Datasheet

1
2 R Cp
F F
=
GBP
4 R Cp
F D
GBP
2 R Cp
F D
f =
-3dB
I 1
OUT
DAC2904
I 1
OUT
V 1
OUT
V
DC
R
3
R
4
R
5
I
OUT
1
I
OUT
1
V
OUT
1
I
OUT
DAC2904
25W
50W
50W
I =
OUTFS
20mA
V =0Vto+0.5V
OUT
I
OUT
DAC2904
www.ti.com .............................................................................................................................................. SBAS198C –AUGUST 2001–REVISED OCTOBER 2009
C
F
is added across R
F
to compensate for this noise INTERFACING ANALOG QUADRATURE
gain peaking. To achieve a flat transimpedance MODULATORS
frequency response, the pole in each feedback
One of the main applications for the dual-channel
network should be set to:
DAC is baseband I- and Q-channel transmission for
digital communications. In this application, the DAC is
followed by an analog quadrature modulator,
(8)
modulating an IF carrier with the baseband data, as
with GBP = Gain Bandwidth Product of the OPA
shown in Figure 25. Often, the input stages of these
quadrate modulators consist of npn-type transistors
which will give a corner frequency f
–3dB
of
that require a dc bias (base) voltage greater than
approximately:
0.8V. The wide output compliance range (–10V to
+1.25V) allows for a direct dc-coupling between the
DAC2904 and the quadrature modulator.
(9)
Figure 24 shows an example of a dc-coupled
The full-scale output voltage is simply defined by the
interface with dc level-shifting, using a precision
product of I
OUTFS
× R
F
, and has a negative unipolar
resistor network. An ac-coupled interface (see
excursion. To improve on the ac performance of this
Figure 26) has the advantage that the common-mode
circuit, adjustment of R
F
and/or I
OUTFS
should be
levels at the input of the modulator can be set
considered. Further extensions of this application
independently of those at the output of the DAC.
example may include adding a differential filter at the
Furthermore, no voltage loss is obtained in this setup.
OPA2690 output followed by a transformer, in order
to convert to a single-ended signal.
SINGLE-ENDED CONFIGURATION
Using a single load resistor connected to one of the
DAC outputs, a simple current-to-voltage conversion
can be accomplished. The circuit in Figure 23 shows
a 50Ω resistor connected to I
OUT
, providing the
termination of the further connected 50Ω cable.
Therefore, with a nominal output current of 20mA, the
DAC produces a total signal swing of 0V to 0.5V into
the 25Ω load.
Figure 24. DC-Coupled Interface to Quadrature
Modulator Applying Level Shifting
Figure 23. Driving a Doubly-Terminated 50Ω
Cable Directly
Different load resistor values may be selected as long
as the output compliance range is not exceeded.
Additionally, the output current, I
OUTFS
, and the load
resistor may be mutually adjusted to provide the
desired output signal swing and performance.
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