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AD9122 датащи(PDF) 6 Page - Analog Devices |
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AD9122 датащи(HTML) 6 Page - Analog Devices |
6 / 9 page 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 2 × IFS × R1||{R2+R3||(R4/2)}, 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 |
Аналогичный номер детали - AD9122 |
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Аналогичное описание - AD9122 |
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