ADRF6720-27ACPZ-R7 datasheet(26/44 Pages) AD

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Data Sheet

ADRF6720-27

Rev. B | Page 25 of 43

APPLICATIONS INFORMATION

DAC TO I/Q MODULATOR INTERFACING

The ADRF6720-27 is designed to interface with minimal

components to members of the Analog Devices, Inc., family of

TxDAC® converters. These dual- or quad-channel differential

current sinking DACs provide a current swing from 0 mA to

20 mA. The interface described in this section can be used with

any DAC that has a similar output.

An example of interfacing the dual-channel differential current

sinking TxDAC is shown in Figure 45. The baseband inputs of the

ADRF6720-27 require a dc bias of 2.68 V and the TxDAC outputs

interface seamlessly with ADRF6720-27. Place a shunt 100 Ω

external resistor across the I and Q inputs to match the 100 Ω

impedance of the DAC. With the 50 Ω termination resistors to

the power supply in the DAC outputs , the 100 Ω shunt resistors

across the I and Q inputs with a 20 mA full-scale current and

bleed current , the resulting drive signal from each differential

pair is 1 V p-p differential (with the DAC running at 0 dBFS),

with a 2.68 V dc bias.

Figure 45. Interface Between the TxDAC and ADRF6720-27 with 50 Ω

Resistors to Ground to Establish the 2.68 V DC Bias for the ADRF6720-27

Baseband Inputs

Adjust the voltage swing for a given DAC output current by

placing a different resistance value on RLI and RLQ to the interface

(see Figure 45). This adjustment has the effect of varying the ac

swing without changing the dc bias already established by the

50 Ω resistors. A higher resistance value increases the output

power of the ADRF6720-27 and signal-to-noise ratio (SNR) at

the cost of higher intermodulation distortion.

Figure 46 shows the relationship between the swing-limiting

resistor and the peak-to-peak ac swing that it produces when

50 Ω bias-setting resistors are used. The differential peak-to-

peak swing at the modulator input is

[

]

[

]

SIGNAL

Figure 47 shows the differential input resistance and

capacitance over baseband input frequencies.

Figure 46. Relationship Between the AC Swing Limiting Resistance and the

Peak-to-Peak Voltage Swing with 50 Ω Bias Setting Resistors

Figure 47. Differential Baseband Input Resistance and Input Capacitance

Equivalents (Shunt R, Shunt C)

I/Q Filtering

An antialiasing filter between the DAC and modulator is

necessary to filter out Nyquist images, common-mode noise,

and broadband DAC noise. The interface for setting up the

biasing and ac swing described in the DAC to I/Q Modulator

Interfacing section lends itself well to the introduction of such a

filter. The filter can be inserted between the dc bias setting

resistors and the ac swing limiting resistor. With this configuration,

the dc bias setting resistors set the source impedance, and the

ac swing limiting resistor sets the load impedance with a high

differential I and Q input impedance in parallel for the filter.

3.3V

I–

I+

TxDAC

ADRF6720-27

3.3V

IOUT0–

IOUT0+

IOUT1+

IOUT1–

Q+

Q–

RBI+

Ω

RBI–

Ω

RBQ+

Ω

RBQ–

Ω

RLI

100

Ω

RLQ

100

Ω

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

100

10k

AC SWING LIMITING RESISTANCE (

Ω)

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

100

200

300

400

500

600

700

800

900

1000

FREQUENCY (MHz)

CAPACITANCE

RESISTANCE

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