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THEORY OF OPERATION

D/A SECTION

_

+

Resistor String

DAC Register

External Reference (–)

External Reference (+)

R

R

V

OUT

+ 2

V

D

65536

(1)

RESISTOR STRING

To Output

Amplifier

R

R

R

R

OUTPUT AMPLIFIER

DAC8544

SLAS420 – MAY 2004

The architecture of the DAC8544 consists of four string DACs followed by an output buffer amplifier. Figure 30

shows a block diagram of the DAC architecture.

Figure 30. DAC8544 Architecture

The input coding to the DAC8544 is unsigned binary, which gives the ideal output voltage as:

where

• D = decimal equivalent of the binary code that is loaded to the DAC register; it can range from 0 to 65535.

The resistor string section is shown in Figure 31. It is simply a divide-by-two resistor, followed by a string of

resistors, each of value R. The code loaded into the DAC register determines at which node on the string the

voltage is tapped off, to be fed into the output amplifier, by closing one of the switches connecting the string to

the amplifier. Because it is a string of resistors, it is assured monotonic.

Figure 31. Resistor String

The output buffer is capable of generating rail-to-rail voltages at its output, which gives an output range of 0 V to

the output amplifier can be seen in the typical curves. The slew rate is 1 V/

µs with a full-scale settling time of 10

µs with the output loaded. The feedback and gain setting resistors of the amplifier are in the order of 50 kΩ. Their

absolute value can be off significantly, but they are matched to within 0.1%.

Other signal conditioning circuitry may also be connected between these points for specific applications including

current sourcing.

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