a good idea.
 

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