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ADL5375ACPZ-R7 数据表(PDF) 26 Page - Analog Devices |
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ADL5375ACPZ-R7 数据表(HTML) 26 Page - Analog Devices |
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26 / 36 page  ADL5375 Data Sheet Rev. C | Page 26 of 36 1 0 –1 –2 –3 –4 –5 –6 1 10 100 1k BASEBAND FREQUENCY (MHz) Figure 64. ADL5375-15 Baseband Frequency Response Normalized to Response for 1 MHz In CIF applications, a low-pass filter between the DAC and modulator is still favored to filter out images, noises discussed in the Filtering section as well as to preserve dc bias level from DAC to ADL5375-05. Figure 65 shows a fifth order Butterworth filter with a 300 MHz corner frequency and the frequency response of this filter is shown in Figure 66. Even a purely differential filter can work well, splitting the filter capacitors into two and grounding at filter topology as like C2 and C4 in Figure 65 divert common mode currents to ground and result in additional common-mode rejection of high frequency signals to a purely differential filter. 67 66 IBBN IBBP AD9122 ADL5375-05 59 58 21 22 9 10 IOUT1N IOUT1P IOUT2 IOUT2N QBBP QBBN RBIP 50Ω RBIN 50Ω RBQN 50Ω RBQP 50Ω RSLI 100Ω RSLQ 100Ω C3I 6pF C3Q 6pF C1I 3.6pF C1Q 3.6pF L1PI 33nH L1NI 33nH L2PI 33nH L2NI 33nH L1NQ 33nH L1PQ 33nH C2PI 22pF C2NI 22pF C4PI 3pF C4NI 3pF L2NQ 33nH L2PQ 33nH C2NQ 22pF C2PQ 22pF C4NQ 3pF C4PQ 3pF Figure 65. Recommended DAC Modulator Interface Topology with FC = 300 MHz Fifth-Order, Butterworth Filter 0 –25 –20 –15 –10 –5 1M 500M 100M 10M FREQUENCY (Hz) 4.0 0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Figure 66. Frequency Response for DAC Modulator Interface with 300 MHz Fifth-Order Butterworth Filter Driving the ADL5375-15 with a TXDAC The ADL5375-15 requires a 1500 mV dc bias and therefore requires a slightly more complex interface that performs a dc level shift on the baseband signals. It is necessary to level-shift the DAC output from a 500 mV dc bias to the 1500 mV dc bias that the ADL5375-15 requires. Level-shifting can be achieved with either a passive network or an active circuit. A passive network of resistors is shown in Figure 67. In this network, the dc bias of the DAC remains at 500 mV while the input to the ADL5375-15 is 1500 mV. It should be noted that this passive level-shifting network introduces approximately 2 dB of loss in the ac signal. 67 66 IBBN IBBP AD9122 ADL5375-15 59 58 21 22 9 10 IOUT1N IOUT1P IOUT2 IOUT2N QBBP QBBN RBIP 45.3Ω RBIN 45.3Ω RBQN 45.3Ω RBQP 45.3Ω RLIP 3480Ω RLIN 3480Ω RLQN 3480Ω RLQP 3480Ω RSIN 1kΩ RSIP 1kΩ RSQN 1kΩ RSQP 1kΩ 5V 5V Figure 67. Passive Level-Shifting Network For Biasing ADL5375-15 from TxDAC The active level shifting circuit involves the use of the ADA4938 dual-differential amplifier. This device has a VOCM pin that sets the output dc bias. Through this pin, the output common- mode of the amplifier can be easily set to the requisite 1.5 V for biasing the ADL5375-15 baseband inputs. |
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