AD734

–9–

REV. C

A PRECISION AGC LOOP

The variable denominator of the AD734 and its high gain-

bandwidth product make it an excellent choice for precise

automatic gain control (AGC) applications. Figure 13 shows a

peak amplitude of from 10 mV to 10 V at any frequency from

100 Hz to 10 MHz, is applied to the X input, and a fixed posi-

form an integrator having a current summing node at its invert-

ing input. (The AD712 dual op amp is a suitable choice for this

application.) In the absence of an input, the current in D2 and

R2 causes the integrator output to ramp negative, clamped by

diode D3, which is included to reduce the time required for the

loop to establish a stable, calibrated, output level once the

circuit has received an input signal. With no input to the

denominator (U0 and U2), the gain of the AD734 is very high

(about 70 dB), and thus even a small input causes a substantial

output.

D3

1N914

R2

1M

R1

1M

C1

1 F

R3

1M

OP AMP = AD712 DUAL

1

2

3

4

5

6

7

10

8

9

11

13

12

14

W

ER

VN

VP

DD

Z1

Z2

X1

X2

U1

U2

U0

Y1

Y2

AD734

L

0.1 F

0.1 F

+15V

–15V

A1

NC

C2

1 F

A2

D2

1N914

+1V TO

+10V

D1

1N914

C1

1 F

L

Figure 13. Precision AGC Loop

Diode D1 and C1 form a peak detector, which rectifies the out-

put and causes the integrator to ramp positive. When the

current in R1 balances the current in R2, the integrator output

holds the denominator output at a constant value. This occurs

of +1 V to +10 V. The X input of the AD734, which has finite

offset voltage, could be troublesome at the output at high gains.

The output offset is reduced to that of the X input (one or two

millivolts) by the offset loop comprising R3, C3, and buffer A1.

The low pass corner frequency of 0.16 Hz is transformed to a

high-pass corner that is multiplied by the gain (for example,

160 Hz at a gain of 1000).

In applications not requiring operation down to low frequencies,

amplifier A1 can he eliminated, but the AD734’s input

resistance of 50 k

Ω between X1 and X2 will reduce the time

constant and increase the input offset. Using a non-polar 20

µF

tantalum capacitor for C1 will result in the same unity-gain

high-pass corner; in this case, the offset gain increases to 20, still

very acceptable.

Figure 14 shows the error in the output for sinusoidal inputs at

put error for any frequency between 300 Hz and 300 kHz is

similar to that for 100 kHz. At low signal frequencies and low

input amplitudes, the dynamics of the control loop determine

the gain error and distortion; at high frequencies, the 200 MHz

gain-bandwidth product of the AD734 limit the available gain.

more than +10 V.

+2

+1

0

–1

–2

10m

100m

1

10

INPUT AMPLITUDE – Volts

Figure 14. AGC Amplifier Output Error vs. Input Voltage

WIDEBAND RMS-DC CONVERTER

USING U INTERFACE

The AD734 is well suited to such applications as implicit RMS-

DC conversion, where the AD734 implements the function

V

avg V

IN

2

[]

V

RMS

(13)

using its direct divide mode. Figure 15 shows the circuit.

L

1/2

AD708

V

O

=

V

O

R1

3.32k

1

2

3

4

5

6

7

10

8

9

11

13

12

14

W

ER

VN

VP

DD

Z1

Z2

X1

X2

U1

U2

U0

Y1

Y2

AD734

0.1 F

0.1 F

+15V

–15V

U2a

C1

47 F

U1

L

L

L

C2

1 F

L

L

L

1/2

AD708

U2b

Figure 15. A 2-Chip, Wideband RMS-DC Converter

In this application, the AD734 and an AD708 dual op amp

serve as a 2-chip RMS-DC converter with a 10 MHz bandwidth.

Figure 16 shows the circuit’s performance for square-, sine-,

and triangle-wave inputs. The circuit accepts signals as high as

10 V p-p with a crest factor of 1 or 1 V p-p with a crest factor of

10. The circuit’s response is flat to 10 MHz with an input of

10 V, flat to almost 5 MHz for an input of 1 V, and to almost

1 MHz for inputs of 100 mV. For accurate measurements of

input levels below 100 mV, the AD734’s output offset (Z inter-

face) voltage, which contributes a dc error, must be trimmed out.

In Figure 15’s circuit, the AD734 squares the input signal, and

R1 and C1 and has a corner frequency of 1 Hz. Because of the

and the denominator in Equation (13). U2a and U2b, an

AD708 dual dc precision op amp, serve as unity-gain buffers,

supplying both the output voltage and driving the U interface.