LTSPice op-amp sim, fundamental understanding.

Thread Starter

Sjöholm

Joined Jun 13, 2018
35
Hi Everyone!

I have this basic differential amplifier, it amplifies the voltage over R7 by 100 and the sim works fine. However if I connect V2(-) to V1(-) then it doesn't work, the output of U1 becomes basically zero. So I would like to know why? I think it's my lack of fundamental understanding of LTSpice and/or differential amplifiers that is the problem. And if I can't connect V2(-) to V1(-) and I wanted to do something useful with the U1 output e.g. connect it to the circuit made up of V1/R7/R6 then I would have to use a e.g. optocoupler,? That can't be so..

Screenshot from 2019-05-26 13-20-46.png
 

LvW

Joined Jun 13, 2013
887
Two comments:
1.) Do not use such large resistor values like 10 meg. All the known formulas assume that the opamps internal input resistances can be neglected ...and this requires that all external resistors are much less than these input resistances.
2.) You are using single supply only - hence, your output cannot go to negative output voltages. Ust instead double supply.
 

Thread Starter

Sjöholm

Joined Jun 13, 2018
35
Two comments:
1.) Do not use such large resistor values like 10 meg. All the known formulas assume that the opamps internal input resistances can be neglected ...and this requires that all external resistors are much less than these input resistances.
2.) You are using single supply only - hence, your output cannot go to negative output voltages. Ust instead double supply.
Hi LvW

1) The 10M was suggested by the datasheet, however it doesn't make any difference if I use 10k/100k
2) Yes, I don't need negative output voltage so it's ok with single supply in this case.
 

danadak

Joined Mar 10, 2018
3,726
On further examination, specifically allowable input CM range, you are
violating it - its 3.5V min, eg. the inputs will function up to 3.5 min, 3.8 typ,
but in your case they are almost 5V



upload_2019-5-26_10-34-8.png

Regards, Dana.
 
Last edited:

Thread Starter

Sjöholm

Joined Jun 13, 2018
35
Your original circuit will work if you use a Rail-Rail input type opamp, such as an LT1366.
I actually tried with LMC6482 and LT1884, but I can't seem to make those models work in LTSpice.

For the LMC6482 I took the PSpice model from TI website and imported it into LTSpice.

The LT1884 already came with LTSpice, but I couldn't even make it works as a simple amplifier :-(
 

Thread Starter

Sjöholm

Joined Jun 13, 2018
35
Here's using the LMC6482.

View attachment 178335
I see, so the LMC6482 model is the one from TI? The only thing I did differently was to rename the file from .LIB to .sub and place it in the LTspiceXVII/lib/sub folder, and then created an .asy file which I placed in the LTspiceXVII/lib/sym folder. But I really that can't see that as the reason why it doesn't work for me here.
 

Thread Starter

Sjöholm

Joined Jun 13, 2018
35
I finally managed to install ADIsimPE with wine under ubuntu 18.04, for anyone attempting the same; the trick is to ignore the installers message "The Installation failed, the software was not installed" - as the software IS installed and working.

Do you know the reason why Analog pumped out ADIsimPE when they already had a user base with LTSpice?
 

eetech00

Joined Jun 8, 2013
1,747
I see, so the LMC6482 model is the one from TI? The only thing I did differently was to rename the file from .LIB to .sub and place it in the LTspiceXVII/lib/sub folder, and then created an .asy file which I placed in the LTspiceXVII/lib/sym folder. But I really that can't see that as the reason why it doesn't work for me here.
I did nothing more than download the file from TI and use the opamp2 symbol.

Maybe your symbols pin mapping was incorrect?
 
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