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

[jiangwp]

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?

 

[glenjoy]

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

 

[reltol1]

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

 

[Hughes]

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.