CN-0205 Datasheet, PDF (5/9 Pages) Analog Devices – Interfacing the ADL5375 I/Q Modulator to the AD9122 Dual Channel, 1.2 GSPS High Speed DAC

Circuit Note
CN-0205
Figure 11. Spreadsheet to Calculate Modulator Output Power
Calculating the Output Power of the AD9122 and the
ADL5375
In addition to the bias-setting and signal scaling resistors, the
power level at the output of the ADL5375 is a function of the
DAC’s digital backoff level (dBFS), the signal’s peak-to-average
ratio, the DAC’s full-scale current, the insertion loss of the
Nyquist filter, and I/Q modulator’s voltage gain. The
spreadsheet shown in Figure 11 can be used to make this
calculation.
This spreadsheet can be downloaded using the following URL:
www.analog.com/CN0205-PowerCalculator.
Level Shifting to Drive the ADL5375-15
The ADL5375-15 requires a dc bias level of 1500 mV. Other
than the difference in bias levels, the ADL5375-05 and
ADL5375-15 are identical. To drive the ADL5375-15 from the
AD9122, either a passive or active level-shifting network must
be used. The passive level shifting network shown in Figure 12
uses four series resistors along with four pull-up resistors to
achieve a bias level of 1500 mV at the ADL5375-15 input. This
passive level shifting network introduces a loss of
approximately 2 dB in the signal level.
An active level shifting circuit would use a dual-differential
amplifier, such as ADA4938, where placing 1500 mV on the
VOCM pin sets the output dc bias level. In this approach,
however, the interface bandwidth is limited by the op amp.
AD9122
OUT1_P
OUT1_N
67
RBIP
45.3Ω
RBIN
66 ^45.3Ω
RSIN
1kΩ
RSIP
1kΩ
RLIP
3480Ω
RLIN
3480Ω
ADL5375-15
21
IBBP
5V
22
IBBN
OUT2_N
OUT2_P
59
RBQN
45.3Ω
RBQP
58 45.3Ω
RSQN
1kΩ
RSQP
1kΩ
RLQN
3480Ω
RLQP
3480Ω
9
QBBN
5V
10
QBBP
Figure 12. Passive Level-Shifting Network For Biasing ADL5375-15 from the
AD9122 TxDAC
As previously mentioned, it is necessary to put a filter between
AD9122 and ADL5375-15. The LC filter can be located
anywhere between the DAC termination resistors (R1 in Figure 13)
and the ac swing-limiting resistor (R4 in Figure 13). However,
the circuit in Figure 13 allows flexibility in the design of the
level shifting circuit with low loss by R2 and a high driving
level to modulator. It also allows a matched filter at source and
load impedance. Figure 13 is the recommended passive level-
shifting network with filter.
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