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Design >> Analog Design >> Telescopic amplifier with different input and output CM's
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Message started by thomasross20 on Feb 27^th, 2009, 5:43am

Title: Telescopic amplifier with different input and output CM's
Post by thomasross20 on Feb 27^th, 2009, 5:43am

Hi all,

I'm building a pipeline ADC... usually in the sampling phase, the amplifier is tied in unity-gain configuration. However, I'm using a fully-differential telescopic amplifier with output cmomon mode = 1.3V and input common mode = 0.9V... I've searched the literature but cannot find a thing relating to what the amp is to do when the caps are sampling.... it can't be put into unity gain as normal. I use SC-CMFB which tried to keep the output at 1.3V during the sampling, but right now the amplifier is open-loop during sampling, which is not ideal... any hints?
Putting a cap in the feedback during this phase doesn't do much, right... sure doesn't cature the d.c. offset voltage - only works at high frequencies. Perhaps a source-follower in the feedback? Something else?

I've also found little literature on the topic of amp measurments when using an amp which different input and output common modes. What I do for gain/phase is the usual resistor and cap round the feedback to set up d.c. but block a.c. - though of course there is a little current flowing through the resistor due to the difference in common-mode levels - I don't think this is too much of a problem? Anyway, thanks for any help! :)

EDIT - One paper I see has simply shorted the output of the amplifier to the required output common-mode voltage - good approach? I'm thinking of currents entering the bias circuitry here..

Title: Re: Telescopic amplifier with different input and output CM's
Post by Berti on Feb 27^th, 2009, 6:37am

Instead of a unity-gain feedback configuration you could put a level-shift capacitor into the feedback as done for the summing amplifier of the following paper:

L.E.Larson et. al., "GaAs switched-capacitor circuits for high-speed signal processing", IEEE J. Solid-State Circuit, 1987.

Title: Re: Telescopic amplifier with different input and output CM's
Post by thomasross20 on Feb 27^th, 2009, 7:34am

Thanks - I'll get reading this over the weekend - is there a particular figure I should be looking at?
Much appreciated, Berti..!

Title: Re: Telescopic amplifier with different input and output CM's
Post by Berti on Feb 27^th, 2009, 8:08am

I don't have the paper at hand, but I think you should be able to find the right circuit.
I used that in a 0.13um CMOS implementation, and it worked well.

Regards

Title: Re: Telescopic amplifier with different input and output CM's
Post by thomasross20 on Feb 27^th, 2009, 8:33am

I've found it - a short paragraph under figure 13 stating that a feedback cap can be added in order to compensate for outputs going haywire during clock dead-times. In fact it says the cap can be added between the output and the left hand side of another cap - so some sort of capacitive divider? Can't say I'm overly keen on that as I don't want to disturb my sampling circuit - and if I want to switch out the amp, I get a kick on the inputs which sends the outputs rocketing. For other reasons, I can't have the output going to either rail - it has to sit pretty, and kicks at the input do me no good. :( Tricky..!