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ADL5390-EVAL

RF/IF Vector Multiplier

Analog Devices 

ADL5390

The 3 dB bandwidth is set by choosing CFLT according to the

following equation:

45 kHz × 10 nF

f 3 dB ≈ C external + 0.5 pF

This equation has been verified for values of CFLT from 10 pF to

0.1 µF (bandwidth settings of approximately 4.5 kHz to 43 MHz).

INTERFACING TO HIGH SPEED DACs

The AD977x family of dual DACs is well suited to driving the I

and Q gain controls of the ADL5390 with fast modulating sig-

nals. While these inputs can in general be driven by any DAC,

the differential outputs and bias level of the ADI TxDAC® fam-

ily allows for a direct connection between DAC and modulator.

The AD977x family of dual DACs has differential current out-

puts. The full-scale current is user programmable and is usually

set to 20 mA, that is each output swings from 0 mA to 20 mA.

The basic interface between the AD9777 DAC outputs and the

ADL5390 I and Q gain control inputs is shown in Figure 32.

Resistors R1 and R2 (R1 = R2) set the dc bias level according to

the following equation:

Bias Level = Average Output Current × R1

For example, if the full-scale current from each output is 20 mA,

each output will have an average current of 10 mA. Therefore,

to set the bias level to the recommended 0.5 V, R1 and R2

should be set to 50 Ω each. R1 and R2 should always be equal.

If R3 is omitted, this will result in an available swing from the

DAC of 2 V p-p differential, which is twice the maximum voltage

range required by the ADL5390. DAC resolution can be maxi-

mized by adding R3, which scales down this voltage according

to the following equation:

Full Scale Swing =

( ( )) 2× I MAX R1|| R2 + R3

×

⎡

⎢1 −

⎢⎣

R2

R2 + R3

⎤

⎥

⎥⎦

AD9777

IOUTA1

R1

R2

IOUTB1

ADL5390

OPTIONAL

LOW-PASS

FILTER

IBBP

R3

IBBM

IOUTA2

R1

R2

IOUTB2

OPTIONAL

LOW-PASS

FILTER

QBBP

R3

QBBM

Figure 32. Basic AD9777-to-ADL5390 Interface

1.15

1.13

1.10

1.08

1.05

1.02

1.00

0.97

0.95

0.92

0.90

0.88

0.85

0.82

0.80

0.77

0.75

0.72

0.70

50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130

R3 (Ω)

Figure 33. Peak-Peak DAC Output Swing vs.

Swing Scaling Resistor R3 (R1 = R2 = 50 Ω)

Figure 33 shows the relationship between the value of R3 and

the peak baseband voltage with R1 and R2 equal to 50 Ω. As

shown in Figure 33, a value of 100 Ω for R3 will provide a

peak-peak swing of 1 V p-p differential into the ADL5390’s

I and Q inputs.

When using a DAC, low-pass image reject filters are typically

used to eliminate the Nyquist images produced by the DAC.

They also provide the added benefit of eliminating broadband

noise that might feed into the modulator from the DAC.

Rev. 0 | Page 14 of 24

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