SNAS348G – MAY 2006 – REVISED APRIL 2016
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Using References as Power Supplies (continued)
Like any low dropout regulator, the LP2980 requires an output capacitor for loop stability. This output capacitor
must be at least 1 µF over temperature, but values of 2.2 µF or more provides even better performance. The
ESR of this capacitor must be within the range specified in the LP2980 data sheet. Surface-mount solid tantalum
capacitors offer a good combination of small size and ESR. Ceramic capacitors are attractive due to their small
size but generally have ESR values that are too low for use with the LP2980. Aluminum electrolytic capacitors
are typically not a good choice due to their large size and have ESR values that may be too high at low
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
Figure 41. DAC124S085 Layout Example