AD8313ARM-REEL7(1999) Просмотр технического описания (PDF) - Analog Devices
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AD8313ARM-REEL7 Datasheet PDF : 16 Pages
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AD8313
A positive input step on VSET (indicating a demand for in-
creased power from the PA) will drive VOUT towards ground.
This should be arranged to increase the gain of the PA. The
loop will settle when VOUT settles to a voltage that sets the input
power to the AD8313 to the dB equivalent of VSET.
Input Coupling
The signal may be coupled to the AD8313 in a variety of ways.
In all cases, there must not be a dc path from the input pins to
ground. Some of the possibilities include: dual input coupling
capacitors, a flux-linked transformer, a printed-circuit balun,
direct drive from a directional coupler, or a narrow-band imped-
ance matching network.
Figure 30 shows a simple broadband resistive match. A termina-
tion resistor of 53.6 Ω combines with the internal input imped-
ance of the AD8313 to give an overall resistive input impedance
of approximately 50 Ω. The termination resistor should prefer-
ably be placed directly across the input pins, INHI to INLO,
where it serves to lower the possible deleterious effects of dc
offset voltages on the low end of the dynamic range. At low
frequencies, this may not be quite as attractive, since it necessi-
tates the use of larger coupling capacitors. The two 680 pF
input coupling capacitors set the high-pass corner frequency of
the network at 9.4 MHz.
50⍀ SOURCE
50⍀
C1
680pF
C2
680pF
RMATCH
53.6⍀
AD8313
CIN RIN
Figure 30. A Simple Broadband Resistive Input Termination
The high pass corner frequency can be set higher according to
the equation:
f3 dB
=
2
×
π
1
×C
× 50
where:
C
=
C1×
C1+
C2
C2
In high frequency applications, the use of a transformer, balun
or matching network is advantageous. The impedance match-
ing characteristics of these networks provide what is essentially a
gain stage before the AD8313 that increases the device sensitiv-
ity. This gain effect is further explored in the following match-
ing example.
Figures 31 and 32 show device performance under these three
input conditions at 900 MHz and 1900 MHz.
While the 900 MHz case clearly shows the effect of input
matching by realigning the intercept as expected, little improve-
ment is seen at 1.9 GHz. Clearly, if no improvement in sensitiv-
ity is required, a simple 50 Ω termination may be the best choice
for a given design based on ease of use and cost of components.
3
BALANCED
2
1
MATCHED
0
–1
TERMINATED
DR = 66dB
BALANCED
DR = 71dB
–2
MATCHED
DR = 69dB
–3
–90 –80 –70 –60 –50 –40 –30 –20 –10 0 10
INPUT AMPLITUDE – dBm
Figure 31. Comparison of Terminated, Matched and
Balanced Input Drive at 900 MHz
3
TERMINATED
DR = 75dB
2
MATCHED
1
TERMINATED
0
BALANCED
–1
MATCHED
DR = 73dB
–2
BALANCED
DR = 75dB
–3
–90 –80 –70 –60 –50 –40 –30 –20 –10 0 10
INPUT AMPLITUDE – dBm
Figure 32. Comparison of Terminated, Matched and
Balanced Input Drive at 1900 MHz
A Narrow-Band LC Matching Example at 100 MHz
While numerous software programs are available that allow the
values of matching components to be easily calculated, a clear
understanding of the calculations involved is valuable. A low
frequency (100 MHz) value has been used for this exercise
because of the deleterious board effects at higher frequencies.
RF layout simulation software is useful when board design at
higher frequencies is required.
A narrow-band LC match can be implemented either as a
series-inductance/shunt-capacitance or as a series-capacitance/
shunt-inductance. However, the concurrent requirement that the
AD8313 inputs, INHI and INLO, be ac-coupled, makes a
series-capacitance/shunt-inductance type match more appropri-
ate (see Figure 33).
50⍀ SOURCE
50⍀
C1
C2
LMATCH
AD8313
CIN RIN
Figure 33. Narrow-Band Reactive Match
REV. B
–11–
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