AD8361ART-EVAL(2000) View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD8361ART-EVAL Datasheet PDF : 16 Pages
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AD8361
700
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500
400
300
200
100
0
0
100 200 300 400 500 600 700 800
INPUT – mV
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.
2.0
1.5
1.0
0.5
100MHz
0.0
–0.5
–1.0
100MHz
2.5GHz
1.9GHz
–1.5
–2.0
0.01
0.02
(–21dBm)
900MHz
0.1
(–7dBm)
INPUT – V rms
0.4
1.0
(+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.
RFIN
CC
RSH
RFIN
AD8361
a. Broadband Resistor Match
RFIN
LM
CC
RFIN
AD8361
b. Series Inductor Match
RFIN
CM
CC
RFIN
LM
AD8361
c. Narrowband Reactive Match
RFIN
RSERIES
CC
RFIN
AD8361
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
800 MHz
900 MHz
1800 MHz
1900 MHz
2500 MHz
63.4 Ω Shunt
75 Ω Shunt
75 Ω Shunt
150 Ω Shunt or 4.7 nH Series
150 Ω Shunt or 4.7 nH Series
150 Ω Shunt or 2.7 nH Series
REV. A
–11–
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