AD600/AD602

Rev. E | Page 20 of 28

C1HI

A1CM

A1OP

VPOS

VNEG

A2OP

A2CM

C2HI

C1LO

A1HI

A1LO

GAT1

GAT2

A2LO

A2HI

C2LO

1

2

3

4

5

6

7

14

13

12

11

10

9

8

U2

AD636

VINP

VNEG

CAVG

VLOG

BFOP

BFIN

VPOS

COMM

LDLO

INPUT

1V rms

MAX

(SINE WAVE)

R2 200Ω

R3

133kΩ

U3A

1/2

AD712

R4

3.01kΩ

R5

16.2kΩ

C1

0.1µF

C2

2µF

NC

NC

NC

NC

NC

NC

AF/RF

OUTPUT

C4

4.7µF

+6V DEC

R7

56.2kΩ

R6

3.16kΩ

C3

1µF

U3B

1/2

AD712

+316.2mV

VOUT

+100mV/dB

0V = 0dB (AT 10mV rms)

NC = NO CONNECT

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

REF

A1

A2

+

–

U1

AD600

FB

FB

+6V

–6V

+6V

DEC

–6V

DEC

0.1µF

0.1µF

POWER SUPPLY

DECOUPLING

NETWORK

CAL

0dB

+6V

DEC

–6V

DEC

–6V

DEC

R1

115Ω

15.625mV/dB

Figure 41. The Output of This Three-IC Circuit Is Proportional to the Decibel Value of the rms Input

The output of A2 is ac-coupled via another 12 Hz high-pass

filter formed by C2 and the 6.7 kΩ input resistance of the

AD636. The averaging time constant for the rms-dc converter

is determined by C4. The unbuffered output of the AD636 (at

Pin 8) is compared with a fixed voltage of 316 mV set by the

proportional to this voltage, and systems requiring greater

calibration accuracy should replace the supply dependent

reference with a more stable source.

Any difference in these voltages is integrated by the op amp

U3B, with a time constant of 3 ms formed by the parallel sum

of R6/R7 and C3. Now, if the output of the AD600 is too high,

V rms is greater than the setpoint of 316 mV, causing the output

gain-control inputs of the AD600, so causing the gain to be

reduced, as required, until V rms is exactly equal to 316 mV, at

which time the ac voltage at the output of A2 is forced to be

exactly 316 mV rms. This fraction is set by R4 and R5 such that

a 15.625 mV change in the control voltages of A1 and A2—

which would change the gain of the cascaded amplifiers by

since A2 is forced to operate at an output level well below its

capacity, waveforms of high crest factor can be tolerated

throughout the amplifier.

To check the operation, assume an input of 10 mV rms is

applied to the input, which results in a voltage of 3.16 mV rms

at the input to A1, due to the 10 dB loss in the attenuator. If the

This being the case, the gain of both A1 and A2 is 20 dB and the

output of the AD600 is therefore 100 times (40 dB) greater than

its input, which evaluates to 316 mV rms, the input required at

the AD636 to balance the loop. Finally, note that unlike most

AGC circuits that need strong temperature compensation for

the internal kT/q scaling, these voltages, and thus the output of

this measurement system, are temperature stable, arising

directly from the fundamental and exact exponential

attenuation of the ladder networks in the AD600.

Typical results are presented for a sine wave input at 100 kHz.

Figure 42 shows that the output is held close to the setpoint of

316 mV rms over an input range in excess of 80 dB.

450

300

150

10µ

100µ

10

1

100m

10m

1m

225

375

350

200

275

425

325

175

250

400

INPUT SIGNAL (V rms)

Figure 42. RMS Output of A2 Held Close to the Setpoint 316 mV

for an Input Range of over 80 dB