Datasheet

DAC2902
14
SBAS167C
www.ti.com
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 DAC2902 at lower than 20mA
output currents may be desirable for reasons of reducing the
total power consumption, optimizing the distortion perfor-
mance, or observing the output compliance voltage limita-
tions 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.
GAIN SETTING OPTIONS
The full-scale output current on the DAC2902 can be set two
ways: either for each of the two DAC channels independently or
for both channels simultaneously. For the independent gain set
mode, the GSET pin (pin 42) must be LOW (that is, connected
to AGND). In this mode, two external resistors are required—
one R
SET
connected to the FSA1 pin (pin 44) and the other to the
FSA2 pin (pin 41). In this configuration, the user has the
flexibility to set and adjust the full-scale output current for each
DAC independently, allowing for the compensation of possible
gain mismatches elsewhere within the transmit signal path.
Alternatively, bringing the GSET pin HIGH (that is, con-
nected to +V
A
), the DAC2902 will switch into the simulta-
neous gain set mode. Now the full-scale output current of both
DAC channels is determined by only one external R
SET
resistor connected to the FSA1 pin. The resistor at the FSA2
pin may be removed, however this is not required since this
pin is not functional in this mode and the resistor has no effect
to the gain equation. The formula for deriving the correct R
SET
remains unchanged (for example, R
SET
= 2kΩ will result in a
20mA output for both DACs).
EXTERNAL REFERENCE OPERATION
The internal reference can be disabled by simply applying an
external reference voltage into the REF
IN
pin, which in this
case functions as an input, as shown in Figure 11. The use
of an external reference may be considered for applications
that require higher accuracy and drift performance, or to add
the ability of dynamic gain control.
While a 0.1µF capacitor is recommended to be used with the
internal reference, it is optional for the external reference
operation. The reference input, REF
IN
, has a high input
impedance (1MΩ) and can easily be driven by various
sources. Note that the voltage range of the external reference
should stay within the compliance range of the reference
input (0.5V to 1.25V).
POWER-DOWN MODE
The DAC2902 features a power-down function that can be
used to reduce the total supply current to less than 6mA.
Applying a logic HIGH to the PD pin will initiate the power-
down mode, while a logic LOW enables normal operation.
When left unconnected, an internal active pull-down circuit
will enable the normal operation of the converter.
FIGURE 10. Internal Reference Configuration.
FIGURE 11. External Reference Configuration.
DAC2902
+1.25V Ref.
R
SET
2kΩ
0.1µF
FSA
+5V
+V
A
REF
IN
Current
Sources
I
REF
=
V
REF
R
SET
Ref
Control
Amp
R
SET
External
Reference
I
REF
=
V
REF
R
SET
DAC2902
+1.25V Ref.
FSA
+5V
+V
A
REF
IN
Current
Sources
Ref
Control
Amp