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AD8313ARM 데이터시트(PDF) 11 Page - Analog Devices |
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AD8313ARM 데이터시트(HTML) 11 Page - Analog Devices |
11 / 16 page AD8313 –11– REV. B A positive input step on VSET (indicating a demand for in- creased power from the PA) will drive VOUT towards ground. This should be arranged to increase the gain of the PA. The loop will settle when VOUT settles to a voltage that sets the input power to the AD8313 to the dB equivalent of VSET. Input Coupling The signal may be coupled to the AD8313 in a variety of ways. In all cases, there must not be a dc path from the input pins to ground. Some of the possibilities include: dual input coupling capacitors, a flux-linked transformer, a printed-circuit balun, direct drive from a directional coupler, or a narrow-band imped- ance matching network. Figure 30 shows a simple broadband resistive match. A termina- tion resistor of 53.6 Ω combines with the internal input imped- ance of the AD8313 to give an overall resistive input impedance of approximately 50 Ω. The termination resistor should prefer- ably be placed directly across the input pins, INHI to INLO, where it serves to lower the possible deleterious effects of dc offset voltages on the low end of the dynamic range. At low frequencies, this may not be quite as attractive, since it necessi- tates the use of larger coupling capacitors. The two 680 pF input coupling capacitors set the high-pass corner frequency of the network at 9.4 MHz. RMATCH 53.6 C1 680pF C2 680pF CIN RIN AD8313 50 50 SOURCE Figure 30. A Simple Broadband Resistive Input Termination The high pass corner frequency can be set higher according to the equation: f C dB 3 1 250 = ×× × π where: C CC CC = × + 12 12 In high frequency applications, the use of a transformer, balun or matching network is advantageous. The impedance match- ing characteristics of these networks provide what is essentially a gain stage before the AD8313 that increases the device sensitiv- ity. This gain effect is further explored in the following match- ing example. Figures 31 and 32 show device performance under these three input conditions at 900 MHz and 1900 MHz. While the 900 MHz case clearly shows the effect of input matching by realigning the intercept as expected, little improve- ment is seen at 1.9 GHz. Clearly, if no improvement in sensitiv- ity is required, a simple 50 Ω termination may be the best choice for a given design based on ease of use and cost of components. INPUT AMPLITUDE – dBm –80 –70 –60 –50 –40 –30 –20 –10 3 2 1 0 –1 –2 –3 TERMINATED DR = 66dB –90 10 0 BALANCED MATCHED BALANCED DR = 71dB MATCHED DR = 69dB Figure 31. Comparison of Terminated, Matched and Balanced Input Drive at 900 MHz INPUT AMPLITUDE – dBm –80 –70 –60 –50 –40 –30 –20 –10 0 3 2 1 0 –1 –2 –3 –90 10 TERMINATED DR = 75dB BALANCED BALANCED DR = 75dB MATCHED DR = 73dB MATCHED TERMINATED Figure 32. Comparison of Terminated, Matched and Balanced Input Drive at 1900 MHz A Narrow-Band LC Matching Example at 100 MHz While numerous software programs are available that allow the values of matching components to be easily calculated, a clear understanding of the calculations involved is valuable. A low frequency (100 MHz) value has been used for this exercise because of the deleterious board effects at higher frequencies. RF layout simulation software is useful when board design at higher frequencies is required. A narrow-band LC match can be implemented either as a series-inductance/shunt-capacitance or as a series-capacitance/ shunt-inductance. However, the concurrent requirement that the AD8313 inputs, INHI and INLO, be ac-coupled, makes a series-capacitance/shunt-inductance type match more appropri- ate (see Figure 33). LMATCH C1 C2 CIN RIN AD8313 50 50 SOURCE Figure 33. Narrow-Band Reactive Match |
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