Virtual op amps CC does not function.

[jtex]

Problem Description :

The virtual op amps do not function properly. I entered the parameters for the Max9910 op amp to check the GBW product. I had to increase the Fu to 2x the stated value.

The Compensation Capacitance value has not effect on the simulation.

Also, please define the (VSW+) and (VSW-) parameters in the virtual model?

Steps To Reproduce :

See attached file

Workaround :

Trying to use an op amp model Max494 and change the model parameters to match the Max 9910 opamp, but I do not know which parameters to change for Fu?

What

are G1, G2, and G3? R1, R2, and R3? Max494 spice model model is below.

* EWB Version 4 - 3 Terminal Opamp Model

* A= 251188 RI= 3e+006 RO= 1000 VSW+= 6.85 VSW-= -20

* Vos= 0.0002 Ibs= 2.5e-008 Ios= 5e-010 SR+= 200000

* fu= 500000 fp2= 1e+032 CC= 3.9e-011

Vos 4 1 DC 0.0002V

Ib1 4 0 2.525e-008A

Ib2 2 0 2.475e-008A

G1 0 5 4 2 0.0630957

G2 0 6 5 0 0.0630957

G3 0 3 6 0 0.0630957

Ri 4 2 3e+006ohm

R1 5 0 1000ohm

R2 6 0 1000ohm

R3 3 0 1000ohm

C1 5 0 7.99556e-005

C2 6 0 1.59155e-036

Cc 5 0 3.9e-011

[lacy]

If you could attach your circuit file it would help in troubleshooting your situation. Without the circuit file we can not make a determination as to what might be causing a problem.

[jtex]

see the attachment

[lacy]

There is a problem with this. The VSW+ and VSW- parameters messed up somehow. In your top circuit nothing works until I set the op amp up to the same way the default op amp is set. This would make VSW- set to 3 while VSW+ is set  to -3. When I try the same set up with the lower circuit then it gives the incorrect output. It inverts it and that shouldn't be. I can reverse these two parameters on the second circuit and it works fine, but won't for the tiop circuit. Also, it sems to me that htis amp requires a negative voltage swing and a positive voltage swing to be entered and won't function corrrectly if they are set to zero.

These are my observations and I could be wrong, but this is what I see in your circuits.

One of the NI Support Team memebers will have to address this as to why this is occurring and what can be done about it.

[jtex]

Thanks.  Also, the compensation capacitance has no effect on the op amp too.

[jtex]

By the way, ALL the Virtual Amps have this problem.

[lacy]

One other option here is to use the OP AMP 5T_Virtual. This way you can apply power to the two terminals and this would set the VSW parameters and you wouldn't have to be concerned about that anymore.

[lacy]

What i see is a differentiated waveform on the output of the first circuit when I get it working. This tells me that there is a capacitor in the circuit creating this waveform and most likely it is across the feedback resistor. You have to take into consideration that these are perfect models with no parasitic capacitance modelled with in them and you have no other capacitance in your circuit  Also you are applying a square wave to this op amp and the compensation capacitor, with no other capacitance in the circuit, is causing the output waveform to differentiate.

It seem to be there and is working, but not maybe the way you would want it to.

Anyway, I would like to hear form NI and see what they think about both of these situations when it comes to the virtual op-amps

Message Edited by lacy on 12-15-2007 11:28 AM

[jtex]

In the non-inverting op amp, the gain is 40 dB.  I should be able to adjust the CC cap, to get match any op amp, but changing the CC value in the model does not alter the first pole.

If you switch, the VSW+ to zero and VSW- to 3 the opamp does seem to work.

[lacy]

In my last post I made an error. What it should have said is that I see an integrated wavefomr and not a diferentiated waveform.

Yes it does work that way on the first circuit, but look at your second circuit and you will find that it is set the same as the first circuit and there is no output. It should work the same for both circuits if this parameter was right. To get the second circuit right you would have to flip the values for it. What I am saying is that both circuits op amp parameters should be equal and both work the way they are supposed to. This is with the 3T Op Amp. The five terminal seems to work just fine. It is just this VSW parameters on the 3T that don't seem to be right.

I am not clearly understanding why you need to adjust a perfect model. Perfect models are just what they say they are. There is no variance from op amp to op amp since they are all the same.