AD9712B/AD9713B

REV. B

–7–

D

(LSB)

12

ANALOG

RETURN

D

(MSB)

1

LATCH ENABLE

REFERENCE

GROUND

REFERENCE

OUT

CONTROL

AMP IN

REFERENCE

IN

CONTROL

AMP OUT

AD9712B

AD9713B

0.1µF

0.01µF

20

Ω

SYSTEM

GROUND

DIGITAL

GROUND

–5.2V

13

22

27

3

4

26

11

5

6

7

8

9

10

14

12,21

15,25

20

16

24

19

18

17

28

1

2

ECL

DRIVE

LOGIC

0.1µF

0.01µF

0.1µF

Figure 4. Typical Resistive Load Connection

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

conversion of the DAC output. Figure 5 shows an example of a

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

amplifier.

CONTROL

AMP IN

AD9712B

AD9713B

19

20

14

16

10k

10k

400

200

REF

OUT

25

–

+

1/2 AD708

R

FF

25

V

±2.048V

OUT

100

–

+

1/2 AD708

–

+

AD9617

12.5

Figure 5. I/VConversion Using Current Feedback

limits the current used in the I-to-V conversion, and provides an

output voltage swing within the specifications of the AD9617.

current. This offset current is based on the reference of the

AD9712B/AD9713B, to avoid coupling noise into the output

signal.

The resistor values in Figure 5 provide a 4.096 V swing, cen-

tered at ground, at the output of the AD9617 amplifier.

Power and Grounding

Maintaining low noise on power supplies and ground is critical

for obtaining optimum results with the AD9712B or AD9713B.

DACs are most often used in circuits which are predominantly

digital. To preserve 12-bit performance, especially at conversion

speeds up to 100 MSPS, special precautions are necessary for

power supplies and grounding.

Ideally, the DAC should have a separate analog ground plane.

All ground pins of the DAC, as well as reference and analog

output components, should be tied directly to this analog

ground plane. The DAC’s ground plane should be connected to

the system ground plane at a single point.

Ferrite beads such as the Stackpole 57-1392 or Amidon

FB-43B-101, along with high frequency, low-inductance decou-

pling capacitors, should be used for the supply connections to

isolate digital switching currents from the DAC supply pins.

Separate isolation networks for the digital and analog supply

connections will further reduce supply noise coupling to the

output.

Molded socket assemblies should be avoided even when

prototyping circuits with the AD9712B or AD9713B. When

the DAC cannot be directly soldered into the board, individual

pin sockets such as AMP #6-330808-0 (knock-out end), or

#60330808-3 (open end) should be used. These have much

less effect on inter-lead capacitance than do molded assemblies.

DDS Applications

Numerically controlled oscillators (NCOs) are digital devices

which generate samples of a sine wave. When the NCO is com-

bined with a high performance D/A converter (DAC), the com-

bination system is referred to as a Direct Digital Synthesizer

(DDS).

The digital samples generated by the NCO are reconstructed by

the DAC and the resulting sine wave is usable in any system

which requires a stable, spectrally pure, frequency-agile refer-

ence. The DAC is often the limiting factor in DDS applications,

since it is the only analog function in the circuit. The AD9712B/

AD9713B D/A converters offer the highest level of performance

available for DDS applications.

DC linearity errors of a DAC are the dominant effect in low-

frequency applications and can affect both noise and harmonic

content in the output waveform. Differential Nonlinearity

(DNL) errors determine the quantization error between adja-

cent codes, while Integral Nonlinearity (INL) is a measure of

how closely the overall transfer function of the DAC compares

with an ideal device. Together, these errors establish the limits

of phase and amplitude accuracy in the output waveform.