AD8307
0.1µF
4.7Ω
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.ꢀ dB.
V , 2.7V TO 5.5V
P
AT ~8mA
C1
NC
5
8
7
6
50Ω INPUT
–88dBm TO
+3dBm
INP VPS ENB INT
L
M
AD8307
INM COM OFS OUT
Z
= 50Ω
1
2
3
4
IN
NC
OUTPUT
25mV/dB
NARROW-BAND MATCHING
C2
NC = NO CONNECT
Transformer coupling is useful in broadband applications.
However, 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.
Figure 35. High Frequency Input Matching Network
14
13
12
11
10
9
GAIN
This has several advantages. The same voltage gain is achieved,
providing increased sensitivity, but now 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.
8
7
6
5
4
3
INPUT
2
Figure 3ꢀ 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.
1
0
–1
60
70
80
90
100
110
120
130
140
150
FREQUENCY (MHz)
Figure 36. Response of 100 MHz Matching Network
Table 4 provides solutions for a variety of center frequencies
(FC) 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; this 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)
±60
82
30
±5
C2 (pF)
±50
75
27
±3
8.2
6.8
5.6
3.3
9±
LM (nH)
3300
±600
680
330
220
±50
±00
39
5600
2700
±000
430
260
±80
Voltage Gain (dB)
±0
20
50
±00
±50
200
250
500
±0
45
44
46
50
57
57
50
54
±03
±02
99
98
±0±
95
±3.3
±3.4
±3.4
±3.4
±3.2
±2.8
±2.3
±0.9
±0.4
±0.4
±0.6
±0.5
±0.3
±0.3
9.9
±0
7.5
6.2
3.9
±00
5±
22
±±
7.5
5.6
4.3
2.2
20
50
43
±8
±00
±50
200
250
500
9.±
6.2
4.7
3.9
2.0
92
±±4
±30
47
6.8
Rev. C | Page ±8 of 24
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