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AD600

Dual, Low Noise, Wideband Variable Gain Amplifiers

Analog Devices 

APPLICATIONS INFORMATION

The full potential of any high performance amplifier can be

realized only by careful attention to details in its applications.

The following pages describe fully tested circuits in which many

such details have already been considered. However, as is always

true of high accuracy, high speed analog circuits, the schematic

is only part of the story; this is no less true for the AD600/

AD602. Appropriate choices in the overall board layout and the

type and placement of power supply decoupling components

are very important. As explained previously, the input grounds

A1LO and A2LO must use the shortest possible connections.

The following circuits show examples of time-gain control for

ultrasound and sonar, methods for increasing the output drive,

and AGC amplifiers for audio and RF/IF signal processing

using both peak and rms detectors. These circuits also illustrate

methods of cascading X-AMPs for either maintaining the

optimal SNR or maximizing the accuracy of the gain-control

voltage for use in signal measurement. These AGC circuits can

be modified for use as voltage-controlled amplifiers in sonar

and ultrasound applications by removing the detector and

substituting a DAC or other voltage source for supplying the

control voltage.

TIME-GAIN CONTROL (TGC) AND TIME-VARIABLE

GAIN (TVG)

Ultrasound and sonar systems share a similar requirement: both

need to provide an exponential increase in gain in response to a

linear control voltage, that is, a gain control that is linear in dB.

Figure 33 shows the AD600/AD602 configured for a control

voltage ramp starting at −625 mV and ending at +625 mV for a

gain-control range of 40 dB. The polarity of the gain-control

voltage can be reversed, and the control voltage inputs, C1HI

and C1LO, can be reversed to achieve the same effect. The gain-

control voltage can be supplied by a voltage output DAC, such

as the AD7244, which contains two complete DACs, operates

from ±5 V supplies, has an internal reference of +3 V, and

provides ±3 V of output swing. As such, it is well suited for use

with the AD600/AD602, needing only a few resistors to scale

the output voltage of the DACs to the levels needed by the

AD600/AD602.

AD600/AD602

CONTROL VOLTAGE,

VG

+625mV

0dB

40dB A1

GAIN

–625mV

VOLTAGE-OUTPUT

DAC

VG

C1LO

1

A1HI

2+

A1LO

3–

GAT1

4

GAT2 5

C1HI

16

A1CM

15

A1

A1OP

14

VPOS

13

+5V

REF

VNEG

12

–5V

A2LO 6 –

A2HI 7 +

11 A2OP

A2

10 A2CM

C2LO 8

AD600 OR

AD602

9 C2HI

Figure 33. The Simplest Application of the X-AMP Is as a TGC or TVG Amplifier

in Ultrasound or Sonar (Only A1 Connections Shown for Simplicity)

INCREASING OUTPUT DRIVE

The AD600/AD602 output stage has limited capability for

negative-load driving capability. For driving loads less than

500 Ω, the load drive can be increased by approximately 5 mA

by connecting a 1 kΩ pull-down resistor from the output to the

negative supply (see Figure 34).

DRIVING CAPACITIVE LOADS

For driving capacitive loads of greater than 5 pF, insert a 10 Ω

resistor between the output and the load. This lowers the

possibility of oscillation.

GAIN-CONTROL

VOLTAGE

C1LO

1

C1HI

16

A1HI

A1CM

VIN

2+

A1LO

15

A1

A1OP

3–

14

GAT1

4

VPOS

13

+5V 1kΩ

GAT2 5

REF

VNEG

12

ADDED

–5V PULL-DOWN

A2LO 6 –

11 A2OP

RESISTOR

A2

A2HI 7 +

10 A2CM

C2LO 8

AD600/

AD602

9 C2HI

Figure 34. Adding a 1 kΩ Pull-Down Resistor Increases the X-AMP Output

Drive by About 5 mA (Only A1 Connections Shown for Simplicity)

Rev. F | Page 15 of 32

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