Data Sheet

AD8307

Rev. F | 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 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.

TO

ANTENNA

VP

604Ω

100kΩ

1/2W

NC

2kΩ

VR1

2kΩ

INT ±3dB

51pF

51pF

0.1µF

NC

OUTPUT

LEAD-

THROUGH

CAPACITORS,

1nF

1nF

NC = NO CONNECT

+5V

VOUT

AD8307

INP VPS ENB INT

INM COM OFS OUT

87

6

5

23

4

1

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.

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

NC

R6

20kΩ

R7

80.6kΩ

0.1µF

BANDPASS

FILTER*

VR1

5kΩ

INT

±8dB

1nF

NC

0.3V

TO

2.3V

C1

150pF

L1

750nH

R3

330Ω

R4

464Ω

R5

100kΩ

*FOR EXAMPLE: MURATA SFE10.7MS2G-A

NC = NO CONNECT

50Ω

INPUT

–105dBm

TO

+15dBm

1

2

3

4

8

7

6

5

R2

28kΩ

R1

187kΩ

4.7Ω

AD8307

INP VPS ENB INT

INM COM OFS OUT

87

6

5

23

4

1

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, 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 log10(50/0.2)

or 24 dB, assuming a 50 MHz bandwidth from the AD603. An

LC filter can be used in place of the ceramic filter used in this

example.