Datasheet

AD9122
Rev. B | Page 48 of 60
The control registers for the I and Q auxiliary DACs are
Register 0x42, Register 0x43, Register 0x46, and Register 0x47.
INTERFACING TO MODULATORS
The AD9122 interfaces to the ADL537x family of modulators
with a minimal number of components. An example of the
recommended interface circuitry is shown in Figure 73.
RBIP
50Ω
RBIN
50Ω
67
66
IBBN
IBBP
AD9122 ADL537x
RBQN
50Ω
RBQP
50Ω
59
58
RLI
100Ω
RLQ
100Ω
IOUT1N
IOUT1P
IOUT2P
IOUT2N
QBBP
QBBN
08281-041
Figure 73. Typical Interface Circuitry Between the AD9122 and the ADL537x
Family of Modulators
The baseband inputs of the ADL537x family require a dc bias
of 500 mV. The nominal midscale output current on each output
of the DAC is 10 mA (one-half the full-scale current). Therefore,
a single 50 Ω resistor to ground from each of the DAC outputs
results in the desired 500 mV dc common-mode bias for the
inputs to the ADL537x. The signal level can be reduced through
the addition of the load resistor in parallel with the modulator
inputs. The peak-to-peak voltage swing of the transmitted signal is
)2(
)2(
LB
LB
FSSIGNAL
RR
RR
IV
+×
××
×=
BASEBAND FILTER IMPLEMENTATION
Most applications require a baseband anti-imaging filter between
the DAC and the modulator to filter out Nyquist images and
broadband DAC noise. The filter can be inserted between the
I-V resistors at the DAC output and the signal level setting
resistor across the modulator input. This establishes the input
and output impedances for the filter.
Figure 75 shows a fifth-order, low-pass filter. A common-mode
choke is used between the I-V resistors and the remainder of
the filter. This removes the common-mode signal produced by
the DAC and prevents the common-mode signal from being
converted to a differential signal, which can appear as unwanted
spurious signals in the output spectrum. Splitting the first filter
capacitor into two and grounding the center point creates a
common-mode low-pass filter, providing additional common-
mode rejection of high frequency signals. A purely differential
filter can pass common-mode signals.
DRIVING THE ADL5375-15
The ADL5375-15 requires a 1500 mV dc bias and, therefore,
requires a slightly more complex interface than most other
Analog Devices modulators. It is necessary to level-shift the
DAC output from a 500 mV dc bias to the 1500 mV dc bias
required by the ADL5375-15. Level-shifting can be achieved
with a purely passive network, as shown in Figure 74. In this
network, the dc bias of the DAC remains at 500 mV, whereas
the input to the ADL5375-15 is 1500 mV. This passive, level-
shifting network introduces approximately 2 dB of loss in the
ac signal.
67
66
IBBN
IBBP
AD9122 ADL5375-15
59
58
21
22
9
10
RBIP
45.3Ω
RBIN
45.3Ω
RBQN
45.3Ω
RBQP
45.3Ω
RLIP
3480Ω
RLIN
3480Ω
RLQN
3480Ω
RLQP
3480Ω
IOUT1N
IOUT1P
IOUT2P
IOUT2N
QBBP
QBBN
RSIP
1kΩ
RSIN
1kΩ
RSQN
1kΩ
RSQP
1kΩ
5V
5V
08281-043
Figure 74. Passive, Level-Shifting Network for Biasing the ADL5375-15
AD9122
50Ω
50Ω
33nH
33nH
2pF
56nH
56nH
140Ω6pF
3pF
3pF
22pF
22pF
ADL537x
08281-042
Figure 75. DAC Modulator Interface with Fifth-Order, Low-Pass Filter