Datasheet

DAC2902
13
SBAS167C
www.ti.com
Figure 8 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 9, has the advantage
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 is obtained in this setup.
INTERNAL REFERENCE OPERATION
The DAC2902 has an on-chip reference circuit that consists
of a 1.25V bandgap reference and two control amplifiers,
one for each DAC. The full-scale output current, I
OUTFS
, of
the DAC2902 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
(10)
The external resistor R
SET
connects to the FSA pin (Full-
Scale Adjust), see Figure 10. 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.
I
OUT
1
I
OUT
1
DAC2902
I
OUT
1
I
OUT
1
V
OUT
1
V
DC
R
3
R
4
R
5
V
OUT
1
FIGURE 8. DC-Coupled Interface to Quadrature Modulator
Applying Level Shifting.
FIGURE 9. AC-Coupled Interface to Quadrature Modulator Applying Level Shifting.
I
OUT
1
I
OUT
1
DAC2902
I
OUT
1
I
OUT
1
V
OUT
1
0.01µF
0.01µF
V
DC
R
1
R
2
V
OUT
1
R
LOAD
50Ω 50Ω