By Len Staller
Embedded Systems Design
(02/24/05, 05:24:00 PM EST)
Embedded Systems Design
(02/24/05, 05:24:00 PM EST)
Signal-to-noise ratio
The signal-to-noise ratio (SNR) is the ratio of the root mean square (RMS) power of the input signal to the RMS noise power (excluding harmonic distortion), expressed in decibels (dB), as shown in Equation 3.
SNR(dB) = 20 log | Vsignal (rms) / V noise (rms) (Equation 3)
SNR is a comparison of the noise to be expected with respect to the measured signal. The noise measured in an SNR calculation doesn't include harmonic distortion but does include quantization noise (an artifact of quantization error) and all other sources of noise (for example, thermal noise). This noise floor is depicted in the FFT plot in Figure 9. For a given ADC resolution, the quantization noise is what limits an ADC to its theoretical best SNR because quantization error is the only error in an ideal ADC. The theoretical best SNR is calculated in Equation 4.
Figure 9: SNR— A measure of the signal compared to the noise floor
SNR(dB) = 6.02N + 1.76 (4) (Equation 4)
Where N is the ADC resolution
Quantization noise can only be reduced by making a higher-resolution measurement (in other words, a higher-resolution ADC or oversampling). Other sources of noise include thermal noise, 1/ƒ noise, and sample clock jitter.
Reference:
- http://www.embended.com/
- http://en.wikipedia.org
