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Design >> Analog Design >> How to arrange VinCM in an opamp
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Message started by eng on Apr 12^th, 2007, 9:50am

Title: How to arrange VinCM in an opamp
Post by eng on Apr 12^th, 2007, 9:50am

Hi,
Is it a common practice to put input common mode voltage of an opamp in the middle of input common mode range?
For instance my VinCM_min is 540 mV and VinCM_max is 1V (which is vdd). I will use the opamp in a filter (namely cascade them) so output CM level should be equal to input CM level. If I arrange VinCM=770 mV make output same that makes input happy but output swing is very limited from the top. (btw output is a CS stage. )

So which one is more important? Driving input with a voltage in the middle ICM range. Or arranging VinCM as close as possible to Vdd/2 so that output can swing more? If second one is the case how close can I be to VinCM_min.

thanks in advance
best regards
eng

Title: Re: How to arrange VinCM in an opamp
Post by ywguo on Apr 13^th, 2007, 7:28am

Hi, eng,

The input common mode voltage depends on your configuration and application. If the input of opamp is virtual ground, it is OK to choose VinCM = 540mV in your example. However, if the opamp is used as voltage follower, the VinCM should be set in the middle of input range.

If you need large output swing, why don't choose AC coupling between the adjacent opamps? Another choice is design a rail-to-rail input opamp. The problem is how large output swing do you need?

Best regards,
Yawei

Title: Re: How to arrange VinCM in an opamp
Post by eng on Apr 13^th, 2007, 10:51am

Hi ywguo,
The opamps will be used in a low pass filter and cutoff freq is will be a fraction of a MHz so AC coupling would require big capacitors. Actually I do not have a spec about how large the swing should be. The application is baseband filter of a UHF direct conversion receiver.
Do you think it's needed to put some margin for input and make VinCM=670 to be on the safe side?

P.S.: the design should be low voltage and low power therefore I chose the well-known fully differential 2 stg miller opamp topology.

Title: Re: How to arrange VinCM in an opamp
Post by ywguo on Apr 15^th, 2007, 9:20pm

Hi, eng,

The topolgy selection and configuration are sure derived from the spec. Obviously, you are not clear how much swing the opamp need. About the low-power low-voltage application, what voltage supply is low voltage? Do you design an opamp operating at 1.8V or 1.2V supply? Or others? Do you need a rail-to-rail input to meet the swing requirement?

Best regards,
Yawei