Datasheet


SBAS279D − AUGUST 2003 − REVISED JULY 2005
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20
Figure 35 shows an example of a dc-coupled interface
with dc level-shifting, using a precision resistor network.
An ac-coupled interface, as shown in Figure 36, has the
advantage in that the common-mode levels at the input of
the modulator can be set independently of those at the
output of the DAC. Furthermore, no voltage loss occurs in
this setup.
I
OUT
1
I
OUT
1
DAC2932
I
OUT
1
I
OUT
1
V
OUT
1
V
DC
R
3
R
4
R
5
V
OUT
1
Figure 35. DC-Coupled Interface to a Quadrature
Modulator Applying Level Shifting
I
OUT
1
I
OUT
1
DAC2932
I
OUT
1
I
OUT
1
V
OUT
1
0.01
μ
F
0.01
μ
F
V
DC
R
1
R
2
V
OUT
1
R
LOAD
250
Ω
R
LOAD
250
Ω
Figure 36. AC-Coupled Interface to a Quadrature
Modulator Applying Level Shifting
INTERNAL REFERENCE OPERATION
The DAC2932 has an on-chip reference circuit that
comprises a 1.22V bandgap reference and two control
amplifiers, one for each DAC. The full-scale output current,
I
OUTFS
, of the DAC2932 is determined by the reference
voltage, V
REF
, and the value of resistor R
SET
. I
OUTFS
can
be calculated by:
I
OUTFS
+ 32 I
REF
+ 32
V
REF
R
SET
The external resistor R
SET
connects to the FSA pin
(full-scale adjust) as shown in Figure 37. 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,
as shown in Equation 10. The full-scale
output current, I
OUTFS
, results from multiplying I
REF
by a
fixed factor of 32.
DAC2932
+1.22V Ref.
R
SET
19.6k
Ω
0.1
μ
F
FSA
+3V
+V
A
REF
IN
Current
Sources
I
REF
=
V
REF
R
SET
Ref
Control
Amp
Figure 37. Internal Reference Configuration
Using the internal reference, a 19.6kΩ resistor value
results in a full-scale output of approximately 2mA.
Resistors with a tolerance of 1% or better should be
considered. Selecting higher values, the output current
can be adjusted from 2mA down to 0.5mA. Operating the
DAC2932 at lower than 2mA output currents may be
desirable for reasons of reducing the total power
consumption 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 0.1MHz.
GAIN SETTING OPTIONS
The full-scale output current on the DAC2932 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, GSET (pin 19) must be high
(that is, connected to +V
A
). In this mode, two external
resistors are required—one R
SET
connected to the FSA1
pin (pin 24) and the other to the FSA2 pin (pin 23). 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.
(10)