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2.2 Clock Oscillator
The DAC1220EVM can be used with its on-board 2.4576MHz crystal or with
an external clock source. A commonly-available crystal type in a through-hole
HC49 package was used.
Selector switch S2 is inserted between the crystal and the DAC. The double-
pole switch disconnects the crystal from the device entirely, to better simulate
actual application conditions when using an external clock.
As with any CMOS Pierce crystal oscillator circuit, load capacitance is critical.
On the DAC1220EVM, slightly lower load capacitor values are used to com-
pensate for the added parasitic capacitance incurred by the switch and longer
trace distance between the DAC and the crystal.
2.3 Voltage Reference
The voltage reference circuit is based on the Burr-Brown REF1004-2.5 volt-
age reference. This device offers low cost, low power consumption, and a rela-
tively uniform noise characteristic well-suited to the DAC1220.
The reference input of the DAC1220 is somewhat demanding, so the
REF1004-2.5 is buffered by an OPA350. The output of this is filtered by the
large filter capacitor at the amplifier output for low noise. For even lower noise,
a tantalum capacitor can be installed at C4.
An external reference voltage can be switched in using switch S1.
Clock Oscillator
Circuit Description
2-3
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