AD9751AST datasheet(9/26 Pages) AD

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AD9751

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FUNCTIONAL DESCRIPTION

Figure 3 shows a simplified block diagram of the AD9751. The

AD9751 consists of a PMOS current source array capable of

providing up to 20 mA of full-scale current, IOUTFS. The array is

divided into 31 equal sources that make up the five most signifi-

cant bits (MSBs). The next four bits, or middle bits, consist of

15 equal current sources whose value is 1/16th of an MSB cur-

rent source. The remaining LSB is a binary weighted fraction of

the middle bit current sources. Implementing the middle and

lower bits with current sources, instead of an R-2R ladder,

enhances dynamic performance for multitone or low-amplitude

signals and helps maintain the DAC’s high output impedance

(i.e., >100 k

Ω).

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

two outputs (i.e., IOUTA or IOUTB) via PMOS differential current

switches. The switches are based on a new architecture that

drastically improves distortion performance. This new switch

architecture reduces various timing errors and provides match-

ing complementary drive signals to the inputs of the differential

current switches.

The analog and digital sections of the AD9751 have separate

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

independently over a 3.1 V to 3.5 V range. The digital section,

which is capable of operating at a 300 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 refer-

ence and a reference control amplifier.

The 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. 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 the value of IREF.

REFERENCE OPERATION

The AD9751 contains an internal 1.20 V bandgap reference.

This can easily be overdriven 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 less than 100 nA should be used. An example

of the use of the internal reference is given in Figure 4.

A low impedance external reference can be applied to REFIO

as shown in Figure 5. The external reference may provide either

a fixed reference voltage to enhance accuracy and drift perfor-

mance or a varying reference voltage for gain control. Note that

the 0.1

µF compensation capacitor is not required since the inter-

nal reference is overdriven, and the relatively high input impedance

of REFIO minimizes any loading of the external reference.

REFERENCE CONTROL AMPLIFIER

The AD9751 also contains an internal control amplifier that is

used to regulate the DAC’s full-scale output current, IOUTFS.

The control amplifier is configured as a voltage-to-current con-

verter as shown in Figure 4, so that its current output, IREF, is

determined by the ratio of VREFIO and an external resistor, RSET,

as stated in Equation 4. IREF is applied to the segmented current

sources with the proper scaling factor to set IOUTFS as stated in

Equation 3.

The control amplifier allows a wide (10:1) adjustment span of

IOUTFS over a 2 mA to 20 mA range by setting IREF between

62.5

µA and 625 µA. The wide adjustment span of IOUTFS provides

several application benefits. The first benefit relates directly to

the power dissipation of the AD9751, which is proportional to

IOUTFS (refer to the Power Dissipation section). The second

benefit relates to the 20 dB adjustment, which is useful for sys-

tem gain control purposes.

The small signal bandwidth of the reference control amplifier is

approximately 500 kHz and can be used for low frequency, small

signal multiplying applications.

AD9751

IOUTA

IOUTB

SEGMENTED

SWITCHES FOR

DB0 TO DB9

DAC

FSADJ

REFIO

1.2V REF

DIV0

PLLLOCK

DIGITAL DATA INPUTS

0.1 F

RSET

RLOAD

DB0 – DB9

DCOM

PMOS CURRENT

SOURCE ARRAY

AVDD

3.1V TO 3.5V

DVDD

2 -1 MUX

PORT 1 LATCH

DAC LATCH

ACOM

PORT 2 LATCH

DIV1

PLL

CIRCUITRY

PLLVDD

CLKVDD

CLK+

CLK–

CLKCOM

RESET

LPF

DB0 – DB9

VOUTB

RLOAD

VOUTA

VDIFF = VOUTA – VOUTB

Figure 3. Simplified Block Diagram

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