Neither.Or should the non-inverting pin be tied to the output?
Which is correct? Left? Or Right?
View attachment 220549
Start with the left image, but connect the output to the inverting input (voltage follower) and the non-inverting input to GND.
The solution I posted is documented in Figure 2 in the second link @upand_at_them posted.
How about if the common mode range doesn't include the negative supply?
Note that there is no requirement that an unused component (opamp, NAND gate, whatever) be tied to a DC potential. If it is more convenient in the pc layout, tie the opamp input to a nearby signal voltage, or tie the gate inputs to a nearby digital signal. Yes, the output will move, but in the vast majority of situations this will have zero consequences.To prevent this, connect the + input to some other DC voltage in the circuit that is within both the input and output operating voltage ranges.
The feedback should go to the "-" inverting input, with the non-inverting tied to GND
Last night as I was watching the World Series (what a game!) in the back of my mind I thought of this option and wondered if it was going to be the best way to go. From your comments (and others) I could tie the non-inverting (+) to anything stable and the output will follow that level. Space is not going to be a concern and this is a one-off PCB. No point in doing a gerber file and have someone make a board; or to go through the process of etching my own board. Does anyone do that anymore?connect the output to the inverting input (voltage follower) and the non-inverting input to GND.
Its important to note that the common mode voltage ranges needs to be considered. This configuration is OK for the LM358 because the LM358 common mode range includes ground.Last night as I was watching the World Series (what a game!) in the back of my mind I thought of this option and wondered if it was going to be the best way to go. From your comments (and others) I could tie the non-inverting (+) to anything stable and the output will follow that level. Space is not going to be a concern and this is a one-off PCB. No point in doing a gerber file and have someone make a board; or to go through the process of etching my own board. Does anyone do that anymore?
Anyway, from the comments received - this is the approach I'll take. Not all will agree 100% but I just don't want the thing to fly off at some unknown frequency and either self destruct or more simply waste battery power. This is just a part of a freezer door alarm I'm building. The other day we put stuff in and closed the door. Some of that stuff shifted and pushed the door open slightly and the next morning we had to do a lot of smoking (sausages and hot dogs that had thawed on the door).
This is what I'm hearing: (below). A voltage divider on pin 5 will act as a slight drain, so I will be tying pin 5 to ground.
View attachment 220592
The second link describes the solution I posted in #12Hello,
Have a look what TI and MAXIM have to say about the issue:
https://e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/11/27/the-unused-op-amp-what-to-do
https://www.maximintegrated.com/en/design/technical-documents/app-notes/1/1957.html
Bertus
by Aaron Carman
by Duane Benson
by Duane Benson
by Jeff Child