adl5386 Analog Devices, Inc., adl5386 Datasheet - Page 26

• Manufacturer Part Number: adl5386
• Description: 50 Mhz To 2200 Mhz Quadrature Modulator With Integrated Detector And Vva
• Manufacturer: Analog Devices, Inc.

ADL5386
Driving the ADL5386 with an Analog Devices TxDAC®
An example of the interface using the
in Figure 46. The baseband inputs of the ADL5386 require a dc
bias of 500 mV. The average output current on each of the outputs
of the AD9788 is 10 mA. Therefore, a single 50 Ω resistor to
ground from each of the DAC outputs results in an average current
of 10 mA flowing through each of the resistors, thus producing
the desired 500 mV dc bias for the inputs to the ADL5386.
The AD9788 output currents source from 0 mA to 20 mA. With
the 50 Ω resistors in place, the ac voltage swing going into the
ADL5386 baseband inputs ranges from 0 V to 1 V. A full-scale
sine wave out of the AD9788 can be described as a 1 V p-p single-
ended (or 2 V p-p differential) sine wave with a 500 mV dc bias.
The AD9788 also has the capability of easily compensating for
gain, offset, and phase mismatch in the IQ signal path; therefore,
optimizing performance of the ADL5386.
Limiting the AC Swing
There are situations in which it is desirable to reduce the ac voltage
swing for a given DAC output current. To reduce the ac voltage
swing, add an additional resistor to the interface. This resistor is
placed in shunt between each side of the differential pair, as shown
in Figure 47. It has the effect of reducing the ac swing without
changing the dc bias already established by the 50 Ω resistors.
Ground to Establish the 500 mV DC Bias for the ADL5386 Baseband Inputs
Figure 46. Interface Between AD9788 and ADL5386 with 50 Ω Resistors to
TxDAC
OUT1_P
OUT1_N
OUT2_N
OUT2_P
RBQN
RBQP
RBIP
RBIN
50Ω
50Ω
50Ω
50Ω
CHANNEL
CHANNEL
Figure 49. 39 MHz, 5-Pole Chebychev Filter with In-Band Ripple of 0.1 dB for a 155 MSPS, 128 QAM Transmitter
AD9788
AD9788
Q
I
1/2
1/2
AD9788
50Ω
50Ω
50Ω
50Ω
50Ω LINE
50Ω LINE
50Ω LINE
50Ω LINE
67.5pF
67.5pF
67.5pF
67.5pF
TxDAC is shown
29
30
25
26
ADL5386
QBBN
QBBP
IBBN
IBBP
317.4nH
317.4nH
317.4nH
317.4nH
Rev. 0 | Page 26 of 36
156.9pF
156.9pF
156.9pF
156.9pF
372.5nH
372.5nH
372.5nH
372.5nH
The value of this ac voltage swing-limiting resistor is chosen based
on the desired ac voltage swing. Figure 48 shows the relationship
between the swing-limiting resistor and the peak-to-peak ac
swing that it produces when 50 Ω bias-setting resistors are used.
Filtering
When driving a modulator from a DAC, it is necessary to introduce
a low-pass filter between the DAC and the modulator to reduce
the DAC images. The interface for setting up the biasing and ac
swing lends itself well to the introduction of such a filter. The
filter can be inserted between the dc bias setting resistors and the ac
swing-limiting resistor, thus establishing the input and output
impedances for the filter. A filter example is shown in Figure 49.
124.7pF
124.7pF
124.7pF
124.7pF
TxDAC
Figure 47. AC Voltage Swing Reduction Through Introduction of Shunt
100Ω LINE
100Ω LINE
100Ω LINE
100Ω LINE
OUT1_P
OUT1_N
OUT2_N
OUT2_P
Figure 48. Relationship Between AC Swing-Limiting Resistor and
Peak-to-Peak Voltage Swing with 50 Ω Bias-Setting Resistors
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
10
0Ω
0Ω
0Ω
0Ω
RBQN
RBQP
RBIP
RBIN
50Ω
50Ω
50Ω
50Ω
Resistor Between Differential Pair
200Ω
200Ω
100
IBBP
IBBN
QBBP
QBBN
ADL5386
R
L
(Ω)
RSLQ
100Ω
100Ω
RSLI
1000
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ADL5386
QBBN
QBBP
IBBP
IBBN
10000