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How to calculate SNR of a tested circuit?

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May 8, 2001
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snr calculation sine

how do i calculate snr of a circuit i have tested. say i hav constructed a ic741 inverting amplifer how do i calculate the snr.

let the input be a normal sine wae of ampl 0.04v


It is a dificult task because NSC didn't provide information about noise spec of this IC. 741 is not used in low noise circuit, because is dificult to obtain snr more than 45...50dB..

Procerure for calculating SNR in general is
You need:
1. equivalent input noise voltage Enoise - given most freqently in nV/sqrt(Hz)
1a. If sorceof the signal has a big output impedance
equivalent input noise current - in nA/sqrt(Hz)

2 gain of circuit ( with feedback )
3. mean value of output sigmal
4.bandwith of input signal

For example let concider inverting amplifier with gain -2, Enoise =20exp-9 BW=20KHz output signal 0.1V.
Output noise will be |gain|*sqrt(BW)*Enoise= 2*141*20exp-9 = 5.64uV
ratio between siganl and noise is 0.1/0.00000564=17730
in dB SNR= 20log17730 = 85dB.
In this example I didn't calculate Inoise * Rsource.

cascaded stages

thank u moss

this is ok for single stage.

i am using a casacded ampliferi transistor stage then one opamp for head phone output and then a amplifier before the adc.

should i calculate snr for each stage or should i calculate for the entire circuit. if i have to calculate snr for each stage then how to compute the entire snr of the circuit


ps: do u hav a yahoo messenger i need to dicuss more thing abt this
my id is snsriram79

consider the case for amplifier with three stages . Each stage have gain G1=2, G2=5, G3=3. In the output of the amplifier we have noise from first stage multiplied at G2*G3 plus noise from second stage multiplied at G3 plus noise from the last stage itself.
General output noise = Output_noise1*|G2*G3|+Output_noise2*|G3| +Output_noise3

After that calculate ratio
SNR = 20log (mean value of output signal)/( General outpt noise)

I have only ICQ at the moment.

individual stage calculations

Linear Technology has a program for calculating the noise of a one op amp stage. This program can be found at **broken link removed** . It is old and may not have the op amp you are using, but you could find one listed that has similar noise specs to the one you are using.

snr calculation


Try to use A/pp/CAD, you can find it at A/gi/lent Web site,
it's free.

Generally speeaking, the formula for the input noise figure of cascaded 4-poles is :

F=F1+(F2-1)/G1 + (F3-1)/(G1.G2)+...;


Fn is the noise figure of the n-th stage,
Gn is the power gain of the n-th stage.
To calculate the cascaded stages noise figure you must know individual noise factors and power gain of tha stages. Noise factors are usually given in a bandwidth of 1Hz.
After this it's easy to convert the result to output power or voltage, taking into account the bandwith of interest.
Take into account also the frequency - the density of noice, generated for example by a bipolar transistor, is higher at very low frequency, because of the recombination,

I hope, above few words are usefull.

Best of luck,


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