DAC250Y BURR-BROWN [Burr-Brown Corporation], DAC250Y Datasheet - Page 12

• Manufacturer Part Number: DAC250Y
• Description: Dual, 12-Bit, 125MSPS DIGITAL-TO-ANALOG CONVERTER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

FIGURE 7. Generic Interface to a Quadrature Modulator. Signal Conditioning (Level-Shifting) May Be Required to Ensure
The DC gain for this circuit is equal to feedback resistor R
At high frequencies, the DAC output impedance (C
will produce a 0 in the noise gain for the OPA2680 that may
cause peaking in the closed-loop frequency response. C
added across R
To achieve a flat transimpedance frequency response, the
pole in each feedback network should be set to:
which will give a corner frequency f
The full-scale output voltage is simply defined by the prod-
uct of I
improve on the ac performance of this circuit, adjustment of
R
this application example may include adding a differential
filter at the OPA2680’s output followed by a transformer, in
order to convert to a single-ended signal.
SINGLE-ENDED CONFIGURATION
Using a single-load resistor connected to the one of the DAC
outputs, a simple current-to-voltage conversion can be ac-
complished. The circuit in Figure 6 shows a 50
connected to I
connected 50
current of 20mA, the DAC produces a total signal swing of
0V to 0.5V into the 25
12
F
and/or I
with GBP = Gain Bandwidth Product of OPA,
OUTFS
DAC2902
OUTFS
Correct DC Common-Mode Levels At the Input of the Quadrature Modulator.
• R
OUT
F
F
to compensate for this noise gain peaking.
should be considered. Further extensions of
cable. Therefore, with a nominal output
, and has a negative unipolar excursion. To
, providing the termination of the further
2 R
I
I
I
I
OUT
OUT
OUT
OUT
V
f
OUT
1
1
2
2
1
3dB
F
~ 0Vp to 1.20Vp
C
load.
F
2 R
4 R
GBP
F
GBP
C
Conditioning
F
D
C
-3dB
Signal
D
of approximately:
V
IN
~ 0.6Vp to 1.8Vp
Q
D1
I
resistor
REF
Q
REF
I
IN
IN
, C
F
(8)
D2
(9)
Quadrature Modulator
F
is
)
.
FIGURE 6. Driving a Doubly Terminated 50 Cable Directly.
Different load resistor values may be selected as long as the
output compliance range is not exceeded. Additionally, the
output current, I
ally adjusted to provide the desired output signal swing and
performance.
INTERFACING ANALOG
QUADRATURE MODULATORS
One of the main applications for the dual-channel DAC is
baseband I- and Q-channel transmission for digital commu-
nications. In this application, the DAC is followed by an
analog quadrature modulator, modulating an IF carrier with
the baseband data, as shown in Figure 7. Often, the input
stages of these quadrate modulators consist of npn-type
transistors that require a DC bias (base) voltage of > 0.8V.
The wide output compliance range (–10V to +1.25V) allows
for a direct DC–coupling between the DAC2902 and the
quadrature modulator.
DAC2902
I
IN
RF
I
I
OUT
OUT
OUTFS
I
OUTFS
, and the load resistor, may be mutu-
25
= 20mA
50
V
OUT
DAC2902
= 0V to +0.5V
50
SBAS167A
I
REF