This is the first time i have used LTspice so the schematic may not be drawn conventionally but my problem or the thing i don't understand is when i make R4 110R , i get -1.14v at point A, -3.65v at point B and 3.11v output which in itself doesn't make any sense to me, if i make R4 2k, then i get 2.23v at point A, 1.15v at point B and -3.3v output which is perfect.

my negative probe is on v.ground (4.5v) for all readings

My question is why does the summing circuit not work with R4 = 110R? why doesn't the op amp give me the negative output from my two inputs?

The OPO7 by the op amp is there to stay, i can't delete it for some reason, LTspice won't let me.

 

Post you .asc simulation file.

 

This is the first time i have used LTspice so the schematic may not be drawn conventionally but my problem or the thing i don't understand is when i make R4 110R , i get -1.14v at point A, -3.65v at point B and 3.11v output which in itself doesn't make any sense to me, if i make R4 2k, then i get 2.23v at point A, 1.15v at point B and -3.3v output which is perfect.

my negative probe is on v.ground (4.5v) for all readings

My question is why does the summing circuit not work with R4 = 110R? why doesn't the op amp give me the negative output from my two inputs?

The OPO7 by the op amp is there to stay, i can't delete it for some reason, LTspice won't let me.

You opamp power pins are not connected to anything, as indicated by the open square at the pin ends.

Do you have a voltage source somewhere on your schematic? Is it connected to nodes labeled "9V" and "-4.5"? Even if it is, and even if having a minus sign is a legal character in a node label, the nodes named "-4.5" and "-4.5v" would be two distinct and unrelated nodes?

Is the top of R2 at 9 V relative to your ground, or is it at 4.5 V relative to the virtual ground.

Are you actually creating a virtual ground from your supplies? Or are you just connecting the non-inverting input of your opamp to the simulator's Node 0 and hoping for the best?

If you want us to be able to help you effectively, you really need to provide complete information. At least show your entire schematic so that we can see how everything is actually connected, including your sources. Don't make us guess. The best thing is to post your .asc file, since many of us have LTSpice and can open it up and play with it.

I'm not even going to try to determine if your results make sense, because there is just to much about your schematic that requires guessing and engineering is not about guessing.

 

"Post you .asc simulation file. "

I don't know what that is.

 

You opamp power pins are not connected to anything, as indicated by the open square at the pin ends.

Do you have a voltage source somewhere on your schematic? Is it connected to nodes labeled "9V" and "-4.5"? Even if it is, and even if having a minus sign is a legal character in a node label, the nodes named "-4.5" and "-4.5v" would be two distinct and unrelated nodes?

Is the top of R2 at 9 V relative to your ground, or is it at 4.5 V relative to the virtual ground.

Are you actually creating a virtual ground from your supplies? Or are you just connecting the non-inverting input of your opamp to the simulator's Node 0 and hoping for the best?

If you want us to be able to help you effectively, you really need to provide complete information. At least show your entire schematic so that we can see how everything is actually connected, including your sources. Don't make us guess. The best thing is to post your .asc file, since many of us have LTSpice and can open it up and play with it.

I'm not even going to try to determine if your results make sense, because there is just to much about your schematic that requires guessing and engineering is not about guessing.

ok

 

"Post you .asc simulation file. "

I don't know what that is.

It's the file for the simulation that resides wherever the LTspice files are stored on your computer.
You can do a "save as" to store it wherever you want, so you can then attach it to your post.

 

It's the file for the simulation that resides wherever the LTspice files are stored on your computer.

I am not doing a simulation, i just use LTspice to draw the schematic.

 

You opamp power pins are not connected to anything, as indicated by the open square at the pin ends.

Do you have a voltage source somewhere on your schematic? Is it connected to nodes labeled "9V" and "-4.5"? Even if it is, and even if having a minus sign is a legal character in a node label, the nodes named "-4.5" and "-4.5v" would be two distinct and unrelated nodes?

Is the top of R2 at 9 V relative to your ground, or is it at 4.5 V relative to the virtual ground.

Are you actually creating a virtual ground from your supplies? Or are you just connecting the non-inverting input of your opamp to the simulator's Node 0 and hoping for the best?

If you want us to be able to help you effectively, you really need to provide complete information. At least show your entire schematic so that we can see how everything is actually connected, including your sources. Don't make us guess. The best thing is to post your .asc file, since many of us have LTSpice and can open it up and play with it.

I'm not even going to try to determine if your results make sense, because there is just to much about your schematic that requires guessing and engineering is not about guessing.

