AD8307AN datasheet(22/24 Pages) AD

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AD8307

Rev. C | Page 22 of 24

2.50

2.25

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

–100

–80

–60

–40

–20

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 in order to desensitize

the log amp to HF signals. Figure 44 shows a simple arrangement,

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 resistance

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 post-

demodulation ripple by lowering the low-pass filter time constant.

This 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.

10µF

AD8307

INP VPS ENB INT

INM COM OFS OUT

NC = NO CONNECT

5kΩ

0.1µF

1µF

VOUT

25mV/dB

VIN

0.5mV TO

20V SINE

AMPLITUDE

1µF

750pF

4.7Ω

Figure 44. Connections for Low Frequency Operation

DC-COUPLED APPLICATIONS

It may occasionally be necessary to provide response to dc inputs.

Since the AD8307 is internally dc-coupled, there is no fundamental

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

constraint since its inputs must be positioned about 2 V above the

COM potential for proper biasing of the first stage. If the source is a

differential signal at this level, it can be directly connected to the

input. For example, a microwave detector can be ac-coupled at its

RF input and its baseband load then automatically provided by the

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

Usually, the source is a single sided ground-referenced signal;

thus, it is necessary to provide a negative supply for the

AD8307. This can be achieved as shown in Figure 45. The

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

necessary to provide an output interface that performs a

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

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

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

2.2 μV, for an input of 4 V. The device 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

VP INT NC

AD830

0.1µF

1µF

VIN

TEMP

+5V

4.7Ω

3.3kΩ

VR1

2kΩ

VR3

50kΩ

250Ω

VR2

50kΩ

32.4kΩ

1kΩ

R5*

*51kΩ FOR 20mV/dB; 5kΩ FOR 100mV/dB

20mV/dB

+5V FOR 20mV/dB

+10V FOR 50mV/dB

+15V FOR 100mV/dB

VOUT

AD589

2N3904

–5V

NC = NO CONNECT

–2V

0.1µF

Figure 45. Connections for DC-Coupled Applications

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