Re: Pspice Filter Analysis
- From: jpopelish@xxxxxxxx
- Date: 31 Jan 2006 10:23:26 -0800
Noway2 wrote:
(snip)
>...I suspect
> that there is some form of error in the way pspice is handling the
> loops with capacitors or the feedaback due to a lack of some required
> 'phantom' component. For example, In the Sallen-Key, one of the
> feedback loops is from the output to the non-inverting terminal. Of
> course pspice won't let you model this a a short, so you need to play
> the game of using a very small resistance instead.
Something is not adding up. Such direct connectionsshouldbe no problem
forspice, if they work in the real world. But in the Sallen-Key
configuation (gain one low pass) the direct connection is between the
output and the inverting input. The feedback to the noninverting input
is through a series R and a shunt to ground C.
(snip)
> I see what you are saying about putting a large resistor to ground at
> the input (I was thinking along the same lines but thinking about
> putting the resistor at the opamp input terminals) and how this would
> have a neglible effect on the mux output.
If you put the resistor at the noninverting opamp input, it adds a zero
to the filter's response. At the mux end, it just adds a little
attenuation.
.
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