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Data Sheet

AD5313R

Rev. B | Page 17 of 26

THEORY OF OPERATION

20B

DIGITAL-TO-ANALOG CONVERTER (DAC)

The AD5313R is a dual 10-bit, serial input, voltage output DAC

with an internal reference. The part operates from supply voltages

of 2.7 V to 5.5 V. Data is written to the AD5313R in a 24-bit word

format via a 3-wire serial interface. The AD5313R incorporates a

power-on reset circuit to ensure that the DAC output powers up to

a known output state. The device also has a software power-down

mode that reduces the typical current consumption to 4 µA.

21B

TRANSFER FUNCTION

The internal reference is on by default. To use an external

reference, only a nonreference option is available. Because the

input coding to the DAC is straight binary, the ideal output

voltage when using an external reference is given by









REF

OUT

Gain

where:

Gain is the output amplifier gain and is set to 1 by default. It can

be set to ×1 or ×2 using the gain select pin. When the GAIN pin

is tied to GND, both DAC outputs have a span from 0 V to VREF.

If the GAIN pin is tied to VLOGIC, both DACs output a span of

0 V to 2 × VREF.

D is the decimal equivalent of the binary code that is loaded to

the DAC register as follows: 0 to 1,024 for the 10-bit device.

N is the DAC resolution.

22B

DAC ARCHITECTURE

The DAC architecture consists of a string DAC followed by an

output amplifier. Figure 38 shows a block diagram of the DAC

architecture.

Figure 38. Single DAC Channel Architecture Block Diagram

The resistor string structure is shown in Figure 39. It is a string

of resistors, each of Value R. The code loaded to the DAC register

determines the node on the string where the voltage is to be tapped

off and fed into the output amplifier. The voltage is tapped off

by closing one of the switches connecting the string to the

amplifier. Because it is a string of resistors, it is guaranteed

monotonic.

Figure 39. Resistor String Structure

44B

Internal Reference

The AD5313R on-chip reference is on at power-up but can be

disabled via a write to a control register. See the Internal

Reference Setup section for details.

The AD5313R has a 2.5 V, 2 ppm/°C reference, giving a full-

scale output of 2.5 V or 5 V, depending on the state of the

GAIN pin. The internal reference associated with the device is

available at the VREF pin. This buffered reference is capable of

driving external loads of up to 10 mA.

45B

Output Amplifiers

The output buffer amplifier can generate rail-to-rail voltages on

its output, which gives an output range of 0 V to VDD. The actual

range depends on the value of VREF, the GAIN pin, the offset

error, and the gain error. The GAIN pin selects the gain of the

output, as follows:

•

If the GAIN pin is tied to GND, both DAC outputs have

a gain of 1, and the output range is 0 V to VREF.

•

If the GAIN pin is tied to VLOGIC, both DAC outputs have

a gain of 2, and the output range is 0 V to 2 × VREF.

These amplifiers are capable of driving a load of 1 kΩ in parallel

with 2 nF to GND. The slew rate is 0.8 V/µs with a ¼ to ¾ scale

settling time of 5 µs.

INPUT

2.5V

REF

DAC

RESISTOR

STRING

REF (+)

VREF

GND

REF (–)

VOUTX

GAIN

(GAIN = 1 OR 2)

TO OUTPUT

AMPLIFIER

VREF

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