AD602 AD [Analog Devices], AD602 Datasheet - Page 8

• Manufacturer Part Number: AD602
• Description: Dual, Low Noise, Wideband Variable Gain Amplifiers
• Manufacturer: AD [Analog Devices]

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 and
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 for sonar, methods for increasing the output
drive, and AGC amplifiers for audio and RF/IF signal process-
ing using both peak and rms detectors. These circuits also illus-
trate methods of cascading X-AMPs for either maintaining the
optimal S/N ratio or maximizing the accuracy of the gain-
control voltage for use in signal measurement. These AGC cir-
cuits may be modified for use as voltage-controlled amplifiers
for use 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 11 shows the AD600/AD602 configured for a con-
trol voltage ramp starting at –625 mV and ending at +625 mV
for a gain-control range of 40 dB. For simplicity, only the A1
connections are shown. The polarity of the gain-control voltage
may be reversed and the control voltage inputs C1HI and
C1LO reversed to achieve the same effect. The gain-control
voltage can be supplied by a voltage-output DAC such as the
AD7242, which contains two complete DACs, operates from
AD600/AD602, needing only a few resistors to scale the output
voltage of the DACs to the levels needed by the AD600/AD602.
Figure 11. 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.
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
CONTROL VOLTAGE,
+625mV
0dB
V
G
–625mV
40dB
A1
GAIN
C1LO
A1LO
GAT1
GAT2
A2LO
C2LO
A1HI
A2HI
1
2
3
4
5
6
7
8
VOLTAGE-OUTPUT
AD600 or AD602
DAC
A1
A2
REF
15
14
13
12
16
11
10
9
C1HI
A1CM
A1OP
V+
V–
A2OP
A2CM
C2HI
+5V
–5V
V
G
–8–
Increasing Output Drive
The AD600/AD602’s output stage has limited capability for
negative-load driving capability. For driving loads less than
500 , the load drive may be increased by about 5 mA by con-
necting a 1 k pull-down resistor from the output to the nega-
tive supply (Figure 12).
Driving Capacitive Loads
For driving capacitive loads of greater than 5 pF, insert a 10
resistor between the output and the load. This lowers the possi-
bility of oscillation.
Figure 12. Adding a 1 k Pull-Down Resistor Increases the
X-AMP’s Output Drive by About 5 mA. Only the A1 Con-
nections Are Shown for Simplicity.
Realizing Other Gain Ranges
Larger gain ranges can be accommodated by cascading amplifi-
ers. Combinations built by cascading two amplifiers include
–20 dB to +60 dB (using one AD602), –10 dB to +70 dB (1/2
of an AD602 followed by 1/2 of an AD600), and 0 dB to 80 dB
(one AD600). In multiple-channel applications, extra protection
against oscillations can be provided by using amplifier sections
from different packages.
An Ultralow Noise VCA
The two channels of the AD600 or AD602 may be operated in
parallel to achieve a 3 dB improvement in noise level, providing
1 nV/ Hz without any loss of gain accuracy or bandwidth.
In the simplest case, as shown in Figure 13, the signal inputs
A1HI and A2HI are tied directly together, the outputs A1OP
and A2OP are summed via R1 and R2 (100
control inputs C1HI/C2HI and C1LO/C2LO operate in paral-
lel. Using these connections, both the input and output resis-
tances are 50 . Thus, when driven from a 50
terminated in a 50
gain range becomes –12 dB to +28 dB for the AD600 and
remains unaffected (1 V rms at the IC pins, or 2 V rms from an
unloaded 50
50
shared between the two channels, so the overall amplifier still
meets its specified maximum output and distortion levels for a
200
power of +10 dBm to the load.
–22 dB to +18 dB for the AD602. The peak input capability
V
IN
load, is effectively 200 , because the load current is
load. This amplifier can deliver a maximum sine wave
A1LO
GAT1
C1LO
GAT2
A2LO
C2LO
A1HI
A2HI
source). The loading on each output, with a
1
2
3
4
5
6
7
8
load, the gain is reduced by 12 dB, so the
GAIN-CONTROL
VOLTAGE
AD600
A1
A2
REF
15
14
13
12
10
16
11
9
C1HI
A1CM
A1OP
VPOS
VNEG
A2OP
A2CM
C2HI
+5V
each), and the
source and
–5V
1k
PULL-DOWN
RESISTOR
ADDED
REV. A