The power supply pin is bypassed with a capacitor network located close to the ADC. Because the reference for the ADC is the supply voltage, any noise on the supply degrades device noise performance. To keep noise off the supply, use a dedicated linear regulator for this device, or provide sufficient decoupling from other circuitry to keep noise off the ADC supply pin. Because of the ADC's low power requirements, it is also possible to use a precision reference as a power supply to maximize performance.

10.1 Power Management

The ADC takes time to power up, either after first applying V_A, or after returning to normal mode from shutdown mode. This corresponds to one dummy conversion for any SCLK frequency within the specifications in this document. After this first dummy conversion, the ADC performs conversions properly. Note that the t_QUIET time must still be included between the first dummy conversion and the second valid conversion.

When the V_A supply is first applied, the ADC may power up in either of the two modes: normal or shutdown. As such, one dummy conversion must be performed after start-up, as described in the previous paragraph. The part may then be placed into either normal mode or the shutdown mode, as described in Normal Mode and Shutdown Mode.

When the ADC is operated continuously in normal mode, the maximum throughput is f_SCLK / 20 at the maximum specified f_SCLK. Throughput may be traded for power consumption by running f_SCLK at its maximum specified rate and performing fewer conversions per unit time, raising the ADC CS line after the 10th and before the 15th fall of SCLK of each conversion. Figure 15 shows a plot of typical power consumption versus throughput. To calculate the power consumption for a given throughput, multiply the fraction of time spent in the normal mode by the normal mode power consumption and add the fraction of time spent in shutdown mode multiplied by the shutdown mode power consumption. Note that the curve of power consumption vs throughput is essentially linear. This is because the power consumption in the shutdown mode is so small that it can be ignored for all practical purposes.

10.2 Power Supply Noise Considerations

The charging of any output load capacitance requires current from the power supply, V_A. The current pulses required from the supply to charge the output capacitance causes voltage variations on the supply. If these variations are large enough, they could degrade SNR and SINAD performance of the ADC. Furthermore, 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 causes 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 is the noise coupled into the analog channel, degrading noise performance.

To keep noise out of the power supply, keep the output load capacitance as small as practical. It is good practice to use a 100-Ω series resistor at the ADC output, placed as close to the ADC output pin as practical. This limits the charge and discharge current of the output capacitance and improves noise performance.