AD600/AD602
REALIZING OTHER GAIN RANGES
Larger gain ranges can be accommodated by cascading
amplifiers. Combinations built by cascading two amplifiers
include −20 dB to +60 dB (using one AD602), −10 dB to +70 dB
(using �½ of an AD602 followed by �½ of an AD600), and 0 dB to
80 dB (using one AD600). In multiple-channel applications,
extra protection against oscillation can be provided by using
amplifier sections from different packages.
A LOW NOISE, 6 dB PREAMPLIFIER
In some ultrasound applications, a high input impedance
preamplifier is needed to avoid the signal attenuation that
results from loading the transducer by the 100 Ω input resistance
of the X-AMP. High gain cannot be tolerated because the
peak transducer signal is typically ±0.5 V, while the peak input
capability of the AD600 or AD602 is only slightly more than
±1 V. A gain of 2 is a suitable choice. It can be shown that if the
preamplifier’s overall referred-to-input (RTI) noise is the same
as that due to the X-AMP alone (1.4 nV/√Hz), the input noise
of nX2 preamplifier must be √(3/4) times as large, that is,
1.2 nV/√Hz.
+5V
R1
49.9Ω
R2
174Ω
Q1
MRF904
1µF
R4
42.2Ω
V
IN
R5
42.2Ω
1µF
+5V
R6
562Ω
Q2
MM4049
R7
174Ω
R8
49.9Ω
–5V
R3
562Ω
–5V
INPUT
GROUND
0.1µF
100Ω
R
IN
OF X-AMP
1µF
AN ULTRALOW NOISE VCA
The two channels of the AD600 or AD602 can operate 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 35, the signal inputs
A1HI and A2HI are tied directly together. The outputs A1OP
and A2OP are summed via R1 and R2 (100 Ω each), and the
control inputs C1HI/C2HI and C1LO/C2LO operate in parallel.
Using these connections, both the input and output resistances
are 50 Ω. Thus, when driven from a 50 Ω source and terminated
in a 50 Ω load, the gain is reduced by 12 dB, so the gain range
becomes –12 dB to +28 dB for the AD600 and −22 dB to
+18 dB for the AD602. The peak input capability remains
unaffected (1 V rms at the IC pins, or 2 V rms from an
unloaded 50 Ω source). The loading on each output, with a
50 Ω load, is effectively 200 Ω, because the load current is
shared between the two channels, so the overall amplifier still
meets its specified maximum output and distortion levels for a
200 Ω load. This amplifier can deliver a maximum sine wave
power of 10 dBm to the load.
GAIN-CONTROL
VOLTAGE
V
G
– +
C1LO
A1HI
A1LO
GAT1
V
IN
GAT2
A2LO
A2HI
C2LO
C1HI
A1CM
A1OP
VPOS
VNEG
A2OP
A2CM
C2HI
00538-033
0.1µF
OUTPUT
GROUND
1µF
1
2
3
4
REF
5
–
6
7
8
A2
+
–
A1
16
15
14
13
12
11
10
Figure 36. A Low Noise Preamplifier for the AD600/AD602
100Ω
+5V
–5V
100Ω
V
OUT
50Ω
AD600 OR
AD602
9
Figure 35. An Ultralow Noise VCA Using the AD600 or AD602
An inexpensive circuit using complementary transistor types
chosen for their low r
bb
is shown in Figure 36. The gain is
determined by the ratio of the net collector load resistance to
the net emitter resistance. It is an open-loop amplifier. The gain
is ×2 (6 dB) only into a 100 Ω load, assumed to be provided by
the input resistance of the X-AMP; R2 and R7 are in shunt
with this load, and their value is important in defining the gain.
For small-signal inputs, both transistors contribute an equal
transconductance that is rendered less sensitive to signal level
by the emitter resistors, R4 and R5. They also play a dominant
role in setting the gain.
Rev. E | Page 16 of 28
00538-034
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AD [ ANALOG DEVICES ]
