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Design >> Analog Design >> question on the dc offset correction loop in LA
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Message started by john_xu on Feb 6^th, 2006, 6:34pm

Title: question on the dc offset correction loop in LA
Post by john_xu on Feb 6^th, 2006, 6:34pm

Hi,
I have aquestion on the dc offset loop for a high gain amplifier, e.g., a limiting amplifier.

We know in the high gain amplifier design, we always need a dc offset loop to reduce the mismatch effect. It consists of a low pass filter which is to extract the dc level and feed abck to the input port. I noticed in many designs, there is a amplifire located in this dc offset loop path. Pls. see the attached file, Fig.1.

Can anyone help me to understand the function of this amplifier? Is it to isolate the crosstalk of the input/output? Or it is to isolate the twop poles of the input and feedback loop? Or anyother functions???

Thanks in advance

Title: Re: question on the dc offset correction loop in L
Post by Paul on Feb 7^th, 2006, 4:26am

Hi John,

some figures on limiting amplifier offset compensation show passive feedback mechanisms to do so, but frequently you build a two-input LA input stage, where the first input receives the signal and the second input the offset compensation signal (see document below, p.10, on the left). By two-input input stage I understand a differential input in case your TIA output is single ended, respectively a dual-differential input stage if your TIA delivers a differential signal.
http://www.imec.be/esscirc/esscirc2001/C01_Presentations/41.pdf
Notice that the amplifier shown in your schematic corresponds to the right half of the input differential pair, not to the amplifier shown below. The latter is added to minimize the loading of LA output, respectively in this case of the intermediate stage.

Paul