AD9732

–9–

REV. A

APPLICATION NOTES

THEORY OF OPERATION

The AD9732 high speed digital-to-analog converter utilizes most

significant bit decoding and segmentation techniques to reduce

glitch impulse and deliver high dynamic performance on lower

power consumption than previous bipolar DAC technologies.

The design is based on four main subsections: the decode/driver

circuits, the edge-triggered data register, the switch network and

the control amplifier. An internal bandgap reference is included

to allow operation of the device with minimum external support

components.

Digital Inputs/Timing

The AD9732 has PECL high speed single-ended inputs for data

inputs and clock. The switching threshold is +2.0 V.

In the decode/driver section, the three MSBs are decoded to

seven “thermometer code” lines. An equalizing delay is included

for the seven least significant bits and the clock signals. This

delay minimizes data skew and data setup-and-hold times at the

register inputs.

The on-board register is rising-edge triggered and should be

used to synchronize data to the current switches by applying a

pulse with proper data setup-and-hold times as shown in the

timing diagram. Although the AD9732 is designed to provide

isolation of the digital inputs to the analog output, some cou-

pling of digital transitions is inevitable. Digital feedthrough can

be minimized by forming a low-pass filter at the digital input by

using a resistor in series with the capacitance of each digital

input. This common high speed DAC application technique has

the effect of isolating digital input noise from the analog output.

References

The internal bandgap reference, control amplifier and reference

input are pinned out to provide maximum user flexibility in

configuring the reference circuitry for the AD9732. When using

the internal reference, REF OUT (Pin 25) should be connected

to CONTROL AMP IN (Pin 26). CONTROL AMP OUT (Pin

24) should be connected to REF IN (Pin 23). A 0.1

µF ceramic

capacitor connected from Pin 23 to GND improves settling time

by decoupling switching noise from the current sink baseline. A

reference current cell provides feedback to the control amplifier

Full-scale current is determined by CONTROL AMP IN and

The internal reference is nominally –1.25 V (referenced to

temperature of 150 ppm/

°C. If greater accuracy or temperature

stability is required, an external reference can be used. The

AD589 reference features 10 ppm/

°C drift over the 0°C to

+70

°C temperature range.

Two modes of multiplying operation are possible with the

AD9732. Signals with bandwidths up to 2.5 MHz and input

swings from 3.8 V to 4.4 V can be applied to the CONTROL

AMP IN pin as shown in Figure 18. Because the control ampli-

fier is internally compensated, the 0.1

µF capacitor discussed

above can be reduced to maximize the multiplying bandwidth.

However, it should be noted that output settling time, for

changes in the digital word, will be degraded.

AD9732

CONTROL

AMP IN

CONTROL

AMP OUT

REFERENCE IN

3.8V TO 4.4V

2.5MHz TYPICAL

0.1 F

Figure 18. Lower Frequency Multiplying Circuit

The REFERENCE IN pin can also be driven directly for wider

bandwidth multiplying operation. The analog signal for this

mode of operation must have a signal swing in the range of

0.95 V to 1.9 V. This can be implemented by capacitively cou-

by dividing REFERENCE IN with a low impedance op amp

whose signal swing is limited to the stated range.

AD9732

REFERENCE IN

APPROX

1.4V

Figure 19. Wideband Multiplying Circuit

Analog Output

The switch network provides complementary current outputs

cal current source matching, which provides a 10-bit linearity

portion to the digital input word. The sum of the two currents is

always equal to the full-scale output current. The current can be

converted to a voltage by resistive loading as shown in Figure

all performance. The voltage that is developed is the product of

the output current and the value of the load resistor.

EVALUATION BOARD

The performance characteristics of the AD9732 make it ideally

suited for direct digital synthesis (DDS) and other waveform

synthesis applications. The AD9732 evaluation board provides a

platform for analyzing performance under optimum layout con-

ditions. The AD9732 also provides a reference for high speed

circuit board layout techniques.