Designer’s Guide Community

New Member

Offline

Posts: 4

Op Amp Input Capacitance
Oct 05^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

Back to top

IP Logged

buddypoor

Community Fellow

Offline

Posts: 529
Bremen, Germany

Re: Op Amp Input Capacitance
Reply #1 - 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).

Back to top

LvW (buddypoor: In memory of the great late Buddy Rich)

IP Logged

raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Op Amp Input Capacitance Reply #2 - Oct 6^th, 2011, 1:33am otherwise you can find S-parameter. Thanks, raj.
Back to top	IP Logged

Daniel Lai Junior Member Offline Posts: 11 Taipei	Re: Op Amp Input Capacitance Reply #3 - 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)
Back to top	IP Logged

weber8722 Community Member Offline Posts: 95	Re: Op Amp Input Capacitance Reply #4 - Nov 17^th, 2011, 8:15am Look up : 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.
Back to top	IP Logged