AD9122 datasheet(46/56 Pages) AD

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AD9122

Rev. 0 | Page 46 of 56

Figure 73 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.

The control registers for controlling the I and Q auxiliary DACs

are in Register 0x42, Register 0x43, and Register 0x46.

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 71.

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 (½ 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

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, Inc, modulators. It is necessary to level shift

the DAC output from a 500 mV dc bias to the 1500 mV dc bias

that the ADL5375-15 requires. Level shifting can be achieved

with a purely passive network, as shown in Figure 72. In this

network, the dc bias of the DAC remains at 500 mV while the

input to the ADL5375-15 is 1500 mV. This passive, level shifting

network introduces approximately 2 dB of loss in the ac signal.

[]

SIGNAL

RBIP

Ω

RBIN

Ω

IBBN

IBBP

AD9122

ADL537x

RBQN

Ω

RBQP

Ω

RLI

100

Ω

RLQ

100

Ω

IOUT1N

IOUT1P

IOUT2P

IOUT2N

QBBP

QBBN

IBBN

IBBP

AD9122

ADL5375-15

RBIP

45.3

Ω

RBIN

45.3

Ω

RBQN

45.3

Ω

RBQP

45.3

Ω

RLIP

3480

Ω

RLIN

3480

Ω

RLQN

3480

Ω

RLQP

3480

Ω

IOUT1N

IOUT1P

IOUT2P

IOUT2N

QBBP

QBBN

RSIN

Ω

RSIP

Ω

RSQN

Ω

RSQP

Ω

Figure 71. Typical Interface Circuitry Between the AD9122 and the ADL537x

Family of Modulators

BASEBAND FILTER IMPLEMENTATION

Figure 72. Passive, Level Shifting Network for Biasing ADL5375-15

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. Doing this establishes the input and

output impedances for the filter.

AD9122

Ω

33nH

2pF

56nH

140

Ω

6pF

3pF

22pF

ADL537x

Figure 73. DAC Modulator Interface with Fifth-Order, Low Pass Filter

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