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[SOLVED] about relation between amplitude of input signal and SNR

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J

jiangwp

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We know the

SNR=(the RMS of input signal)/(the RMS of noise) (I)

or . a input sine signal: Vpsin(wt)

SNR=(6.02N+1.76) dB (II)

It is not related between the SNR and the amplitude , only effected by the N.

In the mean while, we estimate the SNR from the above the equation (I) ,

also there is no realtion between the amplitue and the SNR.

but , we test the DAC/ADC chip , we will find the SNR increase with the increase of the input signal amplitude unlil overloading . Why?
 

Amplitude of a signal can be the Average or the Maximum Amplitude, therefore you can derive the RMS from it.
 

Eq. (II) comes when the input signal is full scale.
Then surely SNR increases when you use a larger amplitude input.
 

The equation 'SNR=(6.02N+1.76) dB' represents the relation in ideal ADC/DAC. In ideal ADC/DAC, the N (noise) is quantization noise only. But in a pratical ADC/DAC, there will be other noise sources, such as thermal noise. These noise sources are independent on the signal amplitude. As a result, the SNR in a pratical ADC/DAC may be worse than that predicted by the above equation, and the SNR increases as the signal amplitude increases.
 

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