Figure 3-2 shows all ADC connections. Each power-supply and reference connection have a 100-nF decoupling capacitor. Place these capacitors as close as possible to the ADC pins and make sure each component has a low-impedance connection to the GND plane.

Each digital pin of the ADC has a 49.9-Ω series resistor. These resistors smooth the edges of the digital signals so that the signals have minimal overshoot and ringing. A resistor is most important on the serial clock (SCLK) trace because the SCLK signal can toggle as quickly as 8 MHz. Resistors on the other pins are not strictly required, though these components can be included in the final design to improve digital signal integrity.

Pullup resistors R8, R9, R10, and R11 prevent CS, START, RESET, and PWDN, respectively, from floating when not driven. The differential voltage across test points TP4 and ground is the reference voltage used by the ADC.

Figure 3-2 ADS1258EVM ADC Power-Supply Decoupling and Analog and Digital Connections