AD9744ARU datasheet(13/32 Pages) AD

13 / 32 page

AD9744

Rev. B | Page 13 of 32

FUNCTIONAL DESCRIPTION

Figure 24 shows a simplified block diagram of the AD9744. The

AD9744 consists of a DAC, digital control logic, and full-scale

output current control. The DAC contains a PMOS current

source array capable of providing up to 20 mA of full-scale cur-

rent (IOUTFS). The array is divided into 31 equal currents that

make up the five most significant bits (MSBs). The next four

bits, or middle bits, consist of 15 equal current sources whose

value is 1/16th of an MSB current source. The remaining LSBs

are binary weighted fractions of the middle bits current sources.

Implementing the middle and lower bits with current sources,

instead of an R-2R ladder, enhances its dynamic performance

for multitone or low amplitude signals and helps maintain the

DAC’s high output impedance (that is, >100 kΩ).

All of these current sources are switched to one or the other of

the two output nodes, that is, IOUTA or IOUTB, via PMOS

differential current switches. The switches are based on the

architecture that was pioneered in the AD9764 family, with

further refinements to reduce distortion contributed by the

switching transient. This switch architecture also reduces vari-

ous timing errors and provides matching complementary drive

signals to the inputs of the differential current switches.

The analog and digital sections of the AD9744 have separate

power supply inputs, that is, AVDD and DVDD, that can

operate independently over a 2.7 V to 3.6 V range. The

digital section, which is capable of operating at a rate of up

to 210 MSPS, consists of edge-triggered latches and segment

decoding logic circuitry. The analog section includes the PMOS

current sources, the associated differential switches, a 1.2 V

band gap voltage reference, and a reference control amplifier.

The DAC full-scale output current is regulated by the reference

control amplifier and can be set from 2 mA to 20 mA via an

external resistor, RSET, connected to the full-scale adjust

(FS ADJ) pin. The external resistor, in combination with both

the reference control amplifier and voltage reference VREFIO, sets

the reference current IREF, which is replicated to the segmented

current sources with the proper scaling factor. The full-scale

current, IOUTFS, is 32 times IREF.

REFERENCE OPERATION

The AD9744 contains an internal 1.2 V band gap reference. The

internal reference cannot be disabled, but can be easily overrid-

den by an external reference with no effect on performance.

Figure 25 shows an equivalent circuit of the band gap reference.

REFIO serves as either an output or an input depending on

whether the internal or an external reference is used. To use the

internal reference, simply decouple the REFIO pin to ACOM

with a 0.1 µF capacitor and connect REFLO to ACOM via a

resistance less than 5 Ω. The internal reference voltage will be

present at REFIO. If the voltage at REFIO is to be used any-

where else in the circuit, an external buffer amplifier with an

input bias current of less than 100 nA should be used. An ex-

ample of the use of the internal reference is shown in Figure 26.

AVDD

REFIO

REFLO

µA

Ω

Figure 25. Equivalent Circuit of Internal Reference

150pF

+1.2V REF

AVDD

REFLO

CURRENT

SOURCE

ARRAY

3.3V

REFIO

FS ADJ

Ω

0.1

µF

AD9744

ADDITIONAL

LOAD

OPTIONAL

EXTERNAL

REF BUFFER

Figure 26. Internal Reference Configuration

An external reference can be applied to REFIO, as shown in

Figure 27. The external reference may provide either a fixed

reference voltage to enhance accuracy and drift performance or

a varying reference voltage for gain control. Note that the 0.1 µF

compensation capacitor is not required since the internal refer-

ence is overridden, and the relatively high input impedance of

REFIO minimizes any loading of the external reference.

150pF

+1.2V REF

AVDD

REFLO

CURRENT

SOURCE

ARRAY

3.3V

REFIO

FS ADJ

RSET

AD9744

EXTERNAL

REF

IREF =

VREFIO/RSET

AVDD

REFERENCE

CONTROL

AMPLIFIER

VREFIO

Figure 27. External Reference Configuration

REFERENCE CONTROL AMPLIFIER

The AD9744 contains a control amplifier that is used to regulate

the full-scale output current, IOUTFS. The control amplifier is

configured as a V-I converter, as shown in Figure 26, so that its

current output, IREF, is determined by the ratio of the VREFIO and

an external resistor, RSET, as stated in Equation 4. IREF is copied

to the segmented current sources with the proper scale factor to

set IOUTFS, as stated in Equation 3.

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