AD9709 datasheet(9/27 Pages) AD

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AD9709

–9–

FUNCTIONAL DESCRIPTION

Figure 20 shows a simpliﬁed block diagram of the AD9709.

The AD9709 consists of two DACs, each one with its own

independent digital control logic and full-scale output current

control. Each DAC contains a PMOS current source array

capable of providing up to 20 mA of full-scale current (IOUTFS).

The array is divided into 31 equal currents that make up the ﬁve

most signiﬁcant bits (MSBs). The three lower bits consist of

seven equal current sources whose value is 1/8th of an MSB

current source. Implementing the lower bits with current sources,

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

for multitone or low-amplitude signals and helps maintain the

DACs high-output impedance (i.e., >100 k

Ω).

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

the two output nodes (i.e., IOUTA or IOUTB) via PMOS differ-

ential current switches. The switches are based on a new archi-

tecture that drastically improves distortion performance. This

new switch architecture reduces various timing errors and pro-

vides matching complementary drive signals to the inputs of the

differential current switches.

The analog and digital sections of the AD9709 have separate

power supply inputs (i.e., AVDD and DVDD) that can operate

independently over a 3 V to 5.5 V range. The digital section,

which is capable of operating up to a 125 MSPS clock rate,

consists of edge-triggered latches and segment decoding logic

circuitry. The analog section includes the PMOS current sources,

the associated differential switches, a 1.20 V bandgap voltage

reference and two reference control ampliﬁers.

The full-scale output current of each DAC is regulated by sepa-

rate reference control ampliﬁers and can be set from 2 mA to

20 mA via an external resistor, RSET, connected to the Full-Scale

Adjust (FSADJ) 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 seg-

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

scale current, IOUTFS, is 32

× IREF.

REFERENCE OPERATION

The AD9709 contains an internal 1.20 V bandgap reference.

This can be easily overridden by an external reference with no

effect on performance. REFIO serves as either an input or output

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. The internal reference

voltage will be present at REFIO. If the voltage at REFIO is to

be used elsewhere in the circuit, an external buffer ampliﬁer

with an input bias current of less than 100 nA should be used.

An example of the use of the internal reference is shown in

Figure 21.

An external reference can be applied to REFIO as shown in

Figure 22. 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.

GAINCTRL MODE

The AD9709 allows the gain of each channel to be set indepen-

dently by connecting one RSET resistor to FSADJ1 and another

RSET resistor to FSADJ2. To add flexibility and reduce system

cost, a single RSET resistor can be used to set the gain of both

channels simultaneously.

When GAINCTRL is low (i.e., connected to AGND), the inde-

pendent channel gain control mode using two resistors is enabled.

In this mode, individual RSET resistors should be connected to

FSADJ1 and FSADJ2. When GAINCTRL is high (i.e., connected

to AVDD), the master/slave channel gain control mode using one

resistor is enabled. In this mode, a single RSET resistor is con-

nected to FSADJ1 and the resistor on FSADJ2 can be removed.

DIGITAL DATA INPUTS

IREF1

IREF2

AVDD

DB0-DB7

GAINCTRL

WRT1/

IQWRT

1.2V REF

RSET2

CHANNEL 1 LATCH

CHANNEL 2 LATCH

PMOS

CURRENT

SOURCE

ARRAY

PMOS

CURRENT

SOURCE

ARRAY

SEGMENTED

SWITCHES

FOR DAC1

SEGMENTED

SWITCHES

FOR DAC2

LSB

SWITCH

LSB

SWITCH

MULTIPLEXING LOGIC

AD9709

DCOM

MODE

SLEEP

CLK2/

IQRESET

CLK1/

IQCLK

FSADJ1

RSET1

REFIO

0.1 F

FSADJ2

DB0-DB7

WRT2/

IQSEL

DVDD

DAC2

LATCH

DAC1

LATCH

CLK

DIVIDER

ACOM

IOUTA1

IOUTB1

RL1A

VOUT1A

RL1B

VOUT1B

RL2A

VOUT2A

RL2B

VOUT2B

IOUTA2

IOUTB2

VDIFF = VOUTA – VOUTB

Figure 20. Simpliﬁed Block Diagram

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