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Design >> Analog Design >> noise analysis
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Message started by singh on Nov 28^th, 2011, 2:30am

Title: noise analysis
Post by singh on Nov 28^th, 2011, 2:30am

for 1st order low pass RC filter
thermal Vnoise = sqrt (4KTR)

what is Vnoise expression for second order low pass RC filter?
is it Vnoise = sqrt [ 4KT (R1 || R2)] ??

thanks in advance,

Title: Re: noise analysis
Post by raja.cedt on Nov 28^th, 2011, 2:54am

hello,
you are not clear, what is sqrt(4ktr), it's the spectral density of the resister, so it doesn't change with the architecture and for the 2nd order low pass filer it's not sqrt(4kt*(r1//r2)..always try to think about integrated noise when you compare diferent architectures.

Thanks,
Raj.

Title: Re: noise analysis
Post by singh on Nov 28^th, 2011, 3:21am

oh ok :) .. ya I am quite new to these concepts
i have sin signal and passing it thru low pass filter so
net_out = (Transfer function signal) * TF filter (for freq of 1 to 100k)
which i then multiply to noise voltage
So in the previous case i had 1st order filter so just took Vn = sqrt ( 4 KTR) to calculate and multipled it with my net_out
so net_out_noise = net_out * Vn ( for freq of 1 to 100 k)

But now as as i have two resister in 2nd order , i mean to say both R1 ,R2 are of different values ,so which i shud take
So how will i proceed with this Vn calculation ?? i am in confusion :(

Title: Re: noise analysis
Post by raja.cedt on Nov 28^th, 2011, 3:33am

hello sir,
thats wht i am telling, try to calculate integrated noise, for a first order system it's KT/C, where as for 2nd order low pass filer it will be KT/c2 (where C2 is o/p cap)..try to derive this equations by integrating filter TF and multiply the PSD.

refere razaavi noise chanper to bulid up some basics

Thanks,
Raj.

Title: Re: noise analysis
Post by loose-electron on Nov 29^th, 2011, 3:52pm

the KT/C equation comes from integrating the noise of a resistor over the bandwidth profile caused by the RC filter.

Is that what you are looking for?