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LT5538IDD-TRPBF Datasheet(PDF) 8 Page - Linear Technology |
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LT5538IDD-TRPBF Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page LT5538 8 5538f The LT5538 is a 40MHz to 3.8 GHz logarithmic RF power detector. It consists of cascaded limiting amplifiers and RF detectors. The output currents from every RF detector are combined and low-pass filtered before applied to the output buffer amplifier. As a result, the final DC output voltage approximates the logarithm of the amplitude of the input signal. The LT5538 is able to accurately measure an RF signal over a 70dB dynamic range (–68dBm to 2dBm at 2.1GHz) with 50Ω single-ended input impedance. The slope of linear to log transfer function is about 17.7mV/dB at 2.1GHz. Within the linear dynamic range, very stable output is achieved over the full temperature range from –40°C to 85°C and over the full operating frequency range from 40MHz to 3.8GHz. The absolute variation over temperature is typically within ±1dB over 65dB dynamic range at 2.1GHz. RF INPUT The simplified schematic of the input circuit is shown in Figure 1. The IN+ and IN– pins are internally biased to VCC –0.5V. The IN– pin is internally coupled to ground via 20pF capacitor. An external capacitor of 1nF is needed to connect this pin to ground for low frequency operation. The impedance between IN+ and IN– is about 394Ω. The RF input pin IN+ should be DC blocked when connected to ground or other matching components. A 56Ω resistor (R1) connected to ground will provide better than 10dB input return loss over the operating frequency range up to 1.5GHz. At higher operating frequency, additional LC matching elements are needed for a proper impedance matching to a 50Ω source as shown in Figure 2. Refer to Figure 6 for the circuit schematic of the input matching network. The input impedance vs frequency of the RF input port IN+ is detailed in Table 1. Table 1. RF Input Impedance FREQUENCY (MHz) RF INPUT IMPEDANCE (Ω) S11 MAG ANGLE(°) 40 47.3 + j129.7 0.800 38.5 100 246.6 + j210.7 0.790 11.5 200 408.7 – j37.8 0.785 –1.5 400 192.9 – j190.9 0.772 –14.9 600 105.6 – j158.4 0.756 –25.3 800 69.3 – j127.4 0.737 –34.4 1000 51.8 – j106.2 0.720 –42.7 1200 41.5 – j90.9 0.707 –50.6 1400 34.2 – j78.7 0.697 –58.2 1600 29.2 – j60.0 0.687 –65.6 1800 25.4 – j60.7 0.678 –73.1 2000 22.6 – j53.8 0.669 –80.4 2200 20.5 – j47.7 0.659 –87.7 2400 18.9 – j42.4 0.649 –94.6 2600 17.9 – j37.6 0.638 –101.5 2800 17.1 – j33.4 0.627 –108.2 3000 16.4 – j29.5 0.615 –114.7 3200 16.1 – j26.0 0.602 –121.0 3400 15.9 – j22.8 0.589 –127.0 3600 15.9 – j20.0 0.574 –132.8 3800 15.9 – j17.5 0.560 –137.9 APPLICATIONS INFORMATION 5.3k 394 IN+ IN– 20p 5.3k VCC + – 5538 F01 Figure 1. Simplified Schematic of the Input Circuit Figure 2. Input Return Loss with Additional LC Matching Network 5538 F02 FREQUENCY (GHz) 0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4.0 –30 –25 –20 –15 –10 0 –5 W/O L1 AND C8 L1 = 1.5nH, C8 = 1pF C4, C11 = 12pF, C8 = 0.7pF |
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