Datasheet

I 1
OUT
I 2
OUT
I
IN
I
REF
Q
IN
Q
REF
QuadratureModulator
V ~0V to1.20V
OUT P P
V ~0.6V to1.8V
IN P P
RF
I
IN
I
REF
DAC2904
S
I
OUT
2
I
OUT
1
Signal
Conditioning
I 1
OUT
DAC2904
I 1
OUT
V 1
OUT
0.01 Fm
0.01 Fm
V
DC
R
1
R
2
R
LOAD
50W
50W
I
OUT
1
I
OUT
1
V
OUT
1
I =32 I =32
OUTFS REF
´ ´
V
R
REF
SET
DAC2904
+1.25VRef.
R
SET
2kW
0.1 Fm
FSA
+5V
+V
A
REF
IN
I =
REF
V
REF
R
SET
Current
Sources
Ref
Control
Amp
DAC2904
SBAS198C –AUGUST 2001–REVISED OCTOBER 2009.............................................................................................................................................. www.ti.com
Signal conditioning (level-shifting) may be required to ensure correct dc common-mode levels at the input of the quadrature modulator.
Figure 25. Generic Interface to a Quadrature Modulator
INTERNAL REFERENCE OPERATION
The DAC2904 has an on-chip reference circuit which
consists of a 1.25V bandgap reference and two
control amplifiers, one for each DAC. The full-scale
output current, I
OUTFS
, of the DAC2904 is determined
by the reference voltage, V
REF
, and the value of
resistor R
SET
. I
OUTFS
can be calculated by:
(10)
Figure 26. AC-Coupled Interface to Quadrature
As shown in Figure 27, the external resistor R
SET
Modulator Applying Level Shifting
connects to the FSA pin (Full-Scale Adjust). The
reference control amplifier operates as a V-to-I
converter producing a reference current, I
REF
, which
is determined by the ratio of V
REF
and R
SET
(see
Equation 10). The full-scale output current, I
OUTFS
,
results from multiplying I
REF
by a fixed factor of 32.
Using the internal reference, a 2kΩ resistor value
results in a full-scale output of approximately 20mA.
Resistors with a tolerance of 1% or better should be
considered. Selecting higher values, the output
current can be adjusted from 20mA down to 2mA.
Operating the DAC2904 at lower than 20mA output
currents may be desirable for reasons of reducing the
total power consumption, improving the distortion
performance, or observing the output compliance
voltage limitations for a given load condition.
It is recommended to bypass the REF
IN
pin with a
ceramic chip capacitor of 0.1μF or more. The control
amplifier is internally compensated, and its
small-signal bandwidth is approximately 0.3MHz.
Figure 27. Internal Reference Configuration
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