ADRF6720ACPZ-R7 datasheet(25/44 Pages) AD

25 / 44 page

Data Sheet

ADRF6720

APPLICATIONS INFORMATION

DAC TO I/Q MODULATOR INTERFACING

The ADRF6720 is designed to interface with minimal

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

TxDAC® converters. These dual-channel differential current

output DACs provide an output 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 an interface using the AD9142A TxDAC is shown

in Figure 45. The baseband inputs of the ADRF6720 require a dc

bias of 500 mV. The nominal midscale output current on each of

the outputs of the AD9142A is 10 mA. Therefore, an average

current of 10 mA flowing through a single 50 Ω resistor to

ground from each of the DAC outputs produces the desired

500 mV dc bias for the inputs to the ADRF6720. Place a shunt

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

100 Ω impedance of the DAC. The external resistor reduces the

voltage swing for a given DAC output current. The AD9142A

output currents have a swing ranging from 0 mA to 20 mA.

With the 50 Ω termination resistors to ground in the DAC

outputs and the 125 Ω shunt resistors in place, the resulting

drive signal from each differential pair is 1 V p-p differential

(with the DAC running at 0 dBFS) with a 500 mV dc bias.

Figure 45. Interface Between the AD9142A and ADRF6720 with 50 Ω

Resistors to Ground to Establish the 500 mV DC Bias for the ADRF6720

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 and signal-to-noise ratio (SNR) at the

cost of higher intermodulation distortion.

When setting the size of resistor to adjust swing level, take the input

impedance of the I and Q inputs into account. The I and Q inputs

have a differential input resistance of 500 Ω. As a result, the

effective value of the resistance is 500 Ω in parallel with the

chosen shunt resistor. For example, if a 100 Ω resistance is desired

(based on Figure 45), the value of RLI or RLQ must be set such that

100 Ω = (500 × RLI)/(500 + RLI)

100 Ω = (500 × RLQ)/(500 + RLQ)

resulting in a value for RLI and RLQ of 125 Ω.

Figure 47 shows the differential input resistance and

capacitance over baseband input frequencies.

Figure 46. Relationship Between the Effective 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 500 Ω differential I and Q input impedance

in parallel for the filter.

BASEBAND BANDWIDTH

The ADRF6720 can be used with a DAC generating a complex

IF (CIF), as well as a zero IF signal (ZIF). The 1 dB bandwidth

of the ADRF6720 is more than 1000 MHz. Figure 48 shows the

I–

I+

AD9142A

ADRF6720

IOUT1N

IOUT1P

IOUT2P

IOUT2N

Q+

Q–

RBI+

Ω

RBI–

Ω

RBQ+

Ω

RBQ–

Ω

RLI

125

Ω

RLQ

125

Ω

500

Ω

500

Ω

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

100

10k

EFFECTIVE AC SWING LIMITING RESISTANCE (

Ω)

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

100

150

200

250

300

350

400

450

500

100

200

300

400

500

600

700

800

900

1000

FREQUENCY (MHz)

RESISTANCE

CAPACITANCE

Rev. 0 | Page 25 of 44

ADRF6720ACPZ-R7 数据表(PDF) 25 Page - Analog Devices

ADRF6720ACPZ-R7 数据表(HTML) 25 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

数据表下载