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LT5537

Wide Dynamic Range RF/IF Log Detector

Linear Technology 

LT5537

APPLICATIO S I FOR ATIO

The LT5537 provides a log-linear relationship between an

RF/IF input voltage and its output. The input signal is

amplified successively by limiting amplifier stages. A series

of detector cells rectify the signals and produce an output

current which is log-linearly related to the input power with

a coefficient (ISLOPE) of 3.4µA/dB at 200MHz (independent

of the input termination impedance). This coefficient is

almost constant below 200MHz, but rises at higher fre-

quency. The normalized slope variation plot in Figure 1 can

be used to determine the log-linear coefficient at any

frequency. The slope of the output voltage curve is deter-

mined by the total load resistance at the output terminal.

VSLOPE = 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.

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.

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FREQUENCY (MHz)

5537 F01

Figure 1. Slope Variation over Frequency

VCC

DETECTOR

OUTPUT

8 OUT

7.2k

5537 F02

Figure 2. Simplified Output Circuit

5537fa

8

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