AD9148BBCZ datasheet(61/73 Pages) AD

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Preliminary Technical Data

AD9148

Rev. PrA | Page 61 of 73

In addition, the P or N output can act as a current source or a

current sink. When sourcing current, the output compliance

voltage is 0 V to 1.6 V. When sinking current, the output compliance

voltage is 0.8 V to 1.6 V. The auxiliary DAC current direction is

programmable via Bit 6, Register 0x33 and Bit 6, Register 0x37

(DAC SPI select is 0 to control AUX1 and AUX2, and DAC SPI

select is 1 to control AUX3 and AUX4). The choice of sinking or

sourcing should be made at circuit design time. There is no

advantage to switching between sourcing and sinking current

after the circuit is in place.

These auxiliary DACs can be used for local oscillator (LO)

cancellation when the DAC output is followed by a quadrature

modulator. More information and example application circuits

are given in the Interfacing to Modulators section.

INTERFACING TO MODULATORS

The AD9148 interfaces to the ADL537x family of with a minimal

number of components. An example of the recommended

interface circuitry is shown in Figure 79.

RBIP

50Ω

RBIN

50Ω

IBBN

IBBP

AD9148

ADL537x

RBQN

50Ω

RBQP

50Ω

RLI

100Ω

RLQ

100Ω

IOUT1_N

IOUT1_P

IOUT2_P

IOUT2_N

QBBP

QBBN

Figure 79. Typical Interface Circuitry Between the AD9148 and 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 (1/2 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 by the

addition of the load resistor in parallel with the modulator

inputs (RLI, RLQ). The peak-to-peak voltage swing of the

transmitted signal is

[

]

[]

SIGNAL

Baseband Filter Implementation

Most applications require a baseband anti-imaging filter between

the DAC and modulator to filter out Nyquist images and broadband

DAC noise. The filter can be inserted between the I-to-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.

Figure 81 shows a fifth-order low-pass filter. A common-mode

choke is used between the I-to-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 would appear as unwanted

spurious signals in the output spectrum. The common-mode

choke or balun may not be needed if the layout between the

DAC and IQ modulator is optimized and balanced. Splitting the

second 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 will pass common-mode signals.

Driving the ADL5375-15 with the AD9148

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 80. In this

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

input to the ADL5375-15 is 1500 mV. Note that this passive

level shifting network introduces approximately 2 dB of loss in

the ac signal.

IBBN

IBBP

AD9148

ADL5375-15

RBIP

45.3Ω

RBIN

45.3Ω

RBQN

45.3Ω

RBQP

45.3Ω

RLIP

3480Ω

RLIN

3480Ω

RLQN

3480Ω

RLQP

3480Ω

IOUT1_N

IOUT1_P

IOUT2_P

IOUT2_N

QBBP

QBBN

RSIN

1kΩ

RSIP

1kΩ

RSQN

1kΩ

RSQP

1kΩ

Figure 80. Passive Level Shifting Network for Biasing ADL5375-15 from AD9148

IDAC

QDAC

50Ω

MABACT0043

(OPTIONAL)

33nH

2pF

56nH

100Ω

6pF

3pF

22pF

ADL537x

Figure 81. DAC Modulator Interface with Fifth-Order, Low Pass Filter

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