Design of a Low-cost Gaussian White Noise Source for ADC Dither Experiments

Analog-to-digital converters (ADC) dither technology improves the quality of digital signals by introducing a random noise. Dither signals have been applied in various fields, such as control systems, LCD displays, scanning optical microscopes, and more. Dither is a powerful tool that can be useful in reducing the spurious performance of a data converter. Although dither can provide some remarkable gains in converter performance, circuits to generate dither can be relatively uncomplicated. Given that dither is essentially Gaussian noise, the crucial factor is having a reliable noise source. There are noise chips in the market that can easily produce Gaussian noise, but they tend to be costly and challenging to acquire. In this paper, a method that employs diodes to generate noise in real time is introduced. Experiment results indicate that reverse-biased diode can generate shot noise. Shot noise is a type of current noise generated in electronic tubes and transistor devices due to the uneven passage of charge carriers through the barrier region causing small fluctuations in the current. Both the generation mechanism and experiment results show that shot noise can be considered as "Gaussian white noise process" to a large extent, with the power level of the generated noise depending on the specific circumstances. Therefore, in this paper, Zener diode avalanche breaks down as the noise source to generate Gaussian white noise. However, the noise power level generated by the diode is very low, so some form of gain must be applied. If the system requires a variable dither level to account for changes in system loading over time, some form of noise gain control must be provided. The narrowband dither signals used for ADC dither experiments are generally used near the DC region and only require a bandwidth of several hundred kilohertz. Thus, in this paper, we choose low-cost operational amplifiers to amplify the noise signal into usable dither signals, and control the system gain by adjusting the loop resistance of the operational amplifier. The amplified signal passes through a ninth-order low-pass filter to eliminate the noise in the excess frequency band, retaining the usable narrowband dither signal. Finally, a three-resistor power divider was used to couple the dither signal into the RF signal. The experiment results show that the dither signal generated by this low-cost noise source, priced at only a few dollars, proves effective for ADC dither experiments, typically increasing the Spurious-free Dynamic Range (SFDR) of ADC.