AD600/AD602

REV. A

–12–

INPUT

1V RMS

MAX

(SINE WAVE)

0.1

µF

0.1

µF

FB

–6V

+6V

POWER SUPPLY

DECOUPLING NETWORK

+6V DEC

–6V DEC

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

REF

A1

A2

C1HI

A1CM

A1OP

VPOS

VNEG

A2OP

A2CM

C2HI

C1LO

A1HI

A1LO

GAT1

GAT2

A2LO

A2HI

C2LO

R3

133k

Ω

R2 200

Ω

U1 AD600

+6V DEC

–6V DEC

C2

2

µF

C1

0.1

µF

U3A

1/2

AD712

R1

115

Ω

C4

4.7

µF

R5

16.2k

Ω

1

2

3

4

5

6

7

14

13

12

11

10

9

8

NC

NC

NC

–6V DEC

U3B

R6

3.16k

Ω

R7

56.2k

Ω

+6V DEC

NC

NC

NC

+100mV/dB

0V = 0dB (AT 10mV RMS)

R4

3.01k

Ω

1/2

AD712

V

15.625mV/dB

G

+316.2mV

C3

1

µF

VINP

VNEG

CAVG

VLOG

BFOP

BFIN

VPOS

COMM

LDLO

VRMS

U2

AD636

AF/RF

OUTPUT

NC = NO CONNECT

FB

Figure 19. The Output of This Three-IC Circuit Is Proportional to the Decibel Value of the RMS Input

Note that the peak “log output” of

±4 V requires the use of

±6 V supplies for the dual op amp U3 (AD712) although lower

supplies would suffice for the AD600 and AD636. If only

±5 V

supplies are available, it will be either necessary to use a reduced

be only

±2 V) or restrict the dynamic range of the signal to

about 60 dB.

As in the previous case, the two amplifiers of the AD600 are

used in cascade. However, the 6 dB attenuator and low-pass fil-

ter found in Figure 1 are replaced by a unity gain buffer ampli-

fier U3A, whose 4 MHz bandwidth eliminates the risk of

instability at the highest gains. The buffer also allows the use of

a high impedance coupling network (C1/R3) which introduces a

high-pass corner at about 12 Hz. An input attenuator of 10 dB

(X0.316) is now provided by R1 + R2 operating in conjunction

with the AD600’s input resistance of 100

Ω. The adjustment

critical applications, but R1 and R2 may be replaced by a fixed

resistor of 215

Ω if very close calibration is not needed, since the

input resistance of the AD600 (and all other key parameters of it

and the AD636) are already laser trimmed for accurate opera-

tion. This attenuator allows inputs as large as

±4 V to be ac-

cepted, that is, signals with an rms value of 1 V combined with a

crest factor of up to 4.

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

ter formed by C2 and the 6.7 k

Ω input resistance of the AD636.

The averaging time constant for the rms-dc converter is deter-

mined by C4. The unbuffered output of the AD636 (at Pin 8) is

compared with a fixed voltage of +316 mV set by the positive

portional to this voltage, and systems requiring greater calibra-

tion 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 will be greater than the “setpoint” of 316 mV, causing the

inverting 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 amplifi-

that 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 ap-

plied 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

zero. This being the case, the gain of both A1 and A2 will be

20 dB and the output of the AD600 will therefore be 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, needing strong temperature

compensation for the internal “kT/q” scaling, these voltages,

and thus the output of this measurement system, are tempera-

ture 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 20 shows that the output is held very close to the

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