AD600 Ver la hoja de datos (PDF) - Analog Devices

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componentes Descripción

Fabricante

AD600

Dual, Low Noise, Wideband Variable Gain Amplifiers

Analog Devices 

AD600/AD602

INPUT

1V rms

MAX

(SINE WAVE)

C1

0.1µF

R1

115Ω

CAL

0dB

R2 200Ω

R3

133kΩ

U3A

1/2

AD712

C1LO

1

A1HI

2+

A1LO

3–

GAT1

4

GAT2

5

A1

REF

A2LO

6–

A2HI

A2

7+

C2LO

8

U1

AD600

C1HI

16

A1CM

15

A1OP

14

VPOS

13

VNEG

12

A2OP

11

A2CM

10

C2HI

9

+6V

DEC

–6V

DEC

VG

15.625mV/dB

R4

3.01kΩ

R5

16.2kΩ

Vrms

AF/RF

OUTPUT

C4

4.7µF

+6V DEC

–6V

DEC

C2

2µF

1 VIN

+VS 14

2

NC

U2

AD636

NC

13

3 –VS

NC 12

4 CAV

NC 11

5 dB

COM 10

6 BUF OUT RL 9

7 BUF IN IOUT 8

R6

3.16kΩ

U3B

1/2

AD712

+6V

R7

56.2kΩ

FB

+6V

DEC

–6V

DEC

0.1µF

0.1µF

+316.2mV

C3

1µF

FB

–6V

POWER SUPPLY

DECOUPLING

NETWORK

VOUT

+100mV/dB

0V = 0dB (AT 10mV rms)

NC = NO CONNECT

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

positive supply voltage of 6 V and the R6 and R7 resistors. VREF

is 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 U3B

op amp, with a time constant of 3 ms formed by the parallel

sum of R6/R7 and C3. If the output of the AD600 is too high,

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

of U3B—that is, VOUT—to ramp up (note that the integrator is

noninverting). A fraction of VOUT is connected to the inverting

gain-control inputs of the AD600, 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

1 dB—requires a change of 100 mV at VOUT. Note here that,

because 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 that 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

system operates as claimed, VOUT (and, hence, VG) should be 0.

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

425

400

375

350

325

300

275

250

225

200

175

150

10µ

100µ

1m

10m

100m

1

10

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

Rev. F | Page 20 of 32

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