Translating a physical circuit to Multisim- constant output

[bjw5]

Hey everyone,

  I've been trying to translate a protoboard circuit to Multisim for a while now, and for the life of me can't figure out where I'm going wrong.  I've attached the schematic and the Multisim file, although I am only concerned with the segment between the 2.78V input and the multimeter XMM2.  I have tested the physical circuit and produced the following results: for 2.78V input, the circuit yields 12.56V; for 4.14V input, 4.62V output; and for 7V input, -12.24V output.  The Multisim version, however, returns 1.009mV for every input value I have tried so far.  I have triple-checked the schematic with the physical model and with the Multisim adaptation, but have found no alterations and have no idea where to look.  Has anyone else encountered similar problems?

Much appreciated

[Tayyab_R]

Hi bjw5,

I looked into your circuit and found that there were some differences between the actual physical circuit that you provided and the circuit you had created in Multisim. I made some modifications to the circuit and now I started to see a realistic output. I have attached the modified file to this post.

Hope this helps.

[bjw5]

Tayyab,

I've been fiddling with the circuit you sent, and it does return a much more realistic value, but it still returns a constant value for all values of V4.  Which version of Multisim did you use for this?  Mine is 11.01 if that makes a difference

Many thanks

[Tayyab_R]

Hi bjw5,

I am using version 11.0.2 but it should not really matter. You should be able to configure your circuit in 11.0.1 and simulate properly.

[bjw5]

Tayyab,

I went back to the physical circuit and made a few measurements, and it turns out every op-amp is connected to the +12V power source.  I applied that to your circuit and ran it with a number of input voltages.  Now, the circuit yields an output that changes slightly with the input voltage and stays within the correct range, but doesn't stay still.  That is, the voltage climbs from zero to a value possible with the physical circuit before dropping to about 1.5V, at which point I get a simulation error.  The circuit still doesn't output nearly the range I need it to.  I know I've been a difficult customer so far, but do you have any more words of wisdom I might be able to use?

Many thanks,

Brian W

Rice University