lt5537 Linear Technology Corporation, lt5537 Datasheet - Page 8

• Manufacturer Part Number: lt5537
• Description: Wide Dynamic Range Rf/if Log Detector
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT5537
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 (I
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.
The on-chip pull-down resistor is 7.2k. The total load
resistance (R
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.
Additionally, an off-chip capacitor may be used to reduce
the output time domain voltage ripple.
8
V
Slope = 3.4µA/dB • (7.2//33)kΩ = 20.1mV/dB
SLOPE
= I
SLOPE
LOAD
SLOPE
) can be adjusted by adding external
U
• R
) of 3.4µA/dB at 200MHz (independent
LOAD
U
W
U
150
140
130
120
110
100
Figure 1. Slope Variation over Frequency
70
60
50
90
80
DETECTOR
1
OUTPUT
Figure 2. Simplified Output Circuit
FREQUENCY (MHz)
50
100
V
CC
200
400 600 1000
5537 F01
7.2k
5537 F02
8
OUT
5537fa