AD606

REV. B

–9–

Low Frequency Applications

With reasonably sized input coupling capacitors and an optional

input low-pass filter, the AD606 can operate to frequencies as

low as 200 Hz with good log conformance. Figure 8 shows the

schematic, with the low-pass filter included in the dashed box.

This circuit should be built inside a die cast box and the signal

brought in through a coaxial connector. The circuit must also

have a low-pass filter to reject the attenuated RF signals that

would otherwise be rectified along with the desired signal and

be added to the log output. The shielded and filtered circuit has

a 90 dB dynamic range, as shown in Figure 9.

In this circuit, R4 and R5 form a 20 dB attenuator that extends

the input range to 10 V rms. R3 isolates loads from VLOG.

Capacitors C1 and C2 (4.7

µF each), R1, R2, and the AD606’s

input resistance of 2.5 k

Ω form a 100 Hz high-pass filter that is

before the AD606; the corner frequency of this filter must be

well below the lowest frequency of interest. In addition, the

offset-correction loop introduces another pole at low signal

levels that is transformed into another high-pass filter because it

is in a feedback path. This indicates that there has to be a

gradual transition from a 40 dB roll off at low signal levels to a

20 dB roll off at high signal levels, at which point the feedback

low pass filter is effectively disabled since the incoming signal

swamps the feedback signal.

This low-pass filter introduces some attenuation due to R1 and

R2 in conjunction with the 2.5 k

Ω input resistance of the

AD606. To minimize this effect, the value of R1 and R2 should

be kept as small as possible–100

Ω is a good value since it bal-

ances the need to reduce the attenuation as mentioned above

with the requirement for R1 and R2 to be much larger then the

impedance of C1 and C2 at the low-pass corner frequency, in

our case about 1 MHz.

4

–80

40

3

1

–60

2

020

–20

–40

INPUT SIGNAL – dBm

0

100Hz

3.5V

1kHz – 10MHz

90dB

Figure 9. Performance of Low Frequency Circuit at 100 Hz

and 1 kHz to 10 MHz (Note Attenuation)

C1

4.7 F

TO

DVM

R3

1k

+5V

AD606JN

NC

0.1 F

NC

C4

4.7 F

NC

R1

100

LOW-PASS

FILTER

R2

100

C3

680pF

R4

453

20dB

ATTENUATOR

R5

51.1

AC

INPUT

C2

4.7 F

+

+

DIECAST BOX

NC = NO CONNECT

Figure 8. Circuit for Low Frequency Measurements