Datasheet

REV. B
AD8309
–15–
sensitivity, but now a measure of selectively is simultaneously
introduced. Second, the component count is low: two capacitors
and an inexpensive chip inductor are needed. Third, the net-
work also serves as a balun. Analysis of this network shows that
the amplitude of the voltages at INHI and INLO are quite simi-
lar when the impedance ratio is fairly high (say, 50 Ω to 1000 Ω).
1
2
3
4
5
6
7
8
VLOG
VPS2
PADL
LMHI
LMLO
PADL
FLTR
LMDR
COM2
VPS1
PADL
INHI
INLO
PADL
COM1
ENBL
AD8309
9
10
11
14
15
16
0.1mF
10V
NC
R
LIM
RSSI
LIMITER
OUTPUT
0.1mF
10V
C2 = C
M
Z
IN
NC = NO CONNECT
12
13
V
S
C1 = C
M
L
M
Figure 33. High Frequency Input Matching Network
Figure 34 shows the response for a center frequency of 100 MHz.
The response is down by 50 dB at one-tenth the center frequency,
falling by 40 dB per decade below this. The very high frequency
attenuation is relatively small, however, since in the limiting
case it is determined simply by the ratio of the AD8309’s input
capacitance to the coupling capacitors. Table I provides solu-
tions for a variety of center frequencies f
C
and matching from
impedances Z
IN
of nominally 50 Ω and 100 Ω. Exact values are
shown, and some judgment is needed in utilizing the nearest
standard values.
Table I.
Match to 50 ⍀ Match to 100 ⍀
(Gain = 13 dB) (Gain = 10 dB)
f
C
C
M
L
M
C
M
L
M
MHz pF nH pF nH
10 140 3500 100.7 4790
10.7 133 3200 94.1 4460
15 95.0 2250 67.1 3120
20 71.0 1660 50.3 2290
21.4 66.5 1550 47.0 2120
25 57.0 1310 40.3 1790
30 47.5 1070 33.5 1460
35 40.7 904 28.8 1220
40 35.6 779 25.2 1047
45 31.6 682 22.4 912
50 28.5 604 20.1 804
60 23.7 489 16.8 644
80 17.8 346 12.6 448
100 14.2 262 10.1 335
120 11.9 208 8.4 261
150 9.5 155 6.7 191
200 7.1 104 5.03 125
250 5.7 75.3 4.03 89.1
300 4.75 57.4 3.36 66.8
350 4.07 45.3 2.87 52.1
400 3.57 36.7 2.52 41.8
450 3.16 30.4 2.24 34.3
500 2.85 25.6 2.01 28.6
FREQUENCY – MHz
14
60
DECIBELS
13
12
11
10
9
8
7
6
5
70 80 90 100 110 120 130
4
3
2
1
0
–1
140 150
GAIN
INPUT AT
TERMINATION
Figure 34. Response of 100 MHz Matching Network
General Matching Procedure
For other center frequencies and source impedances, the following
method can be used to calculate the basic matching parameters.
Step 1: Tune Out C
IN
At a center frequency f
C
, the shunt impedance of the input
capacitance C
IN
can be made to disappear by resonating with a
temporary inductor L
IN
, whose value is given by
L
IN
= 1/{(2 π f
C
)
2
C
IN
} = 10
10
/f
C
2
(8)
when C
IN
= 2.5 pF. For example, at f
C
= 100 MHz, L
IN
= 1 µH.
Step 2: Calculate C
O
and L
O
Now having a purely resistive input impedance, we can calculate
the nominal coupling elements C
O
and L
O
, using
C
fRR
L
RR
f
O
CINM
O
IN M
C
=
()
=
()
1
2
2
π
π
;
(9)
For the AD8309, R
IN
is 1 kΩ. Thus, if a match to 50 Ω is
needed, at f
C
= 100 MHz, C
O
must be 7.12 pF and L
O
must be
356 nH.
Step 3: Split C
O
Into Two Parts
Since we wish to provide the fully-balanced form of network
shown in Figure 33, two capacitors C1 = C2
each of nominally
twice C
O
, shown as C
M
in the figure, can be used. This requires
a value of 14.24 pF in this example. Under these conditions, the
voltage amplitudes at INHI and INLO will be similar. A some-
what better balance in the two drives may be achieved when C1
is made slightly larger than C2, which also allows a wider range
of choices in selecting from standard values. For example, ca-
pacitors of C1 = 15 pF and C2 = 13 pF may be used (making
C
O
= 6.96 pF).
Step 4: Calculate L
M
The matching inductor required to provide both L
IN
and L
O
is
just the parallel combination of these:
L
M
= L
IN
L
O
/(L
IN
+ L
O
) (10)
With L
IN
= 1 µH and L
O
= 356 nH, the value of L
M
to complete
this example of a match of 50 Ω at 100 MHz is 262.5 nH. The
nearest standard value of 270 nH may be used with only a slight
loss of matching accuracy. The voltage gain at resonance de-
pends only on the ratio of impedances, as is given by
GAIN
R
R
R
R
IN
S
IN
S
=








=






20 10log log
(11)