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AD9122 Scheda tecnica(PDF) 5 Page - Analog Devices |
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AD9122 Scheda tecnica(HTML) 5 Page - Analog Devices |
5 / 9 page Circuit Note CN-0205 Rev. 0 | Page 5 of 9 Figure 11. Spreadsheet to Calculate Modulator Output Power Calculating the Output Power of the AD9122 and the ADL5375 In addition to the bias-setting and signal scaling resistors, the power level at the output of the ADL5375 is a function of the DAC’s digital backoff level (dBFS), the signal’s peak-to-average ratio, the DAC’s full-scale current, the insertion loss of the Nyquist filter, and I/Q modulator’s voltage gain. The spreadsheet shown in Figure 11 can be used to make this calculation. This spreadsheet can be downloaded using the following URL: www.analog.com/CN0205-PowerCalculator. Level Shifting to Drive the ADL5375-15 The ADL5375-15 requires a dc bias level of 1500 mV. Other than the difference in bias levels, the ADL5375-05 and ADL5375-15 are identical. To drive the ADL5375-15 from the AD9122, either a passive or active level-shifting network must be used. The passive level shifting network shown in Figure 12 uses four series resistors along with four pull-up resistors to achieve a bias level of 1500 mV at the ADL5375-15 input. This passive level shifting network introduces a loss of approximately 2 dB in the signal level. An active level shifting circuit would use a dual-differential amplifier, such as ADA4938, where placing 1500 mV on the VOCM pin sets the output dc bias level. In this approach, however, the interface bandwidth is limited by the op amp. RBIP 45.3Ω RBIN 45.3Ω 67 66 21 22 OUT1_N OUT1_P IBBN IBBP 5V AD9122 ADL5375-15 RLIP 3480Ω RLIN 3480Ω RBQN 45.3Ω RSQP 1kΩ RBQP 45.3Ω 59 58 9 10 OUT2_P OUT2_N QBBP QBBN 5V RLQN 3480Ω RLQP 3480Ω RSQN 1kΩ RSIP 1kΩ RSIN 1kΩ Figure 12. Passive Level-Shifting Network For Biasing ADL5375-15 from the AD9122 TxDAC As previously mentioned, it is necessary to put a filter between AD9122 and ADL5375-15. The LC filter can be located anywhere between the DAC termination resistors (R1 in Figure 13) and the ac swing-limiting resistor (R4 in Figure 13). However, the circuit in Figure 13 allows flexibility in the design of the level shifting circuit with low loss by R2 and a high driving level to modulator. It also allows a matched filter at source and load impedance. Figure 13 is the recommended passive level- shifting network with filter. |
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