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AD8361ARM Datasheet(PDF) 11 Page - Analog Devices |
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AD8361ARM Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page AD8361 –11– REV. A INPUT – mV 700 200 0 500 100 200 300 400 500 400 300 600 100 0 600 700 800 Figure 37. Idealized Output Step Size as Function of Input Voltage Plots of output voltage vs. input voltage result in a straight line. It may sometimes be more useful to plot the error on a logarith- mic scale, as shown in Figure 38. The deviation of the plot for the ideal straight line characteristic is caused by output clipping at the high end and by signal offsets at the low end. It should however be noted that offsets at the low end can be either posi- tive or negative, so that this plot could also trend upwards at the low end. Figures 5, 6, 8, and 9 show a ±3 sigma distribution of device error for a large population of devices. INPUT – V rms 2.0 –0.5 0.01 0.5 0.0 1.5 1.0 –1.0 –1.5 –2.0 1.0 1.9GHz 2.5GHz 900MHz 100MHz 100MHz 0.02 (–21dBm) 0.1 (–7dBm) 0.4 (+5dBm) Figure 38. Representative Unit, Error in dB vs. Input Level, VS = 2.7 V It is also apparent in Figure 38 that the error plot tends to shift to the right with increasing frequency. Because the input impedance decreases with frequency, the voltage actually applied to the input will also tend to decrease (assuming a constant source impedance over frequency). The dynamic range is almost con- stant over frequency, but with a small decrease in conversion gain at high frequency. Input Coupling and Matching The input impedance of the AD8361 decreases with increasing frequency in both its resistive and capacitive components (Figure 13). The resistive component varies from 225 Ω at 100 MHz down to about 95 Ω at 2.5 GHz. A number of options exist for input matching. For operation at multiple frequencies, a 75 Ω shunt to ground, as shown in Figure 39a, will provide the best overall match. For use at a single fre- quency, a resistive or a reactive match can be used. By plotting the input impedance on a Smith Chart, the best value for a resistive match can be calculated. The VSWR can be held below 1.5 at frequencies up to 1 GHz, even as the input impedance varies from part to part. (Both input impedance and input capacitance can vary by up to ±20% around their nominal values.) At very high frequencies (i.e., 1.8 GHz to 2.5 GHz), a shunt resistor will not be sufficient to reduce the VSWR below 1.5. Where VSWR is critical, remove shunt component and insert an inductor in series with the coupling capacitor as shown in Figure 39b. Table II gives recommended shunt resistor values for various frequencies and series inductor values for high frequencies. The coupling capacitor, CC, essentially acts as an ac-short and plays no intentional part in the matching. AD8361 RFIN RFIN RSH CC a. Broadband Resistor Match AD8361 CC RFIN RFIN LM b. Series Inductor Match AD8361 CC RFIN RFIN LM CM c. Narrowband Reactive Match AD8361 CC RFIN RFIN RSERIES d. Attenuating the Input Signal Figure 39. Input Coupling/Matching Options Table II. Recommended Component Values for Resistive or Inductive Input Matching (Figures 39a and 39b) Frequency Matching Component 100 MHz 63.4 Ω Shunt 800 MHz 75 Ω Shunt 900 MHz 75 Ω Shunt 1800 MHz 150 Ω Shunt or 4.7 nH Series 1900 MHz 150 Ω Shunt or 4.7 nH Series 2500 MHz 150 Ω Shunt or 2.7 nH Series |
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