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AD8307

–17–

REV. A

INPUT LEVEL – dBm

2.50

2.25

–100

–80

–20

2.00

1.25

1.00

0.50

WITH FILTER

–60

–40

ERROR

(WITH FILTER)

WITHOUT

FILTER

0.25

0.75

1.50

1.75

–2

–1

Figure 41. Results for 120 dB Measurement System

Operation at Low Frequencies

The AD8307 provides excellent logarithmic conformance at

signal frequencies that may be arbitrarily low, depending only

on the values used for the input coupling capacitors. It may also

be desirable to add a low-pass input filter in order to desensitize

the log amp to HF signals. Figure 42 shows a simple arrange-

ment, providing 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 k

Ω). 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 dy-

namic range is improved by this capacitor, which provides es-

sentially an HF short circuit at the input, thus significantly

lowering the wideband noise; the noise reduction is about 2 dB

compared to the case when the AD8307 is driven from a 50

Ω

source.

To ensure that the output is free of post-demodulation ripple, it

is necessary to lower the low-pass filter time-constant. This is

provided by C5; with the value shown, the output time-constant

is 125 ms. (See also Figure 38 for a more elaborate filter). Finally,

to improve the law-conformance at very low signal levels and at

low frequencies, C4 has been added to the offset compensation

loop.

INP VPS ENB INT

INM COM OFS OUT

AD8307

+5V

750pF

10 F

1 F

10 F

0.1 F

4.7

1 F

VOUT

25mV/dB

0.5mV

TO 20V

SINE

AMPLITUDE

FOR SLOPE AND

INTERCEPT ADJUSTMENTS

SEE FIGURE 35

VIN

NC = NO CONNECT

Figure 42. Connections for Low Frequency Operation

DC-Coupled Applications

It may occasionally be necessary to provide response to dc in-

puts. Since the AD8307 is internally dc-coupled, there is no

fundamental reason why this is precluded. However, there is a

practical constraint, which is that its inputs must be positioned

about 2 V above the COM potential for proper biasing of the

first stage. If it happens that the source is a differential signal at

this level, it may be directly connected to the input. For ex-

ample, a microwave detector can be ac-coupled at its RF input

and its baseband load then automatically provided by the “float-

ing” RIN and CIN of the AD8307, at about VP/2.

Usually, the source will be a single-sided ground-referenced

signal, and it will thus be necessary to provide a negative supply

for the AD8307. This can be achieved as shown in Figure 43.

The output is now referenced to this negative supply, and it is

necessary to provide an output interface that performs a differ-

ential-to-single-sided conversion. This is the purpose of the

AD830. The slope may be arranged to be 20 mV/dB, when the

output ideally runs from zero, for a dc input of 10

µV, to +2.2 V

for an input of 4 V. The AD8307 is fundamentally insensitive to

the sign of the input signal, but with this biasing scheme, the

maximum negative input is constrained to about –1.5 V. The

transfer function after trimming and with R7 = 0, is

VOUT = (0.4 V) log10 (VIN /10 µV)

INP VPS ENB INT

INM COM OFS OUT

AD8307

INT

AD830

0.1 F

1 F

VIN

TEMP

3.3k

+5V

4.7

VR2

50k

* 51k

FOR

20mV/dB

5k FOR

100mV/dB

20mV/dB

+5V FOR 20mV/dB

+10V FOR 50mV/dB

+15V FOR 100mV/dB

VOUT

R7,R8:

SEE TEXT

AD589

2N3904

VR1

0.1 F

–5V

32.4k

250

–5V

VR3

50k

NC = NO CONNECT

–2V

Figure 43. Connections for DC-Coupled Applications

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