Datasheet

R
SET
DAC2904
+1.25VRef.
FSA
+5V
+V
A
REF
IN
I =
REF
V
REF
R
SET
Current
Sources
Ref
Control
Amp
External
Reference
DAC2904
www.ti.com .............................................................................................................................................. SBAS198C –AUGUST 2001–REVISED OCTOBER 2009
GAIN SETTING OPTIONS
The full-scale output current on the DAC2904 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.
Figure 28. External Reference Configuration
Alternatively, bringing the GSET pin high (that is,
connected to +V
A
), the DAC2904 will switch into the
GROUNDING, DECOUPLING AND LAYOUT
simultaneous gain set mode. Now the full-scale
INFORMATION
output current of both DAC channels is determined by
only one external R
SET
resistor connected to the
Proper grounding and bypassing, short lead lengths,
FSA1 pin. The resistor at the FSA2 pin may be
and the use of ground planes are particularly
removed; however, this is not required because this
important for high-frequency designs. Multilayer
pin is not functional in this mode and the resistor has
printed circuit boards (PCBs) are recommended for
no effect on the gain equation. The formula for
best performance because they offer distinct
deriving the correct R
SET
remains unchanged; for
advantages such as minimization of ground
example, R
SET
= 2kΩ will result in a 20mA output for
impedance, separation of signal layers by ground
both DACs.
layers, etc.
The DAC2904 uses separate pins for its analog and
EXTERNAL REFERENCE OPERATION
digital supply and ground connections. The
The internal reference can be disabled by simply
placement of the decoupling capacitor should be such
applying an external reference voltage into the REF
IN
that the analog supply (+V
A
) is bypassed to the
pin, which in this case functions as an input, as
analog ground (AGND), and the digital supply
shown in Figure 28. The use of an external reference
bypassed to the digital ground (DGND). In most
may be considered for applications that require higher
cases 0.1μF ceramic chip capacitors at each supply
accuracy and drift performance, or to add the ability
pin are adequate to provide a low impedance
of dynamic gain control.
decoupling path. Keep in mind that the effectiveness
of these capacitors largely depends on the proximity
While a 0.1μF capacitor is recommended to be used
to the individual supply and ground pins. Therefore,
with the internal reference, it is optional for the
they should be located as close as physically
external reference operation. The reference input,
possible to those device leads. Whenever possible,
REF
IN
, has a high input impedance (1MΩ) and can
the capacitors should be located immediately under
easily be driven by various sources. Note that the
each pair of supply/ground pins on the reverse side of
voltage range of the external reference should stay
the PCB. This layout approach will minimize the
within the compliance range of the reference input.
parasitic inductance of component leads and PCB
runs.
POWER-DOWN MODE
Further supply decoupling with surface-mount
The DAC2904 features a power-down function which
tantalum capacitors (1μF to 4.7μF) may be added as
can be used to reduce the total supply current to less
needed in proximity of the converter.
than 6mA over the specified supply range of 3.0V to
5.5V. Applying a logic high to the PD pin will initiate
Low noise is required for all supply and ground
the power-down mode, while a logic low enables
connections to the DAC2904. It is recommended to
normal operation. When left unconnected, an internal
use a multilayer PCB utilizing separate power and
active pulldown circuit will enable the normal
ground planes. Mixed signal designs require
operation of the converter.
particular attention to the routing of the different
supply currents and signal traces. Generally, analog
supply and ground planes should only extend into
analog signal areas, such as the DAC output signal
and the reference signal. Digital supply and ground
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