So, here is the deal. I am working with this CO sensor http://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 **broken link removed** 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.
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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 http://www.maximintegrated.com/en/design/technical-documents/app-notes/1/1957.html
Analog Devices http://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.
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 http://www.maximintegrated.com/en/design/technical-documents/app-notes/1/1957.html
Analog Devices http://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.