AD8307
characteristic of log amps; indeed, the AD8307 exhibits the
same scaling factor.
1 ꢀW TO 1 kW 50 Ω POWER METER
The front-end adaptation shown in Figure 41 provides the
measurement of power being delivered from a transmitter final
amplifier to an antenna. The range has been set to cover the
power range −30 dBm (7.07 mV rms, or 1 μW) to +ꢀ0 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, now reads 1.5 V for a power level of 100 mW, 2.0 V at
10 W and 2.5 V at 1 kW. The general expression is
The ADꢀ03 has a very low input referred noise: 1.3 nV/√Hz at its
100 Ω input, or 0.9 nV/√Hz when matched to 50 Ω, equivalent to
0.4 μV rms, or −115 dBm, in a 200 kHz bandwidth. It is also
capable of handling inputs in excess of 1.4 V rms, or +1ꢀ 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 is 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. This gain range is 40 dB, which, combined with the 90 dB
range of the AD8307, again corresponds to a 130 dB range.
P (dBm) = 40 (VOUT − 1)
The required attenuation could be implemented using a
capacitive 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
precautions 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 Figure 41. The
coupling capacitors shown here are suitable for f ≥ 10 MHz. A
capacitor can be added across the input pins of the AD8307 to
reduce the response to spurious HF signals, which, as already
noted, extends to over 1 GHz.
V , +5V
P
R1
187kΩ
R2
BANDPASS
FILTER*
4.7Ω
28kΩ
50Ω
0.1µF
INPUT
–105dBm
TO
0.65V
NC
5
+15dBm
8
7
6
1
2
3
8
GPOS VPOS
R3
INP VPS ENB INT
L1
750nH
330Ω
7
6
GNEG VOUT
AD603
VINP
AD8307
R4
464Ω
INM COM OFS OUT
VNEG
1
2
3
4
C1
150pF
VR1
5kΩ
INT
0.3V
TO
2.3V
NC
4
5
COMM FDBK
1nF
±8dB
R7
80.6kΩ
The mismatch caused by the loading of this resistor is trivial;
only 0.05% of the power delivered to the load is absorbed by the
measurement system, a maximum of 500 mW at 1 kW. The
post-demodulation filtering and slope calibration arrangements
are chosen from other applications described in this data sheet
to meet the particular system requirements. The 1 nF capacitor
lowers the risk of HF signals entering the AD8307 via the load.
R6
20kΩ
V
, –5V
N
0.15V TO 1.15V
OUTPUT
10mV/dB
R5
100kΩ
*FOR EXAMPLE: MURATA SFE10.7MS2G-A
NC = NO CONNECT
Figure 42. 120 dB Measurement System
Figure 42 shows how these two parts can work together to
provide state-of-the-art IF measurements in applications such
as spectrum/network analyzers and other high dynamic range
instrumentation. To understand the operation, note first that
the AD8307 is used to generate an output of about 0.3 V to
2.3 V. This 2 V span is divided by 2 in R5/Rꢀ/R7 to provide the
1 V span needed by the ADꢀ03 to vary its gain by 40 dB. Note
that an increase in the positive voltage applied at GNEG (Pin 2
of ADꢀ03) lowers the gain. This feedback network is tapped to
provide a convenient 10 mV/dB scaling at the output node,
which can be buffered if necessary.
TO
ANTENNA
100kΩ
1/2W
0.1µF
V
P
22Ω
+5V
51pF
NC
5
8
7
6
LEAD-
THROUGH
CAPACITORS,
1nF
VR1
2kΩ
INT ±3dB
INP VPS ENB INT
AD8307
50Ω INPUT
FROM P.A.
1µW TO
1kW
INM COM OFS OUT
604Ω
1
2
3
4
2kΩ
NC
V
OUT
51pF
OUTPUT
1nF
The center of the voltage range fed back to the ADꢀ03 is
ꢀ50 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. To absorb the insertion loss
of the filter and center the full dynamic range, the intercept is
adjusted by varying the maximum gain of the ADꢀ03, using
VR1. Figure 43 shows the AD8307 output over the range
−120 dBm to +20 dBm and the deviation from an ideal
logarithmic response. The dotted line shows the increase in the
noise floor that results when the filter is omitted; the decibel
difference is about 10 log10(50/0.2) or 24 dB, assuming a 50
MHz bandwidth from the ADꢀ03. An L-C filter can be used in
place of the ceramic filter used in this example.
NC = NO CONNECT
Figure 41. 1 μW to 1 kW 50 Ω Power Meter
MEASUREMENT SYSTEM WITH 120 dB DYNAMIC
RANGE
The 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 ADꢀ03. This 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
ADꢀ03, this scaling factor is 40 dB/V, or 25 mV/dB. It is
apparent that this property of a linear-in-dB response is
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