AD9142BCPZ datasheet(39/65 Pages) AD

39 / 65 page

Data Sheet

AD9142

Rev. 0 | Page 39 of 64

Figure 56 shows the most basic DAC output circuitry. A pair of

resistors, RO, convert each of the complementary output currents to

a differential voltage output, VOUT. Because the current outputs

of the DAC are high impedance, the differential driving point

impedance of the DAC outputs, ROUT, is equal to 2 × RO. See

Figure 57 for the output voltage waveforms.

Figure 56. Basic Transmit DAC Output Circuit

Figure 57. Output Voltage Waveforms

The common-mode signal voltage, VCM, is calculated as

The peak output voltage, VPEAK, is calculated as

VPEAK = IFS × RO

In this circuit configuration, the single-ended peak voltage is

the same as the peak differential output voltage.

INTERFACING TO MODULATORS

The AD9142 interfaces to the ADL537x family of modulators

with a minimal number of components. An example of the

recommended interface circuitry is shown in Figure 58.

Figure 58. Typical Interface Circuitry Between the AD9142 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 addition of the load resistor in

parallel with the modulator inputs reduces the signal level. 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 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 configuration establishes the

input and output impedances for the filter.

Figure 59 shows a fifth-order, low-pass filter. A common-mode

choke is placed between the I-V resistors and the remainder of

the filter to remove the common-mode signal produced by the

DAC and to prevent 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, which provides additional

common-mode rejection of high frequency signals. A purely

differential filter can pass common-mode signals.

For more details about interfacing the AD9142 DAC to an IQ

modulator, refer to the Circuits from the Lab CN-0205, Interfacing

the ADL5375 I/Q Modulator to the AD9122 Dual Channel,

1.2 GSPS High Speed DAC on the Analog Devices website.

Figure 59. DAC Modulator Interface with Fifth-Order, Low-Pass Filter

VIP +

VIN –

VOUTI

IOUT1P

IOUT1N

VQP +

VQN –

VOUTQ

IOUT2P

IOUT2N

+VPEAK

VCM

–VPEAK

VOUT

RBIP

50Ω

RBIN

50Ω

IBBN

IBBP

AD9142

ADL537x

RBQN

50Ω

RBQP

50Ω

RLI

100Ω

RLQ

100Ω

IOUT1N

IOUT1P

IOUT2P

IOUT2N

QBBP

QBBN

AD9142

50Ω

33nH

3.6pF

33nH

140Ω

6pF

3pF

22pF

ADL537x

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