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Voltage noise issue when computing SNR

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Hi again,
Im trying to compute SNR for my circuit and have some problems with noise. I use INA326 IN-AMP and according to the datasheet :
h**p://focus.ti.com/lit/ds/symlink/ina327.pdf
(page 5) is the voltage noise for Gain=1 about 900nV/sq(Hz) and for G=100 about 35nV/sq(Hz).

1. First question and most important is WHY is the input referred output noise for small Gains so huge and for bigger G much smaller? Input noise is constant and shall be amplified by Gain. Why is this not the case ?

I compute SNR for 1kHz bandwidth , 1mV Input signal, Gain=5. So its on output Vnoise = (900nV * sq(1kHz))=28uV RMS, Vout = (5mV), SNR = 20log(5mV/28uV)=45dB.

2. Is this right how i compute SNR with RMS value of voltage noise ?

Thanx
 

Re: Voltage Noise

1. First question and most important is WHY is the input referred output noise for small Gains so huge and for bigger G much smaller? Input noise is constant and shall be amplified by Gain. Why is this not the case ?
Click to expand...

Gain, usually means differential gain, acts upon wanted signal.
Noise, regarded as common-mode at the input, gets rejected by means of common-mode rejection.

For output noise, you should consider noise injected from amp + noise due to loading.
 

Re: Voltage Noise

> Gain, usually means differential gain, acts upon wanted signal.
>Noise, regarded as common-mode at the input, gets rejected by means of
> common-mode rejection.

Of course, that are basics of in-amps

> For output noise, you should consider noise injected from amp + noise due to loading.

I thought im doing it ... I think that input referred Output noise (as stated in datasheet) is the noise added by the in-amp. There are no others figures considering noise so it must be it ...
The common-mode noise is not so interesting while reduced by 110dB as stated in the datasheet.
 

Voltage Noise

the idea is that the noise is due to diffrent elements in the circuit see diffrent gain to the output ,so not all the noise are enlarged by the large gain of the circuit , while the signal is totally amplified by the circuit gain so the SNR increase with the gain
 

Re: Voltage Noise

indeed u are right in this, but its not SNR given in the figure but the spectral noise density in nV/sq(Hz). The figure desribes Output noise referred to Input with respect to the Gain. So please do not think about SNR in this case, thats my opinion.

Output noise can vary according to gain but WHY is the output noise so huge at low gain, thats my question ...
 

Voltage Noise

well if the noise is input referd then it is divided by the whole gain while each component of the noise is multiplied by only part of the gain when refering to the output so the overall effect is decrease of the input referd noise.


let me b more clear , first get the output noise which is the noise of each element multplied by the each element gain to the output , then divide the output noise by the gain to get the input refered noise then u get lower noise cause the overall gain is bigger than individual elements gain
 

Re: Voltage Noise

> if the noise is input referd then it is divided by the whole gain

In other words , d u mean that the real output noise [noise that i see on my oscilloscope] for Gain =100 is simply 35nV/Hz * 100 in this case ?
Do i must simply multiply the noise stated in the figure by Gain ?
 

Voltage Noise

i am not that sure by i guess so, maybe someone else can comment
 

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