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

2 The impact of the voltage reference on ADC noise

While it is possible to remove some noise by filtering, you cannot realistically filter out noise at low frequencies. In a voltage reference, it is also not possible to filter out flicker noise (which is noise from 0.1Hz to 10Hz) without having a major impact on signal chain performance because of the size of the resistor-capacitor filter components you’ll need. Therefore, noise will almost always be present in your system.

In addition to the voltage reference noise, there will be noise from the ADC itself and the ADC driver. Each one of these components contributes noise to the circuit that generates a digital signal. Figure 1 is a simplified block diagram of this circuit.

Generic ADC circuit configuration with an external voltage reference. Figure 1 Generic ADC circuit configuration with an external voltage reference.

Equation 1 expresses the total noise of this circuit as:

Equation 1. T o t a l   N o i s e =   N o i s e D r i v e r 2 + N o i s e A D C 2 + N o i s e V R E F 2

The amount of noise present in your circuit is important to know when determining your system’s ENOB. In general, selecting low-noise devices is essential to a low-noise design.

In this article, I’ll focus on not only voltage reference selection, but other data processing choices that can help you maximize ADC performance.