Datasheet AD8307 (Analog Devices) - 22

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Data Sheet AD8307 1 μW TO 1 kW 50 Ω POWER METER
The AD603 has a very low input-referred noise: 1.3 nV/√Hz at its The front-end adaptation shown in Figure 41 provides the 100 Ω input, or 0.9 nV/√Hz when matched to 50 Ω, equivalent to measurement of power being delivered from a transmitter final 0.4 μV rms, or −115 dBm, in a 200 kHz bandwidth. It is also amplifier to an antenna. The range has been set to cover the capable of handling inputs in excess of 1.4 V rms, or +16 dBm. It is power range −30 dBm (7.07 mV rms, or 1 μW) to +60 dBm thus able to cope with a dynamic range of over 130 dB in this (223 V rms, or 1 kW). A nominal voltage attenuation ratio of particular bandwidth. 158:1 (44 dB) is used; thus the intercept is moved from −84 dBm to If the gain control voltage for the X-AMP is derived from the −40 dBm and the AD8307, scaled 0.25 V/decade of power, now output of the AD8307, the effect is to raise the gain of this front- reads 1.5 V for a power level of 100 mW, 2.0 V at 10 W, and end stage when the signal is small and lower it when it is large, 2.5 V at 1 kW. The general expression is but without altering the fundamental logarithmic nature of the P (dBm) = 40 (V response. This gain range is 40 dB, which, combined with the 90 dB OUT − 1) range of the AD8307, again corresponds to a 130 dB range. The required attenuation can be implemented using a capacitive
VP, +5V
divider, providing a very low input capacitance, but it is difficult to
R2 R1 187kΩ BANDPASS 4.7Ω 50Ω 28kΩ
ensure accurate values of small capacitors. A better approach is
FILTER* 0.1µF INPUT –105dBm
to use a resistive divider, taking the required precautions to minim-
0.65V TO NC +15dBm
ize spurious coupling into the AD8307 by placing it in a shielded
1 GPOS VPOS 8 R3 8 7 6 5 L1 330Ω INP VPS ENB INT
box with the input resistor passing through a hole in this box, as
750nH 2 GNEG VOUT 7 R4 AD8307
indicated in Figure 41. The coupling capacitors shown in Figure 41
AD603 464Ω 3 VINP VNEG 6 INM COM OFS OUT
are suitable for f ≥ 10 MHz. A capacitor can be added across the
C1 VR1 1 2 3 4 0.3V 150pF 5kΩ NC
input pins of the AD8307 to reduce the response to spurious HF
4 COMM FDBK 5 TO INT 1nF 2.3V ±8dB
signals, which, as previously noted, extends to over 1 GHz.
R7 R6 80.6kΩ VN, –5V 20kΩ
The mismatch caused by the loading of this resistor is trivial;
0.15V TO 1.15V R5 OUTPUT
42 0 only 0.05% of the power delivered to the load is absorbed by the
100kΩ 10mV/dB
2-
*FOR EXAMPLE: MURATA SFE10.7MS2G-A
08 01 measurement system, a maximum of 500 mW at 1 kW. The
NC = NO CONNECT
postdemodulation filtering and slope calibration arrangements Figure 42. 120 dB Measurement System are chosen from other applications described in this data sheet Figure 42 shows how these two parts can work together to to meet the particular system requirements. The 1 nF capacitor provide state-of-the-art IF measurements in applications such lowers the risk of HF signals entering the AD8307 via the load. as spectrum/network analyzers and other high dynamic range
TO ANTENNA
instrumentation. To understand the operation, note first that the AD8307 is used to generate an output of about 0.3 V to
100kΩ 1/2W 0.1µF VP 22Ω
2.3 V. This 2 V span is divided by 2 in R5, R6, and R7 to provide
51pF +5V
the 1 V span needed by the AD603 to vary its gain by 40 dB.
NC 8 7 6 5
Note that an increase in the positive voltage applied at GNEG
VR1 LEAD- INP VPS ENB INT 2kΩ THROUGH
(Pin 2 of the AD603) lowers the gain. This feedback network is
CAPACITORS, INT ±3dB AD8307 1nF
tapped to provide a convenient 10 mV/dB scaling at the output
50Ω INPUT 604Ω INM COM OFS OUT FROM P.A. 1 2 3 4
node, which can be buffered if necessary.
1µW TO 2kΩ NC V 1kW OUT 51pF 1nF OUTPUT
The center of the voltage range fed back to the AD603 is 650 mV, 1 04 and the ±20 dB gain range is centered by R1/R2. Note that the
NC = NO CONNECT
82- 010 intercept calibration of this system benefits from the use of a Figure 41. 1 μW to 1 kW, 50 Ω Power Meter well-regulated 5 V supply. To absorb the insertion loss of the
MEASUREMENT SYSTEM WITH 120 dB DYNAMIC
filter and center the full dynamic range, the intercept is adjusted
RANGE
by varying the maximum gain of the AD603, using VR1. Figure 43 shows the AD8307 output over the range −120 dBm to +20 dBm The dynamic range of the AD8307 can be extended further from and the deviation from an ideal logarithmic response. The 90 dB to over 120 dB by the addition of an X-AMP® such as the dotted line shows the increase in the noise floor that results when AD603. This type of variable gain amplifier exhibits a very exact the filter is omitted; the decibel difference is about 10 log10(50/0.2) exponential gain control characteristic, which is another way of or 24 dB, assuming a 50 MHz bandwidth from the AD603. An stating that the gain varies by a constant number of decibels for LC filter can be used in place of the ceramic filter used in this a given change in the control voltage. For the AD603, this scaling example. 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. Rev. E | Page 21 of 24 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS LOG AMP THEORY PROGRESSIVE COMPRESSION DEMODULATING LOG AMPS INTERCEPT CALIBRATION OFFSET CONTROL EXTENSION OF RANGE INTERFACES ENABLE INTERFACE INPUT INTERFACE OFFSET INTERFACE OUTPUT INTERFACE THEORY OF OPERATION BASIC CONNECTIONS INPUT MATCHING NARROW-BAND MATCHING SLOPE AND INTERCEPT ADJUSTMENTS APPLICATIONS INFORMATION BUFFERED OUTPUT FOUR-POLE FILTER 1 µW TO 1 kW 50 Ω POWER METER MEASUREMENT SYSTEM WITH 120 dB DYNAMIC RANGE OPERATION AT LOW FREQUENCIES OUTLINE DIMENSIONS ORDERING GUIDE