AD9731

–9–

REV. A

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 –3.3 V to

–4.25 V. This can be implemented by capacitively coupling into

by dividing REFERENCE IN with a low impedance op amp

whose signal swing is limited to the stated range.

NOTE: When using an external reference, the external refer-

REFERENCE IN

AD9731

APPROX

–3.8V

Figure 17. 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 minus 1 LSB. The

current can be converted to a voltage by resistive loading as

equally loaded for best overall performance. The voltage that is

developed is the product of the output current and the value of

the load resistor.

An operational amplifier can also be used to perform the I-to-V

conversion of the DAC output. Figure 18 shows an example of a

circuit that uses the AD9617, a high speed, current feedback

amplifier. The resistor values in Figure 18 provide a 4.096 V

swing, centered at ground, at the output of the AD9617 amplifier.

AD9617

AD9731

R1

200

10k

1/2

AD708

10k

R2

100

400

CONTROL

AMP IN

REF

OUT

25

25

1/2

AD708

±2048V

25

Figure 18. I-to-V Conversion Using a Current Feedback

Amplifier

EVALUATION BOARD

The performance characteristics of the AD9731 make it ideally

suited for direct digital synthesis (DDS) and other waveform

synthesis applications. The AD9731 evaluation board provides a

platform for analyzing performance under optimum layout con-

ditions. The AD9731 also provides a reference for high speed

circuit board layout techniques.