如何量化和热噪声确定ADC的有效的噪声图-How Quantization and Thermal Noise Determine an ADC's Effective Noise Figure（推荐）

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Figure 2 shows a direct comparison of the effective noise figure for a Nyquist ADC and an oversampled ADC. The noise density for the oversampled converter is defined in the frequency bandwidth for which the converter is designed, and the SNR is normalized to that particular bandwidth to obtain the spot noise in dBc/Hz.


Figure 2. Compare effective noise figure for Nyquist ADC and oversampling ADC.

Example

1. Assume the full-scale input level for both converters is 2Vp-p and the device input terminating resistance is 200Ω. The full-scale power level is: Signal Power = 10 × log ((Vrms²) / RIN) + 30dB = + 4dBm.
2. Assume the measured SNR value for a 12-bit converter sampled at 65Msps at the user's desired input frequency is 69dB (for a -1dBFS input level).
3. The converter's integrated noise power in the Nyquist band is calculated as: + 4dBm - 1dB - 69dB = -66dBm.
4. Calculate 10 × log (fSAMPLE/2) = 10 × log (65Msps/2) = 75.1dB.
5. The converter's normalized Nyquist band noise power is found by subtracting 75.1dB from -66dBm resulting in -141.1dBm in a 1Hz bandwidth.
6. The resulting effective ADC noise figure = -141.1dBm - KTB = -141.1dBm - (-174dBm) = 32.9dB.
7. As a comparison, the oversampled ADC noise density would be 144.1dBc/Hz or 145.1dBFS/Hz in order to obtain the same effective noise figure.

Conclusion

An RF designer can trade off several parameters to optimize the ADC effective noise figure when using the device in a sampling, sub-sampling, or oversampling receiver architecture. These parameters include ADC clock rate, terminating resistance, clock jitter, and nyquist band filtering, as discussed in this article.

References

1. Digital Techniques for Wideband Receivers, James Tsui, Artech House Publishers, 1995.
   
2. Software Ratio Architecture, Joseph Mitola III, John Wiley & Sons, Inc., 2000.

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，如何量化和热噪声确定ADC的有效的噪声图-How Quantization and Thermal Noise Determine an ADC's Effective Noise Figure（推荐）