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Design >> Analog Design >> Comparator input noise
https://designers-guide.org/forum/YaBB.pl?num=1174395188

Message started by Croaker on Mar 20^th, 2007, 5:53am

Title: Comparator input noise
Post by Croaker on Mar 20^th, 2007, 5:53am

If I have a block A feeding a comparator (block B), and I want to know how much noise is at the input to the comparator, how do I setup the noise analysis?

I'm thinking I have to specify the output as the output of block A. I.e. I don't want to know how much noise the comparator adds, but what noise the comparator input sees.

Title: Re: Comparator input noise
Post by ACWWong on Mar 20^th, 2007, 10:12am

You should replace block B with an impedance which represents the input impedance of block B, but is then de-embedded from the noise analysis.
So replacing your comparator with a large load resistor (and shunt C if needed) to load block A correctly, then doing the noise analysis will work fine. Using spectre you can print the output noise summary and see all the contributors to the noise at output of circuit A, including the load resistor which can be taken off in your subsequent calculations.

Title: Re: Comparator input noise
Post by Croaker on Mar 20^th, 2007, 10:28am

ACWWong wrote on Mar 20^th, 2007, 10:12am:

I'm not sure I follow why the comparator can't be left in the circuit... What is the right way to determine the R and C to represent the comparator?

Title: Re: Comparator input noise
Post by ACWWong on Mar 20^th, 2007, 10:46am

hmm.. good point, should be fine to leave it in there, so long as you examine the output noise summary and remove the contributors that arise from your comparator.... leaving it in at the cost of a bit extra sim time (ususally neglible) and post processing of results time.

I guess I'm just used to individual cell testbenches, which only work correctly if one defines the nature of the sources and loads correctly... anyway I usually determine input Z by means of sp or ac analysis. This is covered in the Analysis->Testbenches section of this website.

Title: Re: Comparator input noise
Post by Croaker on Mar 20^th, 2007, 10:53am

Thanks a lot! ;D

Regarding the concept of removing the load (whether it is the RC or actual comparator), that's to make sure that you are seeing only the noise arriving at the input of the comparator and no noise that the comparator itself contributes, right?

However, I guess I want to know how much noise is appearing at the input no matter where it's coming from...so in that case, it makes sense to leave the comparator in, right? Set the noise analysis output nodes to the input on the comparator, and have both circuit A and B in the schematic...