SLYT854 May   2024 ADS8900B , OPA2320

 

  1.   1
  2. 1Introduction
  3. 2The impact of the voltage reference on ADC noise
  4. 3The impact of the voltage reference on THD
  5. 4How voltage reference noise and THD affect the ENOB
  6. 5How voltage reference noise affects noise-free resolution
  7. 6Conclusion

4 How voltage reference noise and THD affect the ENOB

ENOB measures how the AC characteristics of your circuit affect your ADC resolution. The noise and THD of your circuit are represented by a term known as signal-to-noise ratio and distortion (SINAD). SINAD represents these two AC characteristics in one number, as expressed by Equation 2:

Equation 2. S I N A D   d B =   - 20 l o g 10 - S N R / 10 + 10 T H D / 10

From Equation 2, you can see that as the SNR increases, SINAD also increases. Thus, the less noise and distortion present, the better the SINAD. Using SINAD, you can use Equation 3 to easily find the ENOB of your ADC - especially since:

Equation 3. E N O B =   S I N A D - 1.76 d B 6.02  

Revisiting Equation 1, decreasing the total noise present in the voltage reference would decrease the total noise present in the circuit, leading to an increased SNR. With an increased SNR, the ENOB will also increase. Additionally, adding a high-bandwidth buffer on the output of the voltage reference would also lead to a decreased THD of the ADC, allowing the ENOB to increase.