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AD9122 Arkusz danych(PDF) 3 Page - Analog Devices |
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AD9122 Arkusz danych(HTML) 3 Page - Analog Devices |
3 / 9 page Circuit Note CN-0205 Rev. 0 | Page 3 of 9 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 10 100 1k 10k RL (Ω) Figure 3. Peak-to-Peak Differential Swing and the Swing Limiting Resistor (RL) with 50 Ω Bias-Setting Resistors Baseband Filtering A filter must be inserted between the AD9122 and ADL5375 to remove Nyquist images, spurs, and broadband noise originating from the DAC. The filter should be placed between the dc bias setting resistors and the ac swing-limiting resistor. With this configuration, the dc bias setting resistors (RB in Figure 4) and the signal scaling resistors (RLin Figure 4) conveniently set the source and load resistances for the filter design. Figure 4 shows a third-order Bessel low-pass filter with a −3 dB frequency of 10 MHz. Matching input and output impedances of the filter makes the filter design easy and results in better passband flatness, which allows wide bandwidth filter designs. In this example, the shunt resistor chosen is 100 Ω, producing an ac swing of 1 V p-p differential. The frequency response of this filter is shown in Figure 5. RBIP 50Ω RBIN 50Ω 67 66 21 22 IBBN IBBP AD9122 ADL5375-05 RBQN 50Ω RBQP 50Ω 59 58 9 10 RSLI 100Ω RSLQ 100Ω OUT1_N OUT1_P OUT2_P OUT2_N QBBP QBBN LPI 771.1nH LNI 771.1nH 53.62pF C1I 350.1pF C2I LNQ 771.1nH LPQ 771.1nH 53.62pF C1Q 350.1pF C2Q Figure 4. DAC Modulator Interface with 10 MHz Third-Order, Bessel Filter 1 10 100 –60 –50 –40 –30 –20 –10 0 0 6 12 18 24 30 36 FREQUENCY (MHz) MAGNITUDE GROUP DELAY Figure 5. Frequency Response for DAC Modulator Interface with 10 MHz Third-Order Bessel Filter Filtering for Complex IF (CIF) Applications Figure 6 shows the frequency response of the ADL5375 baseband I and Q inputs. Because this device has a wide and flat frequency response (−3 dB point = 750 MHz), it is well suited to complex IF (CIF) applications where the output signal from the DAC has been digitally upconverted. In CIF applications, a low- pass Nyquist filter is still desirable, primarily because the dc bias level can be preserved from the DAC output to the modulator input. The filter topology shown in Figure 7 is a 5th order Butterworth filter with a 300 MHz corner frequency and is the recommended filter topology. A purely differential filter can reject differential-mode images, spurs, and noise from the DAC. Using two capacitors with their common connection grounded (C2 and C4 in Figure 7) diverts some of the common-mode current to ground and results in better common-mode rejection of high-frequency signals than would be obtained with a purely differential filter. The simulated and measured responses of this filter are shown in Figure 8 and Figure 9. The measured flatness is ±0.6 dB from dc to 250 MHz and ±0.4 dB from 125 MHz to 250 MHz. This data was taken with the AD9122 inverse sinc function on. In this configuration, Figure 10 shows the common-mode rejection performance of the 2 × FDAC common-mode spur vs. common-mode frequency with and without IF filter shown in Figure 7. |
Podobny numer części - AD9122 |
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Podobny opis - AD9122 |
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