12.1 Layout Guidelines

Use a a low-impedance connection for ground, so that return currents flow undisturbed back to their respective sources. For best performance, dedicate an entire PCB layer to a ground plane and route no other signal traces on this layer. Keep connections to the ground plane as short and direct as possible. When using vias to connect to the ground layer, use multiple vias in parallel to reduce impedance to ground.

A mixed signal layout sometimes incorporates separate analog and digital ground planes that are tied together at one location; however, separating the ground planes is not necessary when analog, digital and power supply components are properly placed. Proper placement of components partitions the analog, digital and power supply circuitry into different PCB regions to prevent digital return currents from coupling into sensitive analog circuitry. If ground plane separation is necessary, then make the connection at the ADC. Connecting individual ground planes at multiple locations creates ground loops, and is not recommended. A single ground plane for analog and digital avoids ground loops.

Bypass supply pins with a low-ESR ceramic capacitor. The placement of the bypass capacitors must be as close as possible to the supply pins using short, direct traces. For optimum performance, the ground-side connections of the bypass capacitors must also be low-impedance connections. The supply current flows through the bypass capacitor pin first and then to the supply pin to make the bypassing most effective (also known as a Kelvin connection). If multiple ADCs are on the same PCB, use wide power-supply traces or dedicated power-supply planes to minimize the potential of crosstalk between ADCs.

If external filtering is used for the analog inputs, use C0G-type ceramic capacitors when possible. C0G capacitors have stable properties and low-noise characteristics. Ideally, route differential signals as pairs to minimize the loop area between the traces. Route digital circuit traces (such as clock signals) away from all analog pins. Note the internal reference output return shares the same pin as the AVSS power supply. To minimize coupling between the power-supply trace and reference return trace, route the two traces separately; ideally, as a star connection at the AVSS pin.

It is essential to make short, direct interconnections on analog input lines and avoid stray wiring capacitance, particularly between the analog input pins and AVSS. These analog input pins are high-impedance and extremely sensitive to extraneous noise. Treat the AVSS pin as a sensitive analog signal and connect directly to the supply ground with proper shielding. Leakage currents between the PCB traces can exceed the input bias current of the ADS129x if shielding is not implemented. Keep digital signals as far as possible from the analog input signals on the PCB.

It is important the SCLK input of the serial interface is free from noise and glitches. Even with relatively slow SCLK frequencies, short digital signal rise and fall times may cause excessive ringing and noise. For best performance, keep the digital signal traces short, using termination resistors as needed, and make sure all digital signals are routed directly above the ground plane with minimal use of vias.

Figure 108. System Component Placement

12.2 Layout Example

Figure 109 is an example layout of the ADS129x requiring a minimum of two PCB layers. The example circuit is shown for either a single analog supply or a bipolar-supply connection. In this example, polygon pours are used as supply connections around the device. If a three- or four-layer PCB is used, the additional inner layers can be dedicated to route power traces. The PCB is partitioned with analog signals routed from the left, digital signals routed to the right, and power routed above and below the device.

Figure 109. ADS129x Layout Example