CN-0205

Circuit Note

Rev. 0 | Page 6 of 9

AD9122

LC FILTER

IOUT_P

R1

R1

R2

R2

A

A

B

B

IOUT_N

ADL5375-15

REQUIRED DC LEVEL

B = 1.5V

R3

R4

R3

V1

Figure13. Recommended Passive Level-Shifting Network with LC Filter

The differential source impedance and load impedance of the

filter are

2 × (R1 + R2) and

2 × {R3||(R4/2)}, respectively.

The single-ended impedance seen by DAC is

R1||{R2+R3||(R4/2)}.

R4 acts as the ac load to the DAC. The differential ac swing at

DAC output is

and the differential ac swing at the modulator input is

2 × {R3||(R4/2)}÷{R2+(R3||(R4/2)}

multiplied by the differential ac swing at DAC output.

34.0

218

760

750

20

5.00

31.70

0.50

0.63

1.50

0.34

–5.43

504

502

SETUP

R1 (Ω)

R2 (Ω)

R3(Ω)

R4 (Ω)

IFS (MA)

V1 (V)

DAC R

(SINGLE)

FILTER

INPUT IMPEDANCE (Ω)

OUTPUT IMPEDANCE (Ω)

DAC

DAC

COMMON VOLTAGE (V)

DAC SWING (V p_p)

(SINGLE)

MOD

MODULATOR

COMMON VOLTAGE (V)

MOD INPUT SWING

(V p_p)

(SINGLE)

LOSS BY R2 (DB)

Figure14. Spreadsheet for the Level Shifting Circuit

The LC filter should be placed close to the DAC to allow short

return current path. The 5 V bias supply (V1) should be close

to the modulator because it is shared with the modulator. For

the case when R1, R2, R3, and R4 are 34 Ω, 218 Ω, 760 Ω, and

750 Ω, respectively, the 500 mV dc bias at the AD9122 DAC

output is matched to the 1500 mV dc bias at ADL5375-15.

Actually, it is not necessary to be 500 mV at point A of Figure

13, but it will give flexibility in the ac swing level without

exceeding the compliance voltage of the DAC output. The DAC

load is 31.7 Ω. The input and output impedance of the filter are

504 Ω and 502 Ω. The attenuation by R2, which is the voltage

drop by R2 between the DAC output and modulator input, is

set by the combination of R2 and R3||(R4/2),which is about 5.4 dB.

To calculate dc bias level and ac swing level at the A and B

points (Figure 13), attenuation by R2, and source/load

impedances of the filter, the spreadsheet below can be used.

This can be downloaded at the following URL:

www.analog.com/CN0205-LevelShifter.

The ADIsimRF tool can also be used to perform DAC-

modulator power level calculations. The tool can be

downloaded from www.analog.com/ADIsimRF.

Layout Recommendations

Special care should be taken in the layout of the

DAC/modulator interface. Here are some recommendations.

Figure 15 shows a top-level layout, which follows these

recommendations:

•

Keep all I/Q differential trace lengths well matched.

•

Place filter termination resistor as close as possible to

modulator input.

•

Place DAC output 50 Ω resistors as close as possible to

DAC.

•

Thicken trace widths through the filter network to

reduce signal loss.

•

Place vias around all DAC output traces, filter

networks, modulator output traces, and LO input

traces.

•

Route LO and modulator outputs on different layers

or at 90° angle to each other to prevent coupling.

Figure15. General Layout Recommendations