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AD5624R/AD5644R/AD5664R

Data Sheet

Rev. D | Page 20 of 28

THEORY OF OPERATION

DIGITAL-TO-ANALOG SECTION

The AD5624R/AD5644R/AD5664R DACs are fabricated on a

CMOS process. The architecture consists of a string DAC

followed by an output buffer amplifier. Figure 51 shows a block

diagram of the DAC architecture.

DAC

VDD

GND

VOUT

OUTPUT

AMPLIFIER

(GAIN = +2)

VREFIN

RESISTOR

STRING

REF

Figure 51. DAC Architecture

Because the input coding to the DAC is straight binary, the

ideal output voltage when using an external reference is given

















REFIN

OUT

The ideal output voltage when using the internal reference is

given by

















REFOUT

OUT

where:

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

the DAC register:

0 to 4095 for AD5624R (12 bit).

0 to 16,383 for AD5644R (14 bit).

0 to 65,535 for AD5664R (16 bit).

N is the DAC resolution.

RESISTOR STRING

The resistor string is shown in Figure 52. It is simply a string of

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

determines at which node on the string the voltage is tapped off

to be 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.

OUTPUT AMPLIFIER

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

its output, which gives an output range of 0 V to VDD. It can drive

a load of 2 kΩ in parallel with 1000 pF to GND. The source and

sink capabilities of the output amplifier can be seen in Figure 34

and Figure 35. The slew rate is 1.8 V/μs with a ¼ to ¾ full-scale

settling time of 7 μs.

TO OUTPUT

AMPLIFIER

Figure 52. Resistor String

INTERNAL REFERENCE

The AD5624R/AD5644R/AD5664R on-chip reference is off at

power-up and is enabled via a write to a control register. See the

Internal Reference Setup section for details.

The AD56x4R-3 has a 1.25 V, 5 ppm/°C reference giving a full-

scale output of 2.5 V. The AD56x4R-5 has a 2.5 V, 5 ppm/°C

reference giving a full-scale output of 5 V. The internal

reference associated with each part is available at the VREFOUT

pin. A buffer is required if the reference output is used to drive

external loads. When using the internal reference, it is

recommended that a 100 nF capacitor is placed between

reference output and GND for reference stability.

EXTERNAL REFERENCE

The VREFIN pin on the AD56x4R-3 and AD56x4R-5 allows the

use of an external reference if the application requires it. The

default condition of the on-chip reference is off at power-up.

All devices (AD56x4R-3 and the AD56x4R-5) can be operated

from a single 2.7 V to 5.5 V supply.

SERIAL INTERFACE

The AD5624R/AD5644R/AD5664R have a 3-wire serial interface

(SYNC, SCLK, and DIN) that is compatible with SPI, QSPI, and

MICROWIRE interface standards as well as with most DSPs. See

Figure 2 for a timing diagram of a typical write sequence.

The write sequence begins by bringing the SYNC line low. Data

from the DIN line is clocked into the 24-bit shift register on the

falling edge of SCLK. The serial clock frequency can be as high

as 50 MHz, making the AD5624R/AD5644R/AD5664R compat-

ible with high speed DSPs. On the 24th falling clock edge, the

last data bit is clocked in and the programmed function is

executed, that is, a change in DAC register contents and/or a

change in the mode of operation.

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