Datasheet

DAC128S085

www.ti.com

SNAS407F –AUGUST 2007–REVISED MARCH 2013

LAYOUT, GROUNDING, AND BYPASSING

For best accuracy and minimum noise, the printed circuit board containing the DAC128S085 should have

separate analog and digital areas. The areas are defined by the locations of the analog and digital power planes.

Both of these planes should be located in the same board layer. A single ground plane is preferred if digital

return current does not flow through the analog ground area. Frequently a single ground plane design will utilize

a "fencing" technique to prevent the mixing of analog and digital ground current. Separate ground planes should

only be utilized when the fencing technique is inadequate. The separate ground planes must be connected in

one place, preferably near the DAC128S085. Special care is required to guarantee 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.

For best performance, the DAC128S085 power supply should be bypassed with at least a 1µF and a 0.1µF

capacitor. The 0.1µF capacitor needs to be placed right at the device supply pin. The 1µF or larger valued

capacitor can be a tantalum capacitor while the 0.1µF capacitor needs to be a ceramic capacitor with low ESL

and low ESR. If a ceramic capacitor with low ESL and low ESR is used for the 1µF value and it can be placed

right at the supply pin, the 0.1µF capacitor can be eliminated. Capacitors of this nature typically span the same

frequency spectrum as the 0.1µF capacitor and thus eliminate the need for the extra capacitor. The power supply

for the DAC128S085 should only be used for analog circuits.

It is also advisable to avoid the crossover of analog and digital signals. This helps minimize the amount of noise

from the transitions of the digital signals from coupling onto the sensitive analog signals such as the reference

pins and the DAC outputs.

Product Folder Links: DAC128S085