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參數資料
| 型號： | AD8307AR |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 運動控制電子 |
| 英文描述： | Low Cost DC-500 MHz, 92 dB Logarithmic Amplifier |
| 中文描述： | LOG OR ANTILOG AMPLIFIER, 490 MHz BAND WIDTH, PDSO8 |
| 封裝： | MS-012AA, SOIC-8 |
| 文件頁數： | 16/20頁 |
| 文件大小： | 394K |
| 代理商： | AD8307AR |
AD8307
–16–
REV. A
1
m
W to 1 kW 50
V
Power Meter
T he front-end adaptation shown in Figure 39 provides the mea-
surement of power being delivered from a transmitter final am-
plifier to an antenna. T he range has been set to cover the power
range –30 dBm (7.07 mV rms, or 1
μ
W) to +60 dBm (223 V rms,
or 1 kW). A nominal voltage attenuation ratio of 158:1 (44 dB) is
used; thus the intercept is moved from –84 dBm to –40 dBm and
the AD8307, scaled 0.25 V/decade of power, will now read 1.5 V
for a power level of 100 mW, 2.0 V at 10 W and 2.5 V at 1 kW.
T he general expression is:
P
(
dBm
) = 40 (
V
OUT
– 1)
T he required attenuation could be implemented using a capaci-
tive divider, providing a very low input capacitance, but it is
difficult to ensure accurate values of small capacitors. A better
approach is to use a resistive divider, taking the required precau-
tions to minimize spurious coupling into the AD8307 by placing
it in a shielded box, with the input resistor passing through a
hole in this box, as indicated in the figure. T he coupling capaci-
tors shown here are suitable for f
≥
10 MHz. A capacitor may be
added across the input pins of the AD8307 to reduce the re-
sponse to spurious HF signals which, as already noted, extends
to over 1 GHz.
T he mismatch caused by the loading of this resistor will be
trivial; only 0.05% of the power delivered to the load will be
absorbed by the measurement system, a maximum of 500 mW
at 1 kW. T he post-demodulation filtering and slope-calibration
arrangements should be chosen from other applications described
here, to meet the particular system requirements. T he 1 nF
capacitor lowers the risk of HF signals entering the AD8307 via
the load.
INP VPS ENB INT
AD8307
INM COM OFS OUT
TO
ANTENNA
V
P
604
V
100k
V
1/2W
NC
2k
V
51pF
VR1
2k
V
INT
6
3dB
51pF
22
V
0.1
m
F
NC
OUTPUT
LEAD-
THROUGH
CAPACITORS,
1nF
50
V
INPUT
FROM P.A.
1
m
W TO
1kW
1nF
NC = NO CONNECT
+5V
V
OUT
Figure 39. 1
μ
W to 1 kW 50-
Power Meter
Measurement System with 120 dB Dynamic Range
T he dynamic range of the AD8307 can be extended further—
from 90 dB to over 120 dB—by the addition of an X -AMP
such as the AD603. T his type of variable gain amplifier exhibits
a very exact exponential gain control characteristic, which is
another way of stating that the gain varies by a constant number
of decibels for a given change in the control voltage. For the
AD603, this scaling factor is 40 dB/V, or 25 mV/dB. It will be
apparent that this property of a linear-in-dB response is charac-
teristic of log amps; indeed, the AD8307 exhibits the same
scaling factor.
T he AD603 has a very low input referred noise: 1.3 nV/
√
Hz
at
its 100-
input, or 0.9 nV/
√
Hz
when matched to 50
, equivalent
to 0.4
μ
V rms, or –115 dBm, in a 200 kHz bandwidth. It is also
capable of handling inputs in excess of 1.4 V rms, or +16 dBm.
It is thus able to cope with a dynamic range of over 130 dB in
this particular bandwidth.
Now, if the gain control voltage for the X -AMP is derived from
the output of the AD8307, the effect will be to raise the gain of
this front-end stage when the signal is small and lower it when it
is large, but without altering the fundamental logarithmic nature
of the response. T his gain range is 40 dB, which, combined with
the 90 dB range of the AD8307, again corresponds to a 130 dB
range.
VPOS
GNEG
FDBK
GPOS
VNEG
COMM
VINP
VOUT
AD603
INP VPS ENB INT
AD8307
INM COM OFS OUT
0.65V
0.1
m
F
V
N
, –5V
0.15V TO 1.15V
OUTPUT
10mV/dB
NC
R6
20k
V
BANDPASS
FILTER*
VR1
5k
V
INT
6
8dB
R4
464
V
4.7
V
1nF
R7
80.6k
V
NC
0.3V
TO
2.3V
C1
150pF
L1
750nH
50
V
INPUT
–105dBm
TO
+15dBm
R2
28k
V
R1
187k
V
R3
330
V
R5
100k
V
V
P
, +5V
* E.G., MURATA SFE10.7MS2G-A
NC = NO CONNECT
Figure 40. 120 dB Measurement System
Figure 40 shows how these two parts can work together to pro-
vide state-of-the-art IF measurements in applications such as
spectrum/ network analyzers and other high dynamic range
instrumentation. T o understand the operation, note first that
the AD8307 is used to generate an output of about 0.3 V to
2.3 V. T his 2 V span is divided by 2 in R5/R6/R7 to provide the
1 V span needed by the AD603 to vary its gain by 40 dB. Note
that an increase in the positive voltage applied at GNEG (Pin 2)
lowers the gain. T his feedback network is tapped to provide a
convenient 10 mV/dB scaling at the output node, which may be
buffered if necessary.
T he center of the voltage range fed back to the AD603 is 650 mV
and the
±
20 dB gain range is centered by R1/R2. Note that the
intercept calibration of this system benefits from the use of a
well regulated 5 V supply. T o absorb the insertion loss of the
filter and center the full dynamic range, the intercept is adjusted
by varying the maximum gain of the AD603, using VR1.
Figure 41 shows the AD8307 output over the range –120 dBm
to +20 dBm and the deviation from an ideal logarithmic re-
sponse. T he dotted line shows the increase in the noise floor
that results when the filter is omitted; the decibel difference is
about 10log10(50/0.2) or 24 dB, assuming a 50 MHz band-
width from the AD603. An L-C filter may be used in place of
the ceramic filter used in this example.
X -AMP is a trademark of Analog Devices, Inc.
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