Making sense of ADC specs – Part 2 : SNR Vs DR

Many times in ADC data sheets, both SNR (Signal to Noise ratio) and DR (Dynamic range) are specified. These parameters are usually defined as follows.

“Signal to Noise ratio is the ratio of maximum output signal amplitude to the output noise level not including harmonics or dc.

Dynamic range is ratio of largest to the small signal that can be resolved.”

Looking at these definitions, I couldn’t tell the difference between these parameters. I couldn’t figure out how DR can be larger than SNR. Later I realized that, SNR and DR for ADC should be same for an ADC unless, Noise floor in the ADC changes depending on input level.                                     

This is the case with almost all ADCs. For example take effect of clock jitter on ADC performance. As effect of jitter is more prominent with increase in signal amplitude, noise floor because of clock jitter decreases with reduction in signal amplitude. This will cause DR to be higher than SNR.

Another example is effect of reference noise in differential SAR ADCs. When converting small differential amplitude inputs, reference noise won’t effect output ADC. This is because the transfer function from reference to both inputs of comparator will look alike in that case. So as input amplitude decreases, noise floor because of reference also scales down in this case. This can cause SNR to be lower than DR in case of SAR ADCs.

Another example is with Sigma Delta converters where SNR degrades beyond maximum stable amplitude. So DR will be higher than SNR in this case as well.

SNR Vs DNR figure

SNR and DR for a state of art  Sigma-Delta ADC (DR=103dB and SNR=98.9dB in this example) [1]

So if we realize that noise floor need not stay same for all inputs, it will be clear on why DR and SNR can be different.

References

  1. A. Sukumaran and S.Pavan, “Low Power Design Techniques for Single-Bit Audio Continuous-Time Delta Sigma ADCs Using FIR Feedback,” IEEE Journal of Solid State Circuits, Nov. 2014.

 

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