AD8310ARMZ-REEL71 AD [Analog Devices], AD8310ARMZ-REEL71 Datasheet - Page 16

• Manufacturer Part Number: AD8310ARMZ-REEL71
• Description: Fast, Voltage-Out DC-440 MHz, 95 dB Logarithmic Amplifier
• Manufacturer: AD [Analog Devices]

AD8310
NARROW-BAND MATCHING
Transformer coupling is useful in broadband applications.
However, a magnetically coupled transformer might not be
convenient in some situations. Table 5 lists narrow-band
matching values.
Table 5. Narrow-Band Matching Values
F
(MHz)
10
20
50
100
150
200
250
500
10
20
50
100
150
200
250
500
At high frequencies, it is often preferable to use a narrow-band
matching network, as shown in Figure 31. This has several
advantages. The same voltage gain is achieved, providing
increased sensitivity, but a measure of selectivity is also
introduced. The component count is low: two capacitors and an
inexpensive chip inductor. Additionally, by making these
capacitors unequal, the amplitudes at INP and INM can be
equalized when driving from a single-sided source, that is, the
network also serves as a balun. Figure 32 shows the response for
a center frequency of 100 MHz; note the very high attenuation
at low frequencies. The high frequency attenuation is due to the
input capacitance of the log amp.
C
Z
(Ω)
45
44
46
50
57
57
50
54
103
102
99
98
101
95
92
114
IN
SIGNAL
Figure 31. Reactive Matching Network
INPUT
C1
(pF)
160
82
30
15
10
7.5
6.2
3.9
100
51
22
11
7.5
5.6
4.3
2.2
C1
C2
C2
(pF)
150
75
27
13
8.2
6.8
5.6
3.3
91
43
18
9.1
6.2
4.7
3.9
2.0
L
M
AD8310
INLO
INHI
L
(nH)
3300
1600
680
270
220
150
100
39
5600
2700
1000
430
260
180
130
47
M
8
1
Voltage Gain
(dB)
13.3
13.4
13.4
13.4
13.2
12.8
12.3
10.9
10.4
10.4
10.6
10.5
10.3
10.3
9.9
6.8
Rev. D | Page 16 of 24
GENERAL MATCHING PROCEDURE
For other center frequencies and source impedances, the
following steps can be used to calculate the basic matching
parameters.
Step 1: Tune Out C
At a center frequency, f
capacitance, C
temporary inductor, L
where C
Step 2: Calculate C
Now, having a purely resistive input impedance, calculate the
nominal coupling elements, C
For the AD8310, R
needed, at f
356 nH.
Step 3: Split C
To provide the desired fully balanced form of the network
shown in Figure 31, two capacitors C1 and C2, each of
nominally twice C
14.24 pF in this example. Under these conditions, the voltage
amplitudes at INHI and INLO are similar. A somewhat better
balance in the two drives can 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, capacitors of C1 = 15 pF and C2 = 13 pF can be
used, making C
L
C
14
13
12
11
10
–1
9
8
7
6
5
4
3
2
1
0
IN
O
60
IN
=
=
= 1.4 pF. For example, at f
Figure 32. Response of 100 MHz Matching Network
C
2
ω
π
= 100 MHz, C
70
2
f
IN
C
O
O
, can be made to disappear by resonating with a
1
C
into Two Parts
= 6.96 pF.
IN
O
80
1
R
IN
, can be used. This requires a value of
IN
IN
is 1 kΩ. Therefore, if a match to 50 Ω is
O
IN
and L
R
C
, whose value is given by
90
, the shunt impedance of the input
M
FREQUENCY (MHz)
O
;
100
must be 7.12 pF and L
O
O
L
and L
O
110
=
C
O
= 100 MHz, L
GAIN
INPUT
, using
120
(
R
2
IN
π
f
130
R
C
M
)
140
O
IN
must be
= 1.8 µH.
150