11.1 Layout Guidelines

For best accuracy and minimum noise, the printed-circuit board containing the DAC124S085 must have separate analog and digital areas. The areas are defined by the locations of the analog and digital power planes. Both of these planes must be placed in the same board layer. There must be a single ground plane. A single ground plane is preferred if digital return current does not flow through the analog ground area. Frequently a single ground plane design uses a fencing technique to prevent the mixing of analog and digital ground current. Separate ground planes must only be used when the fencing technique is inadequate. The separate ground planes must be connected in one place, preferably near the DAC124S085. Take special care to ensure that digital signals with fast edge rates do not pass over split ground planes. They must always have a continuous return path below their traces.

The DAC124S085 power supply must be bypassed with a 10-µF and a 0.1-µF capacitor as close as possible to the device with the 0.1 µF right at the device supply pin. The 10-µF capacitor must be a tantalum type and the 0.1-µF capacitor must be a low ESL, low ESR type. The power supply for the DAC124S085 must only be used for analog circuits.

Avoid crossover of analog and digital signals and keep the clock and data lines on the component side of the board. The clock and data lines must have controlled impedances.

11.2 Layout Example