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singh Community Member Offline Posts: 57	noise analysis 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,
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: noise analysis Reply #1 - 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.
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singh

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Posts: 57

Re: noise analysis
Reply #2 - 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 Sad

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: noise analysis Reply #3 - 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.
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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: noise analysis Reply #4 - 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?
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