op amp real input impedance

[houly75]

Hi all
I'm interesting in instrumentation op amp and I would want to know how I can calculate the differential input impedance and the common mode input impedance of an op amp circuit.

I can't find a serious article or application note that show how I can calculate the input impedance of an op amp with considering real input elements of the op amp (resistance not the capacitance).

The first op amp stage is a simple differential op amp :
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how I can calculate the common input impedance with considering the impedance of the op amp ? and the same for differential impedance ?

The second stage is an instrumentation stage :
**broken link removed**
same question...

here is the equivalent circuit that I found on the net
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thanks a lot for your help and sorry for my poor english

 

[aomidee]

look,every where that u want to find common mode input impedance,u should apply two equal dc voltages to the both v1 & v2 and analyze the circuit.(in image1 Zd will have no voltage so it is open. and C.M. impedance is 2Zcm || 2Zcm. in image2:voltage of Zd1 & Zd2 & R0 will be zero and c.m. impedance is as image1.)finding diff. mode impedance is more voluminous! I remember that for image1 it is: 2Zcm||((1+AB)Zd. B=feedback factor=R2/(R2+R1).I suggest u reading this book:Microelectronic Circuits - Adel S. Sedra, Kenneth C. Smith. it is the best!

 

[glias]

Hello Aomidee, and thanks a lot for your interesting answer.
Well I have a question (probably stupid..) for the first schematic, I have just one remark, you said that Zd has no voltage on it, is it because the impedance of R1 and R4 are the same ?

for the second image, I'm OK.

for the differential, you're probably right but what is the method to find differential input impedance ?

Thanks a lot for your help

PS : the book may be interesting but I'm wonder if I would have explanation of analog circuits not in physics terms like i probably find in microelectronic books, I mean that I would prefer a reference book in analog... maybe the title is not the reflect of the content of the book ... ?

 

[aomidee]

1- It is because (R2/R1)=(R3/R4). In other words we have same voltage dividers, so we have same voltages at two input nodes of opamps.Note that it is not necessary to apply a DC voltage,it can be ac but equal at v1 & v2.
2- To find differential input impedance,we apply a signal source across the V1 & V2(without ground) and calculate the amount of impedance that signal senses! In feedbacked circuits There are 2 ways1)KVL/KCL (2)we should first analyze the circuit without feedback,then divide/multiply to K=1+AB.If u don't know about feedback,first read the book!
-Microelectronic circuits(Sedra) is a comprehensive reference book that it's main argument is about analog circuits(explains about physical basis of devices just a bit) It's interesting to know that this book is text book of analog course at some good universities around the world.

 

[glias]

Hello
thanks a lot, you have finished to convince me ! I'm going to buy the book.
Thanks for complement of explanations.

Added after 1 hours 6 minutes:

Re,

I have another question about the datasheet of the AD620. They present a solution to improve the common mode rejection when using a shielded cable. I don't understand how it works, in the datasheet they specified that the AD548 is there to minimize the effects of capacitance of the cable... but how ... ? Do you know this method ?