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Choosing Vref+ and Vref-

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suhas_shiv

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Hi all,

This is with respect to a pipeline ADC I am designing.
I have to choose the comparator thresholds now. My input signal is 1V differential and power supply is 2.5V and common mode level is 1.25V. So the input signal will vary from 0.75V to 1.75V. What value do I choose for Vref+ and Vref-?
How are these references produced in general, like from voltage references??

Also do I have to put in a cap at these reference nodes to suppress transients and if yes, how do I choose the value?

It would help a lot if anyone could answer the questions.

Thanks
 

Well REFP and REFM are the highest and lowest swings in your case. You can generate them using a bandgap and a buffer. Of course, it is a good practice to buffer them with cap loads. The sizing of the caps should be such that, the parasitic capacitors are neglibile. Actually the first stage caps of the pipeline ADC will act as cap loads. The sizing of those caps is dependent on the KT/C noise requriements and the power consumption spec.
 

Hi all,

After going thru some papers, I have found that the Vref+ and Vref- are +Vr/4 and -Vr/4 respectively, where Vr is the input signal range. In my case Vr is 0.5,i.e, input varies from -0.5 to +0.5 with a common mode of 1.25V.
So I am setting my comparator thresholds to Vref-=1.25-Vr/4=1.125V and Vref+=1.25+Vr/4=1.375V.

Am I correct in doing so?

How can I get these reference voltages? I know I will have to use bandgap reference. Should I use some kind of resistor divider circuit to get these voltages from the bandgap voltage?

I would appreciate if anyone can help me out.

Thanks.
 

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