Multiple systems ranging from thermostats to flight control employ an analog-to-digital converter (ADC) to capture real world analog signals which will be processed in the digital domain and take necessary actions based off of the digital results. Each ADC specifies a number of bits to represent the various digital levels it can produce. For a given constant ADC input, the output of the ADC is not a constant digital value due to various errors in a typical signal chain. Thus, it is important to consider the effective number of bits (ENOB) or the noise-free resolution of the signal chain for better comparison and to also extract maximum information from the captured data. Higher precision calls for higher ENOB and noise free resolution.
Typically, signal-to-noise ratio (SNR), total harmonic distortion (THD) and noise of the system play an important role in ENOB calculation. For multiple systems like field transmitters or test and measurement applications, DC input signal accuracy and precision is critical. Therefore, the noise specification becomes most critical. The voltage reference used with an ADC is a critical component in the signal chain that can impact the precision and accuracy.
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.
Equation 1 expresses the total noise of this circuit as:
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.