AD8313 View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD8313 Datasheet PDF : 24 Pages
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Data Sheet
AD8313
Typically, the AD8313 needs to be matched to 50 Ω. The input
impedance of the AD8313 at 100 MHz can be read from the
Smith chart (Figure 26) and corresponds to a resistive input
impedance of 900 Ω in parallel with a capacitance of 1.1 pF.
To make the matching process simpler, the AD8313 input cap-
acitance, CIN, can be temporarily removed from the calculation
by adding a virtual shunt inductor (L2), which resonates away
CIN (Figure 36). This inductor is factored back into the calculation
later. This allows the main calculation to be based on a simple
resistive-to-resistive match, that is, 50 Ω to 900 Ω.
The resonant frequency is defined by the equation
ω=
1
L2 × CIN
therefore,
L2
=
1
ω2 CIN
= 2.3 µH
50Ω SOURCE
50Ω
C1
AD8313
L1
L2
C2
CIN RIN
CMATCH
=
(C1
(C1
×
+
C2)
C2)
LMATCH
=
(C1
(C1
×
+
C2)
C2)
TEMPORARY
INDUCTANCE
Figure 36. Input Matching Example
With CIN and L2 temporarily out of the picture, the focus is now
on matching a 50 Ω source resistance to a (purely resistive) load
of 900 Ω and calculating values for CMATCH and L1. When
RS RIN
=
L1
CMATCH
the input looks purely resistive at a frequency given by
f0 =
1
= 100 MHz
2π L1 × CMATCH
Solving for CMATCH gives
C MATCH =
1 × 1 = 7.5 pF
RS RIN 2πf0
Solving for L1 gives
L1 = RS RIN = 337.6 nH
2πf0
Because L1 and L2 are parallel, they can be combined to give
the final value for LMATCH, that is,
L MATCH
=
L1 × L2
L1 + L2
= 294 nH
C1 and C2 can be chosen in a number of ways. First, C2 can be
set to a large value, for example, 1000 pF, so that it appears as an
RF short. C1 would then be set equal to the calculated value of
CMATCH. Alternatively, C1 and C2 can each be set to twice CMATCH
so that the total series capacitance is equal to CMATCH. By making
C1 and C2 slightly unequal (that is, select C2 to be about 10%
less than C1) but keeping their series value the same, the ampli-
tude of the signals on INHI and INLO can be equalized so that
the AD8313 is driven in a more balanced manner. Any of the
options detailed above can be used provided that the combined
series value of C1 and C2, that is, C1 × C2/(C1 + C2) is equal to
CMATCH.
In all cases, the values of CMATCH and LMATCH must be chosen from
standard values. At this point, these values need now be installed
on the board and measured for performance at 100 MHz.
Because of board and layout parasitics, the component values
from the preceding example had to be tuned to the final values
of CMATCH = 8.9 pF and LMATCH = 270 nH as shown in Table 5.
Assuming a lossless matching network and noting conservation
of power, the impedance transformation from RS to RIN (50 Ω to
900 Ω) has an associated voltage gain given by
Gain dB= 20 × log
RIN = 12.6 dB
RS
Because the AD8313 input responds to voltage and not to true
power, the voltage gain of the matching network increases the
effective input low-end power sensitivity by this amount. Thus,
in this case, the dynamic range is shifted downward, that is, the
12.6 dB voltage gain shifts the 0 dBm to −65 dBm input range
downward to −12.6 dBm to −77.6 dBm. However, because of
network losses, this gain is not be fully realized in practice.
Refer to Figure 33 and Figure 34 for an example of practical
attainable voltage gains.
Table 5 shows recommended values for the inductor and cap-
acitors in Figure 35 for some selected RF frequencies in addition
to the associated theoretical voltage gain. These values for a
reactive match are optimal for the board layout detailed as
Figure 45.
As previously discussed, a modification of the board layout
produces networks that may not perform as specified. At
2.5 GHz, a shunt inductor is sufficient to achieve proper
matching. Consequently, C1 and C2 are set sufficiently high
that they appear as RF shorts.
Table 5. Recommended Values for C1, C2, and LMATCH in
Figure 35
Freq. CMATCH
(MHz) (pF)
C1
C2
LMATCH
(pF) (pF) (nH)
Voltage
Gain(dB)
100 8.9
22
15
270
12.6
1000 270
900 1.5
3
3
8.2
9.0
1.5
1000 8.2
1900 1.5
3
3
2.2
6.2
1.5
1000 2.2
2500 Large
390
390
2.2
3.2
Rev. E | Page 17 of 24
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