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Fully Differential Amplifiers - Application Report SLOA054D

Leonardo Yanzon
Prodigy 20 points
Other Parts Discussed in Thread: THS4121, THS4521

Hi!, I have a few questions about fully differential amplifiers. Im doing an EEG, for my thesis to become an electronic engineer. And I was wondering if it is possible with this amps ( or any configuration with 2 or more of this kind of amps) to make a very high CMRR circuit. At the amp output, the signal mantaines differential, but ones stage CMRR's, acts in chain with the other stage CMRR's increasing it? For example:  

Stage 1: Buffer (voltage follower with Gain=1), CMRR 60dB, fully differential (diff input, differential output)
Stage 2: Insturmentation amplifier, differential input, single-end output, CMRR 80dB, Gain=500

Entire Circuit: Gain=500, CMRR=60dB + 80dB = 140dB, single-ended output

Is this possible? Can i increase the CMRR with more than one fully diff amp like THS4121?

My contact email is leoyanzon@hotmail.com

Thanks!

  • 0
    TI__Expert 4115 points

    Leonardo,

    What you propose is how the design should work in theory. In practice the CMRR is usually dominated by the external gain setting resistors of the fully diff amp (FDA). The required matching can be estimated by taking 20*log (tolerance); so for 60dB you need 0.1% tolerance resistors (or more precisely the matching of the ratio of the gain setting resistors on the 2 sides of the FDA needs to be 0.1%).

    Other concerns to note is the gain bandwidth product; for maximum bandwidth the gain versus bandwidth should be balanced between the 2 stages.

    Also, note we have a new device, THS4521, that is much better than the THS4121

    Jim Karki

    TI High Speed Op Amps

  • 0 in reply to Jim Karki
    Prodigy 20 points

    Hello Jim, thanks for answering, but what i was really wondering.... if i do a voltage follower (with Gain=1), by shortcutting the output and the input (a typical configuration used like buffer), i dont have the resistors tolerance problem. In that circuit, the non-inifinite CMRR is due imperfections or tolerance at the inside of the chip. 

    In a normal (not fully differential) op amp, the input common-mode voltage, at the output, mixes with the differential voltage, because is single-ended output. After that i can´t change CMRR, the only chance that i have is in the first stage.

    My question was... in a fully differential amp, can i still increasing CMRR after the first stage? , it works like Gain?,  Are the common-mode and the differential voltages kept separated and have each stage their own CMR Rejction? My question is actually theoretical.

    Im sorry for my english, is not very good, lol.

    Leonardo.