AD5666 datasheet(20/28 Pages) AD

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AD5666

Rev. A | Page 20 of 28

THEORY OF OPERATION

D/A SECTION

The AD5666 DAC is fabricated on a CMOS process. The archi-

tecture consists of a string of DACs followed by an output buffer

amplifier. The parts include an internal 1.25 V/2.5 V, 5 ppm/°C

reference with an internal gain of 2. Figure 40 shows a block

diagram of the DAC architecture.

DAC REGISTER

REF (+)

VDD

VOUT

GND

REF (–)

RESISTOR

STRING

OUTPUT

AMPLIFIER

(GAIN = +2)

Figure 40. DAC Architecture

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

output voltage when using an external reference is given by

⎟

⎠

⎞

⎜

⎝

⎛

REFIN

OUT

The ideal output voltage when using and internal reference is

given by

⎟

⎠

⎞

⎜

⎝

⎛

REFOUT

OUT

where:

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

DAC register.

0 to 65,535 for AD5666 (16 bits).

N = the DAC resolution.

RESISTOR STRING

The resistor string section is shown in Figure 41. It is simply 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. 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.

TO OUTPUT

AMPLIFIER

Figure 41. Resistor String

INTERNAL REFERENCE

The AD5666 has an on-chip reference with an internal gain of 2.

The AD5666-1 has a 1.25 V 5 ppm/°C reference, giving a full-scale

output of 2.5 V. The AD5666-2 has a 2.5 V 5 ppm/°C reference,

giving a full-scale output of 5 V. The on-board reference is off at

power-up, allowing the use of an external reference. The internal

reference is enabled via a write to a control register.

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 be placed between

the reference output and GND for reference stability.

Individual channel power-down is not supported while using

the internal reference.

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