ADRF6702ACPZ-R7 datasheet(18/36 Pages) AD

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ADRF6702

Data Sheet

Rev. B | Page 18 of 36

DAC-TO-IQ MODULATOR INTERFACING

The ADRF6702 is designed to interface with minimal components

to members of the Analog Devices, Inc., family of TxDACs®. 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 AD9122 TxDAC is shown

in Figure 36. The baseband inputs of the ADRF6702 require

a dc bias of 500 mV. The average output current on each of the

outputs of the AD9122 is 10 mA. Therefore, a single 50 Ω resis-

tor to ground from each of the DAC outputs results in an average

current of 10 mA flowing through each of the resistors, thus

producing the desired 500 mV dc bias for the inputs to the

ADRF6702.

RBIP

50Ω

RBIN

50Ω

OUT1_N

OUT1_P

AD9122

ADRF6702

RBQN

50Ω

RBQP

50Ω

OUT2_P

OUT2_N

Figure 36. Interface Between the AD9122 and ADRF6702 with 50 Ω Resistors

to Ground to Establish the 500 mV DC Bias for the ADRF6702 Baseband Inputs

The AD9122 output currents have a swing that ranges from

0 mA to 20 mA. With the 50 Ω resistors in place, the ac voltage

swing going into the ADRF6702 baseband inputs ranges from

0 V to 1 V (with the DAC running at 0 dBFS). So the resulting

drive signal from each differential pair is 2 V p-p differential

with a 500 mV dc bias.

ADDING A SWING-LIMITING RESISTOR

The voltage swing for a given DAC output current can be

reduced by adding a third resistor to the interface. This resistor

is placed in the shunt across each differential pair, as shown in

Figure 37. It has the effect of reducing the ac swing without

changing the dc bias already established by the 50 Ω resistors.

RBIP

50Ω

RBIN

50Ω

AD9122

ADRF6702

RBQN

50Ω

RBQP

50Ω

RSL

(SEE TEXT)

RSL

(SEE TEXT)

OUT1_N

OUT1_P

OUT2_P

OUT2_N

Figure 37. AC Voltage Swing Reduction Through the Introduction

of a Shunt Resistor Between the Differential Pair

The value of this ac voltage swing limiting resistor (RSL as shown

in Figure 37) is chosen based on the desired ac voltage swing

and IQ modulator output power. Figure 38 shows the relation-

ship between the swing-limiting resistor and the peak-to-peak

ac swing that it produces when 50 Ω bias-setting resistors are

used. A higher value of swing-limiting resistor will increase the

output power of the ADRF6702 and signal-to-noise ratio (SNR)

at the cost if higher intermodulation distortion. For most

applications, the optimum value for this resistor will be between

100 Ω and 300 Ω.

When setting the size of the swing-limiting resistor, the input

impedance of the I and Q inputs should be taken into account.

The I and Q inputs have a differential input resistance of 920 Ω.

As a result, the effective value of the swing-limiting resistance is

920 Ω in parallel with the chosen swing-limiting resistor. For

example, if a swing-limiting resistance of 200 Ω is desired

(based on Figure 37), the value of RSL should be set such that

200 Ω = (920 × RSL)/(920 + RSL)

resulting in a value for RSL of 255 Ω.

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

100

1000

10000

RSL (Ω)

Figure 38. Relationship Between the AC Swing-Limiting Resistor and the

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

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