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參數(shù)資料
| 型號： | AD8307AR-REEL7 |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 10/24頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC LOGARITHMIC AMP 8-SOIC |
| 標準包裝： | 1 |
| 類型： | 對數(shù)放大器 |
| 應用： | 接收器信號強度指示（RSSI） |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 8-SOIC（0.154"，3.90mm 寬） |
| 供應商設備封裝： | 8-SO |
| 包裝： | 標準包裝 |
| 其它名稱： | AD8307AR-REEL7DKR |
AD8307
Rev. D | Page 18 of 24
INPUT MATCHING
Where higher sensitivity is required, an input matching network
is valuable. Using a transformer to achieve the impedance
transformation also eliminates the need for coupling capacitors,
which lowers the offset voltage generated directly at the input,
and balances the drives to Pin INP and Pin INM. The choice of
turns ratio depends somewhat on the frequency. At frequencies
below 50 MHz, the reactance of the input capacitance is much
higher than the real part of the input impedance. In this frequency
range, a turns ratio of about 1:4.8 lowers the input impedance to
50 Ω while raising the input voltage, thus lowering the effect of
the short-circuit noise voltage by the same factor. There is a
small contribution from the input noise current, so the total
noise is reduced by a lesser factor. The intercept is also lowered
by the turns ratio; for a 50 Ω match, it is reduced by 20 log10
(4.8) or 13.6 dB.
NARROW-BAND MATCHING
Transformer coupling is useful in broadband applications. How-
ever, a magnetically coupled transformer may not be convenient in
some situations. At high frequencies, it is often preferable to use
a narrow-band matching network, as shown in Figure 35.
Using a narrow-band matching network 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.
Further, by making these capacitors unequal, the amplitudes at
Pin INP and Pin INM can be equalized when driving from a
single-sided source, that is, the network also serves as a balun.
Figure 36 shows the response for a center frequency of 100 MHz.
Note the very high attenuation at low frequencies. The high fre-
quency attenuation is due to the input capacitance of the log amp.
C2
C1
OUTPUT
25mV/dB
01
08
2-
03
5
AD8307
50 INPUT
–88dBm TO
+3dBm
NC
INP VPS ENB INT
INM COM OFS OUT
NC
NC = NO CONNECT
ZIN = 50
87
6
5
23
4
1
4.7
0.1F
LM
VP, 2.7V TO 5.5V
AT ~8mA
Figure 35. High Frequency Input Matching Network
14
4
–1
3
2
1
0
INPUT
GAIN
9
8
7
6
5
13
12
11
10
01
08
2-
03
6
DE
CI
B
E
L
S
FREQUENCY (MHz)
60
150
140
130
120
110
100
90
80
70
Figure 36. Response of 100 MHz Matching Network
and matching impedances (ZIN) of nominally 50 Ω and 100 Ω.
The unequal capacitor values were chosen to provide a well-
balanced differential drive and to allow better centering of the
frequency response peak when using standard value components,
which generally results in a ZIN that is not exact. The full AD8307
HF input impedance and the inductor losses are included in the
modeling.
Table 4. Narrow-Band Matching Values
fC (MHz)
ZIN (Ω)
C1 (pF)
C2 (pF)
LM (nH)
Voltage Gain (dB)
10
45
160
150
3300
13.3
20
44
82
75
1600
13.4
50
46
30
27
680
13.4
100
50
15
13
330
13.4
150
57
10
8.2
220
13.2
200
57
7.5
6.8
150
12.8
250
50
6.2
5.6
100
12.3
500
54
3.9
3.3
39
10.9
10
103
100
91
5600
10.4
20
102
51
43
2700
10.4
50
99
22
18
1000
10.6
100
98
11
9.1
430
10.5
150
101
7.5
6.2
260
10.3
200
95
5.6
4.7
180
10.3
250
92
4.3
3.9
130
9.9
500
114
2.2
2.0
47
6.8
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