AD600/AD602
APPLICATIONS
The full potential of any high performance amplifier can only
be realized 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.
CONTROL VOLTAGE,
V
G
+625mV
0dB
VOLTAGE-OUTPUT
DAC
A1
GAIN
40dB
V
G
C1LO
A1HI
A1LO
GAT1
GAT2
A2LO
A2HI
C2LO
1
2
3
4
REF
5
–
6
7
8
A2
10 A2CM
+
–
A1
16
15
14
13
12
C1HI
A1CM
A1OP
VPOS
VNEG
+5V
–5V
–625mV
11 A2OP
AD600 OR
AD602
Figure 33. The Simplest Application of the X-AMP Is as a TGC or TVG Amplifier
in Ultrasound or Sonar. Only the A1 connections are shown for simplicity.
INCREASING OUTPUT DRIVE
The AD600/AD602’s 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).
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.
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
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.
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
V
IN
A1HI
A1LO
GAT1
GAT2
A2LO
A2HI
C2LO
1
2
3
4
REF
5
–
6
7
8
A2
+
–
A1
16
15
14
13
12
C1HI
A1CM
A1OP
VPOS
VNEG
–5V
+5V
1kΩ
ADDED
PULL-DOWN
RESISTOR
11 A2OP
10 A2CM
AD600/
AD602
Figure 34. Adding a 1 kΩ Pull-Down Resistor Increases the X-AMP’s Output
Drive by About 5 mA. Only the A1 connections are shown for simplicity.
Rev. E | Page 15 of 28
00538-032
9
C2HI
00538-031
+
9
C2HI
+
AD [ ANALOG DEVICES ]
