LTC5532
12
5532f
APPLICATIO S I FOR ATIO
WUU
U
Operation
The LTC5532 RF detector integrates several functions to
provide RF power detection over frequencies ranging from
300MHz to 7GHz. These functions include an internal fre-
quency compensated buffer amplifier, an RF Schottky di-
ode peak detector and a level shift amplifier to convert the
RF input signal to DC. The LTC5532 has both gain setting
and voltage offset adjustment capabilities.
Buffer Amplifier
The output buffer amplifier is capable of supplying typi-
cally 4mA 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 unity gain stable; however a minimum
gain of two is recommended to improve low output
voltage accuracy. The amplifier has a bandwidth of 2MHz
with a gain of 2. For increased gain applications, the
bandwidth is reduced according to the formula:
Bandwidth = 4MHz/(Gain) = 4MHz • R
B
/(R
A
+ R
B
)
A capacitor can be placed across the feedback resistor R
A
to shape the frequency response. In addition, the ampli-
fier can be used as a comparator. V
M
can be connected to
a reference voltage. When the internal detector output
voltage (which is connected to the positive input of the
buffer amplifier) exceeds the external voltage on V
M
, V
OUT
will switch high.
The V
OS
input controls the DC input voltage to the buffer
amplifier. V
OS
must be connected to ground if the DC
starting voltage is not to be changed. The buffer is initially
trimmed nominally to 120mV (Gain = 2x) 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 LTC5532 output to span the input range
of a variety of analog-to-digital converters. V
OUT
will not
change until V
OS
exceeds 120mV. The starting voltage at
V
OUT
for V
OS
>120mV is:
V
OUT
= 0.5 • V
OS
• Gain
where gain is the output buffer gain. For a buffer gain of 2x,
V
OUT
will exactly track V
OS
above 120mV.
RF Detector
The internal RF Schottky diode peak detector and level
shift amplifier convert the RF input signal to a low fre-
quency signal. The detector demonstrates excellent effi-
ciency and linearity over a wide range of input power. The
Schottky diode is biased at about 55µA and drives a 25pF
internal peak detector capacitor.
Demo Board Schematic
RF
IN
GND
V
OS
V
CC
V
OUT
V
M
LTC5532ES6
1
2
3
6
5
4
C4
39pF
RF
IN
R1
(OPT)
OFFSET
ADJUSTMENT
C2
100pF
C1
0.1µF
C3
(OPT)
V
CC
2.7V TO 6V
V
OUT
GND
R2
10k
1%
R3
10k
1%
5532 DB
