Instrumentation Amplifer from LM123/LM224/LM324

[lk]

Greetings,
For the fun of it i wanted to simulate an instrumentation amplifier to measure the voltage drop over a resistor, but it is not working as i would expect it to work.

This is set up so i have a voltage drop of 1 volt over my sense resistor R5, and as i have set the gain resistor r8, so that the gain of the circuit is approaching 1. So with this setup i would expect that i get some where around 1 volt on my output. But the problem is that the output ends up at  ~6.7 mV.

the big question is what i do wrong here? I have tried in several simulators, so i'm sure that its something that i have screwed up

-lk

[fcb]

100meg (100 meg)!!!!!

try 100 ohms

[qno]

Yes you beat me with the post.

Check your manual. 100 meg without a capital could be read by the simulation package as 100 mili ohms.

Some sim packages cold be difficult about this.
You could try a standard setup with all 10k resistors.

[lk]

Thanks for the replies,

this is ltspcie so MEG = meg = 10^6, so i would imagine that is not the problem.

The reason for R8 being 100meg is the gain approaches 1 as the gain resistor approaches infinity.

fcb: if i set R8 at 100 ohm  the gain of the circuit will be 201,  so my output should be approaching the positive rail, it is not however.

-lk

[Bored@Work]

You have connected one positive input to the positive rail. But that opamp is not a rail-to-rail opamp.

[lk]

Thank you Bored@work,

i selected 100meg, to approach and open r8, and the calculated gain for the was 1.0002, which was close enough for me. But again i must have made a more fundamental mistake in my circuit.

-lasse

[PA0PBZ]

If you put a 5 ohm resistor between the top of R5 and the +12 you no longer have +12 on one of the inputs. (but it still gives the same result)

If you cut R1 and R2 the output of both opamps differ exactly 1 volt but there must be more... time for me to look up instrumental opamps I guess.

[lk]

Good spot Bored@work,

I changed the one positive input to 9 volt, but i still don't get the expected 1 volt on my output. now it just hovers at -78 mV.


-lk

[mrflibble]

Try adding a 100k resistor from output to ground. Also, try something lower for that 100 Meg. Maybe 1 Meg.

[PA0PBZ]

It works if you use dual supply voltages (6 and -6), it doesnt work with single supply voltage.

[Bored@Work]

It works with a single supply. Only that he has the polarity of the input wrong. Upper opamp should have a lower input voltage than the lower opamp. It is the opposite in the schematic.

[lk]

Thanks guys,

i swapped the input on the upper/lower opamp and now i get my ~1 volt out

-lk