Datasheet LT5538 (Analog Devices) - 8

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