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Simulators >> Circuit Simulators >> Op Amp Input Capacitance
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Message started by sunnysunnyho on Oct 5^th, 2011, 12:26pm

Title: Op Amp Input Capacitance
Post by sunnysunnyho on Oct 5^th, 2011, 12:26pm

I am doing a analogue project at the moment.

I am trying to simulate an op-amp using Pspice. Well, Cadence Allegro
to be exact. And I am trying to find the *input* *capacitance* of an op-amp. (power supplies, +ve and -ve inputs and output)

What should I do? How can I find it. Which sort of simulations (or even components on the schematic) I would need?

Cheers guys, I really help you help :)

Title: Re: Op Amp Input Capacitance
Post by buddypoor on Oct 5^th, 2011, 12:58pm

Because you must operate the opamp in open-loop condition, at first you have to find the offset voltage Vo in order to bias the opamp in the linear region. After applying Vo you can feed an ac voltage Vac in one of the inputs and simulate the current vs. frequency (ac analysis).
Then display Vac/Iac. This function gives the input impedance and looks like a first order lowpass. From the cutoff frequency (3 dB) you can derive the input capacitance of the model (if any - some models are pure resistive).

Title: Re: Op Amp Input Capacitance
Post by raja.cedt on Oct 6^th, 2011, 1:33am

otherwise you can find S-parameter.

Thanks,
raj.

Title: Re: Op Amp Input Capacitance
Post by Daniel Lai on Nov 16^th, 2011, 5:50pm

To get the impedance (real part+imaginary part), you may use AC or SP (S-parameter) simulation to do it.

For example at AC:
1. Apply the DC voltage
2. Input an AC voltage source at OPAMP Input
3. Measure the AC current of voltage source output or OPAMP input
4. Zin=Vin/Iin, and result is a complex (Z=R+jX)

Title: Re: Op Amp Input Capacitance
Post by weber8722 on Nov 17^th, 2011, 8:15am

Look up :D: The differential input C and the one measured single-ende or the common-mode C might be different! Usually Cdiif is biggest and more nonlinear than the others.