AD5669RBCBZ-1-R5 datasheet(21/30 Pages) AD

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Data Sheet

AD5629R/AD5669R

Rev. F | Page 21 of 30

THEORY OF OPERATION

DIGITAL-TO-ANALOG CONVERTER (DAC) SECTION

The AD5629R/AD5669R are fabricated on a CMOS process.

The architecture consists of a string of DACs followed by an

output buffer amplifier. Each part includes an internal 1.25 V/

2.5 V, 5 ppm/°C reference with an internal gain of 2. Figure 48

and Figure 49 show block diagrams of the DAC architecture.

OUTPUT

AMPLIFIER

GAIN = ×2

DAC

REF (+)

VREFIN/VREFOUT

VOUT

REF (–)

RESISTOR

STRING

GND

INTERNAL

REFERENCE1

1CAN BE OVERDRIVEN

BY VREFIN/VREFOUT.

Figure 48. DAC Architecture for Internal Reference Configuration

REF

BUFFER

OUTPUT

AMPLIFIER

GAIN = ×2

VREFIN/VREFOUT

GND

REF (+)

REF (–)

RESISTOR

STRING

VOUT

Figure 49. DAC Architecture for External Reference Configuration

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 the internal reference is

given by

















REFOUT

OUT

where:

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

DAC register as follows:

0 to 4095 for AD5629R (12 bits).

0 to 65,535 for AD5669R (16 bits).

N = the DAC resolution.

RESISTOR STRING

The resistor string section is shown in Figure 50. 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 50. Resistor String

INTERNAL REFERENCE

The AD5629R/AD5669R have an on-chip reference with an

internal gain of 2. The AD5629R-1/AD5669R-1 have a 1.25 V,

5 ppm/°C reference, giving a full-scale output of 2.5 V or the

AD5629R-2/AD5629R-3/AD5669R-2/AD5669R-3 have a 2.5 V,

5 ppm/°C reference, working between a supply from 4.5 V to

5.5 V 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 the control register

(see Table 8).

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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