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Old 15th May 2004, 10:19 PM   #1
zahlm is offline zahlm  Spain
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Join Date: Mar 2004
Location: Barcelona
Question common mode questions

I have a couple of more questions about op amps. The first is about common mode:

why do op amps have an input common mode range? or in other words, why do op amps have a limit of input common mode voltage?

I also have seen a capacitor between the balanced inputs of an instrumental op amp used as preamp (i.e. the INA217), to form a RC low pass filter to both inputs. Improve this the CMRR of the circuit? What's the difference if we use a capacitor to ground at each input, forming independent LPF for each input?

Thanks very much, Zahlm
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Old 24th May 2004, 08:38 PM   #2
Neobas is offline Neobas  United States
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Join Date: May 2004
Location: Massachusetts
Post Common mode explained.

If you think about it, no input stage will have the same linear gain between the hard limits of no input signal to saturated ( Vin> V+ or -) BUT there is an area where this gain ( or transconductance versus input current for a bipolar type input stage) is pretty linear. This region is what designers try to make overlap with the common mode region of the device. Think of common mode rejection as having two ,"qualities". A static one measured at 50 or 60 Hz and a fixed input amplitude and another one that represents the rejection over the inputs dynamic range in both frequency and amplitude domains.

Instrumentation amps are specifically made to have very linear transfer function and excellent common mode rejection. Just for an exercise, figure out how closely matched resistors have to be to maintain the 120 db of CMRR. Adding a cap across the two inputs ensures the Xc seen at each input is the same. Caps are not easily matched even to 0.1% wich is a hint, 0.1% is one part in a thousand or 60db.

Neobas



zahlm
I have a couple of more questions about op amps. The first is about common mode:

why do op amps have an input common mode range? or in other words, why do op amps have a limit of input common mode voltage?

I also have seen a capacitor between the balanced inputs of an instrumental op amp used as preamp (i.e. the INA217), to form a RC low pass filter to both inputs. Improve this the CMRR of the circuit? What's the difference if we use a capacitor to ground at each input, forming independent LPF for each input?

Thanks very much, Zahlm
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