DAC2900 Datasheet, PDF (13/21 Pages) Burr-Brown (TI) – Dual, 10-Bit, 125MSPS DIGITAL-TO-ANALOG CONVERTER

GAIN SETTING OPTIONS
The full-scale output current on the DAC2900 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 com-
pensation of possible gain mismatches elsewhere within the
transmit signal path.
Alternatively, bringing the GSET pin HIGH (that is, con-
nected to +V_A), the DAC2900 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, while any present resistor
at the FSA2 pin must be removed. 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 8. 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 (0.1V to 1.25V).
POWER-DOWN MODE
The DAC2900 features a power-down function which can
be used to reduce the total supply current to less than 6mA
over the specified supply range of 3.0V to 5.5V. 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.
External
Reference
I_REF
=
V_REF
R_SET
DAC2900
FSA
REF_IN
Ref
Control
Amp
R_SET
+1.24V Ref.
+5V
+V_A
Current
Sources
FIGURE 8. External Reference Configuration.
DAC2900
13
SBAS166C
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