Datasheet LT5537 (Analog Devices) - 8

LT5537
U U W U APPLICATIO S I FOR ATIO
The LT5537 provides a log-linear relationship between an 150 140 RF/IF input voltage and its output. The input signal is 130 amplified successively by limiting amplifier stages. A series 120 of detector cells rectify the signals and produce an output 110 current which is log-linearly related to the input power with 100 a coefficient (ISLOPE) of 3.4µA/dB at 200MHz (independent µA/dB AT 200MHz 90 of the input termination impedance). This coefficient is 80 almost constant below 200MHz, but rises at higher fre- % OF 3.4 70 quency. The normalized slope variation plot in Figure 1 can 60 be used to determine the log-linear coefficient at any 50 1 50 100 200 400 600 1000 frequency. The slope of the output voltage curve is deter- FREQUENCY (MHz) mined by the total load resistance at the output terminal. 5537 F01 V
Figure 1. Slope Variation over Frequency
SLOPE = ISLOPE • RLOAD The on-chip pull-down resistor is 7.2k. The total load resistance (RLOAD) can be adjusted by adding external load resistance to change the output slope. For example, to achieve a log-linear rate of 20mV/dB, a 33k resistor is connected between the output pin and ground. VCC Slope = 3.4µA/dB • (7.2//33)kΩ = 20.1mV/dB Additionally, an off-chip capacitor may be used to reduce the output time domain voltage ripple. DETECTOR OUTPUT 8 OUT 7.2k 5537 F02
Figure 2. Simplified Output Circuit
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