The charging of any output load capacitance requires current from the digital supply, V_D. The current pulses required from the supply to charge the output capacitance cause voltage variations on the digital supply. If these variations are large enough, they can degrade SNR and SINAD performance of the ADC. Furthermore, if the analog and digital supplies are tied directly together, the noise on the digital supply is coupled directly into the analog supply, causing greater performance degradation than noise alone causes on the digital supply. Similarly, discharging the output capacitance when the digital output goes from a logic high to a logic low dumps current into the die substrate, which is resistive. Load discharge currents cause ground bounce noise in the substrate that degrades noise performance if that current is large enough. The larger the output capacitance, the more current flows through the die substrate and the greater the noise coupled into the analog channel.

The first solution to keeping digital noise out of the analog supply is to decouple the analog and digital supplies from each other or use separate supplies for them. To keep noise out of the digital supply, keep the output load capacitance as small as practical. If the load capacitance is greater than 50 pF, use a 100-? series resistor at the ADC output, located as close to the ADC output pin as practical. This resistor limits the charge and discharge current of the output capacitance and improves noise performance. Because the series resistor and the load capacitance form a low-frequency pole, verify signal integrity when the series resistor is added.