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AD5686R/AD5685R/AD5684R

Data Sheet

Rev. D | Page 20 of 31

THEORY OF OPERATION

19B

DIGITAL-TO-ANALOG CONVERTER

The AD5686R/AD5685R/AD5684R are quad 16-/14-/12-bit,

serial input, voltage output DACs with an internal reference.

The parts operate from supply voltages of 2.7 V to 5.5 V. Data is

written to the AD5686R/AD5685R/AD5684R in a 24-bit word

format via a 3-wire serial interface. The AD5686R/AD5685R/

AD5684R incorporate a power-on reset circuit to ensure that the

DAC output powers up to a known output state. The devices also

have a software power-down mode that reduces the typical

current consumption to typically 4 µA.

20B

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:

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

the DAC register as follows:

0 to 4,095 for the 12-bit device.

0 to 16,383 for the 14-bit device.

0 to 65,535 for the 16-bit device.

N is the DAC resolution.

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

This can be set to ×1 or ×2 using the gain select pin. When this

pin is tied to GND, all four DAC outputs have a span from 0 V

to VREF. If this pin is tied to VDD, all four DACs output a span of

0 V to 2 × VREF.

21B

DAC ARCHITECTURE

The DAC architecture consists of a string DAC followed by an

output amplifier. Figure 48 shows a block diagram of the DAC

architecture.

Figure 48. Single DAC Channel Architecture Block Diagram

The resistor string structure is shown in Figure 49. 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 49. Resistor String Structure

43B

Internal Reference

The AD5686R/AD5685R/AD5684R 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 AD5686R/AD5685R/AD5684R have 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.

44B

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, offset error,

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

•

If this pin is tied to GND, all four outputs have a gain of 1

and the output range is 0 V to VREF.

•

If this pin is tied to VLOGIC, all four 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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