AD9712B/AD9713B
0.1µF
–5.2V
0.1µF
0.01µF
0.01µF
12,21
DIGITAL –V
S
28
1
2
3
4
5
ECL
DRIVE
LOGIC
6
7
8
9
10
11
D
1
(MSB)
D
2
D
3
15,25
ANALOG –V
S
REFERENCE
IN
0.1µF
17
20Ω
CONTROL
AMP OUT
REFERENCE
OUT
CONTROL
AMP IN
R
SET
I
OUT
18
DAC current across feedback resistor R
FB
determines the
AD9617 output swing. A current divider formed by R
L
and R
FF
limits the current used in the I-to-V conversion, and provides an
output voltage swing within the specifications of the AD9617.
Current through R
2
provides dc offset at the output of the
AD9617. Adjusting the value of R
1
adjusts the value of offset
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
D
4
D
5
D
6
D
7
D
8
D
9
D
10
D
11
D
12
(LSB)
20
19
24
16
R
L
V
OUT
=
R
L
I
FS
x R
L
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
AD9712B
AD9713B
14
26 LATCH ENABLE
I
OUT
ANALOG REFERENCE DIGITAL
RETURN
GROUND GROUND
13
22
27
SYSTEM
GROUND
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.
10k
+
1/2 AD708
10k
–
1/2 AD708
200
R
1
–
+
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).
I
OS
400
20
REF
OUT
19
CONTROL
AMP IN
I
OUT
14
I
FS
25
R
FF
100
R
2
R
FB
–
25
R
L
AD9617
V
OUT
±2.048V
AD9712B
AD9713B
+
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.
12.5
I
OUT
16
Figure 5. I/VConversion Using Current Feedback
REV. B
–7–
AD [ ANALOG DEVICES ]
