Help to calculate real model opamp input AC impedance.

ygg-it · 12:35 PM

Hello, I have a doubt to calculate the input AC impedance in the following non-inverting opamp.

Is it:

Zin = RP

or

Zin = RP // (ZIN_diff + (RG // RF) ) = RP // (30k + (RG // RF) )

???

Txs a lot!

godfrey · 6th June 2010, 02:28 PM

Assuming feedback factor is high, then Zin = RP.

ygg-it · 03:21 PM

In this stage gain is 3.
RF = 6k
RG = 3k

What is surprising me is the low 30k

of zin_diff in the LME49860 datasheet, and I have doubts if I have to consider it to calculate the input impedance in a real world...

Zin _diff:

http://www.analog.com/static/importe...als/MT-040.pdf

Shoud i consider it in my real model? Am I missing anything?

jcx · 6th June 2010, 07:35 PM

due to negative feedback the effective Zdiff seen at the + input is the internal Zdiff multiplied by the excess loop gain so it is very big at audio frequencies

Zcm is often more significant at low frequencies

6th June 2010, 03:13 PM	#3
ygg-it diyAudio Member Join Date: Feb 2008	Beta is not high... In this stage gain is 3. RF = 6k RG = 3k What is surprising me is the low 30k of zin_diff in the LME49860 datasheet, and I have doubts if I have to consider it to calculate the input impedance in a real world... Zin _diff: http://www.analog.com/static/importe...als/MT-040.pdf Shoud i consider it in my real model? Am I missing anything? Last edited by ygg-it; 6th June 2010 at 03:21 PM.

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6th June 2010, 02:28 PM	#2
godfrey diyAudio Member Join Date: Nov 2009 Location: Cape Town	Assuming feedback factor is high, then Zin = RP.

6th June 2010, 07:35 PM	#4
jcx diyAudio Member Join Date: Feb 2003 Location: ..	due to negative feedback the effective Zdiff seen at the + input is the internal Zdiff multiplied by the excess loop gain so it is very big at audio frequencies Zcm is often more significant at low frequencies

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