AD8307-EB Analog Devices Inc, AD8307-EB Datasheet - Page 18

• Manufacturer Part Number: AD8307-EB
• Manufacturer: Analog Devices Inc

Specifications of AD8307-EB

• Lead Free Status / Rohs Status: Not Compliant

AD8307
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 log
(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.
Table 4. Narrow-Band Matching Values
f
10
20
50
100
150
200
250
500
10
20
50
100
150
200
250
500
C
(MHz)
Z
45
44
46
50
57
57
50
54
103
102
99
98
101
95
92
114
IN
(Ω)
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
10
Rev. D | Page 18 of 24
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
Table 4 provides solutions for a variety of center frequencies (f
and matching impedances (Z
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 Z
HF input impedance and the inductor losses are included in the
modeling.
–88dBm TO
50Ω INPUT
+3dBm
14
13
12
11
10
–1
9
8
7
6
5
4
3
2
1
0
L
3300
1600
680
330
220
150
100
39
5600
2700
1000
430
260
180
130
47
60
Figure 35. High Frequency Input Matching Network
Figure 36. Response of 100 MHz Matching Network
M
(nH)
NC = NO CONNECT
70
Z
IN
C2
C1
= 50Ω
80
L
M
90
FREQUENCY (MHz)
IN
0.1µF
INM COM OFS OUT
INP VPS ENB INT
IN
8
1
that is not exact. The full AD8307
) of nominally 50 Ω and 100 Ω.
100
AD8307
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
7
2
4.7Ω
110
NC
6
3
GAIN
INPUT
NC
5
4
120
V
AT ~8mA
OUTPUT
25mV/dB
130
P
, 2.7V TO 5.5V
140
150
C
)