AD883B3 データシートの表示（PDF） - Analog Devices
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AD883B3 Datasheet PDF : 20 Pages
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AD600/AD602
GAIN-CONTROL
VOLTAGE
VG
C1LO
1
C1HI
16
A1HI
2
A1CM
15
A1LO
3
A1
A1OP
14
100Ω
GAT1
4
VPOS
13
+5V
VOUT
VIN
GAT2
REF
VNEG
5
12
–5V
A2LO
6
11 A2OP
100Ω
50Ω
A2HI
7
A2
A2CM
10
C2LO
8
C2HI
9
AD600 or AD602
Figure 13. An Ultralow Noise VCA Using the AD600 or
AD602
A Low Noise, 6 dB Preamplifier
In some ultrasound applications, the user may wish to use a
high input impedance preamplifier to avoid the signal attenua-
tion that would result 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 two is a suitable choice. It can be
shown that if the preamplifier’s overall referred-to-input (RTI)
noise is to be the same as that due to the X-AMP alone (1.4 nV/
√Hz), then the input noise of a X2 preamplifier must be √(3/4)
times as large, that is, 1.2 nV/√Hz.
+5V
R1
49.9Ω
1µF
R2
174Ω
Q1
MRF904
1µF
R4
42.2Ω
VIN
R5
42.2Ω
1µF
R3
562Ω
–5V
INPUT
+5V
GROUND
R6
562Ω
Q2
MM4049
0.1µF
0.1µF
100Ω
RIN OF X AMP
OUTPUT
GROUND
R7
174Ω 1µF
R8
49.9Ω
–5V
Figure 14. A Low Noise Preamplifier for the AD600 and
AD602
An inexpensive circuit, using complementary transistor types
chosen for their low rbb, is shown in Figure 14. The gain is de-
termined by the ratio of the net collector load resistance to the
net emitter resistance, that is, it is an open-loop amplifier. The
gain will be X2 (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, which is rendered less sensitive to sig-
nal level by the emitter resistors R4 and R5, which also play a
dominant role in setting the gain.
This is a Class AB amplifier. As VIN increases in a positive di-
rection, Q1 conducts more heavily and its re becomes lower
while that of Q2 increases. Conversely, more negative values of
VIN result in the re Of Q2 decreasing, while that of Q1 increases.
The design is chosen such that the net emitter resistance is es-
sentially independent of the instantaneous value of VIN, result-
ing in moderately low distortion. Low values of resistance and
moderately high bias currents are important in achieving the low
noise, wide bandwidth, and low distortion of this preamplifier.
Heavy decoupling prevents noise on the power supply lines from
being conveyed to the input of the X-AMP.
Table I. Measured Preamplifier Performance
Measurement
Value
Unit
Gain (f = 30 MHz)
Bandwidth (–3 dB)
Input Signal for
1 dB Compression
Distortion
VIN = 200 mV p-p
HD2
HD3
VIN = 500 mV p-p
HD2
HD3
System Input Noise
Spectral Density (NSD)
(Preamp plus X-AMP)
Input Resistance
Input Capacitance
Input Bias Current
Power Supply Voltage
Quiescent Current
6
250
1
0.27
0.14
0.44
0.58
1.03
1.4
15
± 150
±5
15
dB
MHz
V p-p
%
%
%
%
nV/√Hz
kΩ
pF
µA
V
mA
A Low Noise AGC Amplifier with 80 dB Gain Range
Figure 15 provides an example of the ease with which the
AD600 can be connected as an AGC amplifier. A1 and A2 are
cascaded, with 6 dB of attenuation introduced by the 100 Ω
resistor R1, while a time constant of 5 ns is formed by C1 and
the 50 Ω of net resistance at the input of A2. This has the dual
effect of (a) lowering the overall gain range from {0 dB to 80 dB}
to {6 dB to 74 dB} and (b) introducing a single-pole low-pass
filter with a –3 dB frequency of about 32 MHz. This ensures
stability at the maximum gain for a slight reduction in the over-
all bandwidth. The capacitor C4 blocks the small dc offset volt-
age at the output of A1 (which might otherwise saturate A2 at
its maximum gain) and introduces a high pass corner at about
8 kHz, useful in eliminating low frequency noise and spurious
signals which may be present at the input.
REV. A
–9–
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