AD8310 Analog Devices, AD8310 Datasheet - Page 12

• Manufacturer Part Number: AD8310
• Description: Fast Response, DC - 440 Mhz, Voltage Out, 90 DB Logarithmic Amplifier
• Manufacturer: Analog Devices

AD8310
Table I shows the correction factors that should be applied to
measure the rms signal strength of a various signal types. A sine
wave input is used as a reference. To measure the rms power of
a square wave, for example, the mV equivalent of the dB value
given in the table (24 mV/dB times 3.01 dB) should be subtracted
from the output voltage of the AD8310.
Signal Type
Sine Wave
Square Wave or DC
Triangular Wave
GSM Channel (All Time Slots On)
CDMA Channel (Forward Link, 9
CDMA Channel (Reverse Link)
PDC Channel (All Time Slots On) 0.58 dB
Input Matching
Where higher sensitivity is required, an input matching net-
work is useful. Using a transformer to achieve the impedance
transformation also eliminates the need for coupling capacitors,
lowers the offset voltage generated directly at the input, and
balances the drive amplitude to INLO and INHI. The choice of
turns ratio will depend 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 will lower the input impedance
to 50
effect of the short circuit noise voltage by the same factor. The
intercept will also be lowered by the turns ratio; for a 50
match, it will be reduced by 20 log
noise will be reduced by a somewhat smaller factor because
there will be a small contribution from the input noise current.
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 29.
This has several advantages. The same voltage gain is achieved,
providing increased sensitivity, but now a measure of selectively
is also introduced. The component count is low: two capacitors
and an inexpensive chip inductor. Further, by making these
capacitors unequal the amplitudes at INP and INM may be
equalized when driving from a single-sided source; that is, the
network also serves as a balun. Figure 30 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.
Table I. Correction for Signals with Differing Crest Factors
Channels On)
while raising the input voltage, and thus lowering the
10
(4.8) or 13.6 dB. The total
Correction Factor
(Add to Measured Input
Level)
0 dB
–3.01 dB
0.9 dB
0.55 dB
3.55 dB
0.5 dB
–12–
F
MHz
10
20
50
100
150
200
250
500
10
20
50
100
150
200
250
500
C
Figure 30. Response of 100 MHz Matching Network
14
13
12
11
10
–1
9
8
7
6
5
4
3
2
1
0
60
Z
45
44
46
50
57
57
50
54
103
102
99
98
101
95
92
114
Figure 29. Reactive Matching Network
Table II. Narrow-Band Matching Values
IN
70
SIGNAL
INPUT
80
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
90
FREQUENCY – MHz
C1
C2
100
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
110
L
M
GAIN
INPUT
AD8310
INLO
120
INHI
L
nH
3300
1600
680
270
220
150
100
39
5600
2700
1000
430
260
180
130
47
M
130
140
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
150
REV. A