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2.5GHz 45dB RF-Detecting Controllers
impedance of approximately 50 Ω . See the Typical
Operating Characteristics for the input impedance plot to
determine the required external termination at the fre-
quency of interest. The MAX4001/MAX4002 require an
additional external coupling capacitor in series with the
RF input. As the operating frequency increases over
2GHz, input impedance is reduced, resulting in the need
for a larger-valued shunt resistor. Use a Smith Chart for
50Ω SOURCE
50 Ω
C C **
RFIN
C C *
MAX4000
MAX4001
MAX4002
calculating the ideal shunt resistor value.
For high frequencies, use narrowband reactive coupling.
This implementation is shown in Figure 4b. The matching
R S
50 Ω
C IN
R IN
components are drawn as reactances since these can
be either capacitors or inductors depending on the input
impedance at the desired frequency and available stan-
dard value components. A Smith Chart is used to obtain
the input impedance at the desired frequency and then
matching reactive components are chosen. Table 1 pro-
vides standard component values at some common fre-
quencies for the MAX4001. Note that these inductors
must have a high SRF (self-resonant frequency), much
higher than the intended frequency of operation to imple-
ment this matching scheme.
*MAX4000 ONLY INTERNALLY COUPLED
**MAX4001/MAX4002 REQUIRE EXTERNAL COUPLING
Figure 4a. Broadband Resistive Matching
V CC
MAX4000
Device sensitivity is increased by the use of a reactive
matching network, because a voltage gain occurs
50 Ω SOURCE
MAX4001
MAX4002
before being applied to RFIN. The associated gain is
calculated with the following equation:
50 Ω
j X1
C C **
RFIN
C C *
Voltage Gain dB = 20 log 10
R2
R 1
j X2
C IN
R IN
where R1 is the source impedance to which the device
is being matched, and R2 is the input resistance of the
device. The gain is the best-case scenario for a perfect
match. However, component tolerance and standard
value choice often result in a reduced gain.
Figure 4c demonstrates series attenuation coupling.
This method is intended for use in applications where
the RF input signal is greater than the input range of the
device. The input signal is thus resistively divided by
the use of a series resistor connected to the RF source.
Since the MAX4000/MAX4001/MAX4002 log amps offer
*MAX4000 ONLY INTERNALLY COUPLED
**MAX4001/MAX4002 REQUIRE EXTERNAL COUPLING
Figure 4b. Narrowband Reactive Matching
V CC
MAX4000
MAX4001
MAX4002
a wide selection of RF input ranges, series attenuation
coupling is not needed for typical applications.
Table 1. Suggested Components for
MAX4001 Reactive Matching Network
STRIPLINE
R ATTN
C C **
RFIN
C C *
C IN
R IN
FREQUENCY
(GHz)
0.9
1.9
2.5
j X1 (nH)
38
4.4
—
j X2 (nH)
47
4.7
1.8
VOLTAGE
GAIN (dB)
12.8
3.2
-0.3
*MAX4000 ONLY INTERNALLY COUPLED
**MAX4001/MAX4002 REQUIRE EXTERNAL COUPLING
Figure 4c. Series Attenuation Network
V CC
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