AD9731 Datasheet, PDF (9/11 Pages) Analog Devices – 10-Bit, 170 MSPS D/A Converter

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_OUT ≈
22.5 mA) to –4.25 V (I_OUT ≈ 3 mA), as shown in Figure 17, or
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-
ence voltage must be applied prior to applying –V_S.
APPROX
–3.8V
AD9731
REFERENCE IN
–V_S
–V_S
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.
10k⍀
1/2
AD708
10k⍀
1/2
AD708
I_FS
R1
200⍀
R2
100⍀
REF CONTROL
OUT AMP IN
I_OUT
AD9731
R_FF
I_{FS 25⍀}
R_L
25⍀
R_FB
400⍀
AD9617
±2048V
V_OUT
I_OUTB
25⍀
Figure 17. Wideband Multiplying Circuit
Analog Output
The switch network provides complementary current outputs
I_OUT and I_OUTB. The design of the AD9731 is based on statisti-
cal current source matching, which provides a 10-bit linearity
without trim. Current is steered to either I_OUT or I_OUTB in pro-
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_OUT and I_OUTB should be
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.
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.
REV. A
–9–