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AD8361 Datasheet(Fiches technique) 11 Page - Analog Devices

Numéro de pièce AD8361
Description  LF to 2.5 GHz TruPwr™ Detector
Télécharger  16 Pages
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Fabricant  AD [Analog Devices]
Site Internet  http://www.analog.com
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AD8361
–11–
REV. A
INPUT – mV
700
200
0
500
100
200
300
400
500
400
300
600
100
0
600
700
800
Figure 37. Idealized Output Step Size as Function of Input
Voltage
Plots of output voltage vs. input voltage result in a straight line. It
may sometimes be more useful to plot the error on a logarith-
mic scale, as shown in Figure 38. The deviation of the plot for
the ideal straight line characteristic is caused by output clipping
at the high end and by signal offsets at the low end. It should
however be noted that offsets at the low end can be either posi-
tive or negative, so that this plot could also trend upwards at the
low end. Figures 5, 6, 8, and 9 show a
±3 sigma distribution of
device error for a large population of devices.
INPUT – V rms
2.0
–0.5
0.01
0.5
0.0
1.5
1.0
–1.0
–1.5
–2.0
1.0
1.9GHz
2.5GHz
900MHz
100MHz
100MHz
0.02
(–21dBm)
0.1
(–7dBm)
0.4
(+5dBm)
Figure 38. Representative Unit, Error in dB vs. Input Level,
VS = 2.7 V
It is also apparent in Figure 38 that the error plot tends to
shift to the right with increasing frequency. Because the input
impedance decreases with frequency, the voltage actually applied
to the input will also tend to decrease (assuming a constant source
impedance over frequency). The dynamic range is almost con-
stant over frequency, but with a small decrease in conversion gain
at high frequency.
Input Coupling and Matching
The input impedance of the AD8361 decreases with increasing
frequency in both its resistive and capacitive components (Figure
13). The resistive component varies from 225
Ω at 100 MHz
down to about 95
Ω at 2.5 GHz.
A number of options exist for input matching. For operation at
multiple frequencies, a 75
Ω shunt to ground, as shown in Figure
39a, will provide the best overall match. For use at a single fre-
quency, a resistive or a reactive match can be used. By plotting the
input impedance on a Smith Chart, the best value for a
resistive match can be calculated. The VSWR can be held below
1.5 at frequencies up to 1 GHz, even as the input impedance
varies from part to part. (Both input impedance and input
capacitance can vary by up to
±20% around their nominal values.)
At very high frequencies (i.e., 1.8 GHz to 2.5 GHz), a shunt
resistor will not be sufficient to reduce the VSWR below 1.5.
Where VSWR is critical, remove shunt component and insert
an inductor in series with the coupling capacitor as shown in
Figure 39b.
Table II gives recommended shunt resistor values for various
frequencies and series inductor values for high frequencies. The
coupling capacitor, CC, essentially acts as an ac-short and plays
no intentional part in the matching.
AD8361
RFIN
RFIN
RSH
CC
a. Broadband Resistor Match
AD8361
CC
RFIN
RFIN
LM
b. Series Inductor Match
AD8361
CC
RFIN
RFIN
LM
CM
c. Narrowband Reactive Match
AD8361
CC
RFIN
RFIN
RSERIES
d. Attenuating the Input Signal
Figure 39. Input Coupling/Matching Options
Table II. Recommended Component Values for Resistive or
Inductive Input Matching (Figures 39a and 39b)
Frequency
Matching Component
100 MHz
63.4
Ω Shunt
800 MHz
75
Ω Shunt
900 MHz
75
Ω Shunt
1800 MHz
150
Ω Shunt or 4.7 nH Series
1900 MHz
150
Ω Shunt or 4.7 nH Series
2500 MHz
150
Ω Shunt or 2.7 nH Series




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