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AS1504, AS1505

Data Sheet

Supply Bypassing

9 Application Information

Supply Bypassing

The AS1504/AS1505 require a well-filtered power source. In most applications, the AS1504/AS1505 should be pow-

ered directly from the system power supply (+3 to +5V). However, if the logic supply is a switch-mode design, it will

probably generate noise in the 20kHz to 1MHz range. Additionally, fast logic gates can generate transients hundred of

millivolts in amplitude from wiring resistance and inductance.

The circuit shown in Figure 18 isolates the analog section from any logic switching transients. Even if a separate power

supply trace is not available, adequate supply bypassing will reduce supply-line induced errors. Local supply bypass-

ing consisting of a 10µF tantalum electrolytic capacitor in parallel with a 0.1µF ceramic capacitor is recommended (see

Figure 19).

Figure 18. Power Supply Traces

Figure 19. Recommended Supply Bypassing

Output Buffering

For most designs, the nominal 5k

Ω output impedance of the AS1504/AS1505 is sufficient to drive succeeding circuitry.

If a lower output impedance is required, an external amplifier can be added (see Figure 20 on page 13).

A single amplifier should be used as a simple buffer to reduce the output resistance of DAC1. An amplifier with low off-

set voltage, low supply current, and operation at less than 3V is recommended due to its rail-to-rail input and output

operation. DAC2 and DAC3 are configured in a summing arrangement where DAC3 provides the coarse output volt-

age setting and DAC2 is used for fine adjustments.

output of DAC3.

0.1µF

CMOS/TTL

Logic Circuits

+5V Power

Supply

10µF

Tantalum

AS1504/

AS1505

+

0.1µF

10µF

Tantalum

AS1504/

AS1505

+

+5V

16

8

GND