LTC5535
9
5535f
APPLICATIO S I FOR ATIO
WUU
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Operation
The LTC5535 RF detector integrates several functions to
provide RF power detection over frequencies ranging
from 600MHz to 7GHz. These functions include an internal
frequency compensated output amplifier, an RF Schottky
diode peak detector and a level shift amplifier to convert the
RF input signal to DC. The LTC5535 has both gain setting
and voltage offset adjustment capabilities.
Output Amplifier
The output amplifier is capable of supplying typically
20mA into a load. The negative terminal V
M
is brought out
to a pin for gain selection. External resistors connected
between V
OUT
and V
M
(R
A
) and V
M
to ground (R
B
) will set
the gain of this amplifier.
Gain = 1 + R
A
/R
B
The amplifier is not unity gain stable; a minimum gain of
two is required. The output amplifier has a bandwidth of
20MHz with a gain of 2. For increased gain applications,
the bandwidth is reduced according to the formula:
Bandwidth = 40MHz/(Gain) = 40MHz • R
B
/(R
A
+ R
B
)
For stable operation the gain setting resistors should be
low values and the board capacitance on V
M
should be
minimized. R
B
is recommended to be no greater than
500Ω for all gain settings.
The V
OS
input controls the DC input voltage to the output
amplifier. V
OS
must be connected to ground if the DC
output voltage is not to be changed. The output amplifier
is initially trimmed to 200mV (Gain = 2) with V
OS
con-
nected to ground.
The V
OS
pin is used to change the initial V
OUT
starting
voltage. This function, in combination with gain adjust-
ment enables the LTC5535 output to span the input range
of a variety of analog-to-digital converters. V
OUT
will not
change until V
OS
exceeds 200mV. The starting voltage at
V
OUT
for V
OS
>200mV is:
V
OUT
= 0.5 • V
OS
• Gain
where gain is the output amplifier gain. For a gain of 2,
V
OUT
will exactly track V
OS
above 200mV.
RF Detector
The internal RF Schottky diode peak detector and level
shift amplifier converts the RF input signal to a low
frequency signal. The detector demonstrates excellent
efficiency and linearity over a wide range of input power.
The Schottky diode is biased at about 50µA and drives a
5pF internal peak detector capacitor.
RF
IN
GND
V
OS
V
CC
V
OUT
V
M
LTC5535ES6
1
2
3
6
5
4
C4
39pF
RF
IN
R1
(OPT)
OFFSET
ADJUSTMENT
C2
100pF
C1
0.1µF
R
LOAD
(OPT)
V
CC
2.7V TO 5.5V
V
OUT
GND
R2
500Ω
1%
R3
500Ω
1%
5535 DB
Demo Board Schematic
