AD8307

Data Sheet

Rev. F | Page 22 of 24

2.50

2.25

0

2.00

1.25

1.00

0.50

WITH FILTER

ERROR

(WITH FILTER)

WITHOUT

FILTER

0.25

0.75

1.50

1.75

–2

–1

0

1

2

–100

–80

–60

–40

–20

20

0

INPUT LEVEL (dBm)

Figure 43. Results for 120 dB Measurement System

OPERATION AT LOW FREQUENCIES

The AD8307 provides excellent logarithmic conformance at

signal frequencies that can be arbitrarily low, depending only on

the values used for the input coupling capacitors. It can also be

desirable to add a low-pass input filter to desensitize the log

amp to HF signals. Figure 44 shows a simple arrangement, provid-

ing coupling with an attenuation of 20 dB; the intercept is shifted

up by this attenuation, from −84 dBm to −64 dBm, and the

input range is now 0.5 mV to 20 V (sine amplitude).

A high-pass 3 dB corner frequency of nominally 3 Hz is set by

the 10 μF coupling capacitors, C1 and C2, which are preferably

tantalum electrolytics (note the polarity) and a low-pass 3 dB

corner frequency of 200 kHz (set by C3 and the effective resis-

tance at the input of 1 kΩ). The −1% amplitude error points

occur at 20 Hz and 30 kHz. These are readily altered to suit other

applications by simple scaling. When C3 is zero, the low-pass

corner is at 200 MHz. Note that the lower end of the dynamic

range is improved by this capacitor, which essentially provides

an HF short circuit at the input. This significantly lowers the

wideband noise; the noise reduction is about 2 dB compared to

when the AD8307 is driven from a 50 Ω source. Ensure that the

output is free of postdemodulation ripple by lowering the low-pass

filter time constant, which is provided by C5. With the value shown

in Figure 44, the output time constant is 125 ms.

See Figure 40 for a more elaborate filter. To improve the law

conformance at very low signal levels and at low frequencies, add

C4 to the offset compensation loop.

C2

10µF

C1

10µF

AD8307

NC

INP VPS ENB INT

INM COM OFS OUT

NC = NO CONNECT

87

6

5

23

4

1

5V

R1

5kΩ

R2

5kΩ

0.1µF

C5

1µF

VOUT

25mV/dB

VIN

0.5mV TO

20V SINE

AMPLITUDE

C4

1µF

+

+

C3

750pF

4.7Ω

Figure 44. Connections for Low Frequency Operation