AD8307 datasheet(21/24 Pages) AD

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AD8307

Rev. C | Page 21 of 24

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

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.

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.

ANTENNA

604Ω

100kΩ

1/2W

2kΩ

VR1

2kΩ

INT ±3dB

51pF

0.1µF

OUTPUT

LEAD-

THROUGH

CAPACITORS,

1nF

NC = NO CONNECT

+5V

VOUT

AD8307

INP VPS ENB INT

INM COM OFS OUT

50Ω INPUT

FROM P.A.

1µW TO

1kW

22Ω

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 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.

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.

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.

VPOS

GNEG

AD603

FDBK

GPOS

VNEG

COMM

VINP

VOUT

0.65V

0.15V TO 1.15V

OUTPUT

10mV/dB

20kΩ

80.6kΩ

0.1µF

BANDPASS

FILTER*

VR1

5kΩ

INT

±8dB

1nF

0.3V

2.3V

150pF

750nH

330Ω

464Ω

100kΩ

*FOR EXAMPLE: MURATA SFE10.7MS2G-A

NC = NO CONNECT

50Ω

INPUT

–105dBm

+15dBm

28kΩ

187kΩ

4.7Ω

AD8307

INP VPS ENB INT

INM COM OFS OUT

VN, –5V

VP, +5V

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/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

of 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 log10(50/0.2) 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.

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