AD9721 Analog Devices, Inc., AD9721 Datasheet - Page 10

• Manufacturer Part Number: AD9721
• Description: Ad9731 10-bit, 170 Msps
• Manufacturer: Analog Devices, Inc.

AD9731
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
REFERENCE IN a signal with a dc bias of –3.3 V (I
22.5 mA) to –4.25 V (I
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-
ence voltage must be applied prior to applying –V
Analog Output
The switch network provides complementary current outputs
I
cal current source matching, which provides a 10-bit linearity
without trim. Current is steered to either I
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
shown in the block diagram. Both I
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.
OUT
and I
APPROX
OUTB
Figure 4. Wideband Multiplying Circuit
–3.8V
. The design of the AD9731 is based on statisti-
–V
S
REFERENCE IN
OUT
ª 3 mA), as shown in Figure 4, or by
OUT
–V
S
and I
AD9731
OUT
OUTB
or I
S
OUTB
should be
.
OUT
in pro-
ª
–10–
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 that uses the AD9631, a high speed, current feedback
amplifier. The resistor values in Figure 5 provide a 4.096 V
swing, centered at ground, at the output of the AD9631 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.
Figure 5. I-to-V Conversion Using a Current Feedback
Amplifier
OUT
REF
AD708
1/2
AD9731
CONTROL
AMP IN
I
OUTB
I
OUT
10k
AD708
I
FS
1/2
10k
R
25
25
L
25
R
FF
200
R1
I
FS
R2
100
AD9631
400
R
FB
2048V
REV. B
V
OUT