You are way too pedantic! you know what is going on in the circuit.

 

I am not doing a simulation, i just use LTspice to draw the schematic.

So the results are from an actual circuit?
Why aren't you doing a simulation to check out the circuit before you built it?

 

So the results are from an actual circuit.
Why aren't you doing a simulation to check out the circuit before you built it?

because:
a) i don't know how and

b) i prefer to build stuff on a breadboard instead.

 

You are way too pedantic! you know what is going on in the circuit.

Well, simulators tend to be a bit picky about things like that because they don't know how to read your mind and guess at what you are doing.

When you post a simulation schematic, people tend to assume that you are doing a simulation, and therefore it is rather important to get things correct.

Now that I know that you are not actually trying to simulate your circuit, I'm more than willing to assume that you intend for the -4.5V to be connected both to the bottom of R4 and to the negative supply rail of the opamp. I'm even willing to assume that your 9V label is actually meant to be +4.5V relative to your supply rail.

But I have NO idea whether or not you are actually establishing a virtual ground node, or whether your 9 V source is simply connected to the top and bottom of your resistor chain and the opamp supply rails and your non-inverting input is floating and being called a virtual ground. I have no idea because your schematic doesn't show this and you don't provide a picture of your actual circuit set up. But since asking you to provide that information is way too pedantic and burdensome, I will not bother you any further. Good luck and good day.

Assuming you have established a good virtual ground, I would expect your output with R4=110 Ω to be about +3.15 V, with Va=-1.17 V and Vb=-3.67 V. I don't know why those make no sense, they seem reasonably close to what you are seeing to me, but perhaps I'm just being too pedantic by actually expecting the opamp to behave as specified.

 

Well, simulators tend to be a bit picky about things like that because they don't know how to read your mind and guess at what you are doing.

When you post a simulation schematic, people tend to assume that you are doing a simulation, and therefore it is rather important to get things correct.

Now that I know that you are not actually trying to simulate your circuit, I'm more than willing to assume that you intend for the -4.5V to be connected both to the bottom of R4 and to the negative supply rail of the opamp. I'm even willing to assume that your 9V label is actually meant to be +4.5V relative to your supply rail.

But I have NO idea whether or not you are actually establishing a virtual ground node, or whether your 9 V source is simply connected to the top and bottom of your resistor chain and the opamp supply rails and your non-inverting input is floating and being called a virtual ground. I have no idea because your schematic doesn't show this and you don't provide a picture of your actual circuit set up. But since asking you to provide that information is way too pedantic and burdensome, I will not bother you any further. Good luck and good day.

As i said, this is my first play with LTspice and my circuit may not be conventionally drawn but your assumptions are corect with regard to voltage points.

Thank you for the reply and initial interest....good day sir.

 

Assuming you have established a good virtual ground, I would expect your output with R4=110 Ω to be about +3.15 V, with Va=-1.17 V and Vb=-3.67 V. I don't know why those make no sense, they seem reasonably close to what you are seeing to me

Because -1.17v and -3.67v make 4.84v and i expected the output to be 4.84v since it's an inverting amp.

I'm not an engineer, just a hobbyist so my math isn't very good.

 

Because -1.17v and -3.67v make 4.84v and i expected the output to be 4.84v since it's an inverting amp.

I'm not an engineer, just a hobbyist so my math isn't very good.

So you expect the opamp to output a voltage higher than the voltage that is supplying power to it?

You don't have to have either good math skills or be an engineer to look at a part's data sheet and see that, even with a load of only 50 µA, the highest you can typically expect the output voltage to be is 1.35 V below the V+ rail.

https://www.ti.com/lit/ds/symlink/lm358.pdf



So, if the positive rail is 4.5 V, the highest output voltage you can expect, when it is having to supply a few hundred microamps, is going to be something between 3.10 V and 3.15 V. You are measuring 3.11 V, so it is behaving just like the manufacturer has said it will behave.

 

So you expect the opamp to output a voltage higher than the voltage that is supplying power to it?

You don't have to have either good math skills or be an engineer to look at a part's data sheet and see that, even with a load of only 50 µA, the highest you can typically expect the output voltage to be is 1.35 V below the V+ rail.

https://www.ti.com/lit/ds/symlink/lm358.pdf

View attachment 365364

So, if the positive rail is 4.5 V, the highest output voltage you can expect, when it is having to supply a few hundred microamps, is going to be something between 3.10 V and 3.15 V. You are measuring 3.11 V, so it is behaving just like the manufacturer has said it will behave.

Ok that makes sense

Thank you.