AD8307-EB Analog Devices Inc, AD8307-EB Datasheet - Page 21

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

Specifications of AD8307-EB

• Lead Free Status / Rohs Status: Not Compliant

1 μW 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 +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,
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 required attenuation can 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 minim-
ize 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 in Figure 41
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 previously noted, extends to over 1 GHz.
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
postdemodulation 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.
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
AD603. 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 AD603, this scaling
factor is 40 dB/V, or 25 mV/dB. It is apparent that this property
of a linear-in-dB response is characteristic of log amps; indeed,
the AD8307 exhibits the same scaling factor.
ANTENNA
50Ω INPUT
FROM P.A.
1µW TO
1kW
TO
P (dBm) = 40 (V
INT ±3dB
100kΩ
1/2W
Figure 41. 1 μW to 1 kW, 50 Ω Power Meter
NC = NO CONNECT
604Ω
51pF
51pF
VR1
2kΩ
OUT
− 1)
INM COM OFS OUT
INP VPS ENB INT
8
1
AD8307
7
2
0.1µF
NC
6
3
1nF
NC
5
4
CAPACITORS,
22Ω
2kΩ
THROUGH
OUTPUT
LEAD-
1nF
V
P
+5V
V
OUT
Rev. D | Page 21 of 24
The AD603 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 +16 dBm. It is
thus able to cope with a dynamic range of over 130 dB in this
particular bandwidth.
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.
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, R6, and 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 of the AD603) 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.
The 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. 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 AD603, 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 log
or 24 dB, assuming a 50 MHz bandwidth from the AD603. An
L-C filter can be used in place of the ceramic filter used in this
example.
50Ω
INPUT
–105dBm
TO
+15dBm
150pF
750nH
C1
L1
0.65V
R5
100kΩ
28kΩ
1
2
3
4
R2
Figure 42. 120 dB Measurement System
GPOS
GNEG
VINP
COMM
AD603
*FOR EXAMPLE: MURATA SFE10.7MS2G-A
VPOS
VOUT
VNEG
FDBK
187kΩ
R1
8
7
6
5
V
0.15V TO 1.15V
N
NC = NO CONNECT
, –5V
BANDPASS
R3
330Ω
VR1
5kΩ
INT
±8dB
FILTER*
464Ω
R4
1nF
INM COM OFS OUT
INP VPS ENB INT
4.7Ω
8
1
20kΩ
R6
AD8307
7
2
80.6kΩ
NC
AD8307
6
3
R7
OUTPUT
10mV/dB
10
0.1µF
(50/0.2)
NC
V
5
4
P
, +5V
0.3V
TO
2.3V