AD602 AD [Analog Devices], AD602 Datasheet - Page 9

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

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
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/
times as large, that is, 1.2 nV/ Hz.
REV. A
V
Figure 14. A Low Noise Preamplifier for the AD600 and
AD602
IN
Hz), then the input noise of a X2 preamplifier must be (3/4)
V
IN
42.2
42.2
1 F
1 F
R4
R5
49.9
49.9
GAT1
GAT2
C2LO
174
C1LO
A1LO
A2LO
174
A1HI
A2HI
R1
R2
R7
R8
+5V
–5V
–5V
+5V
1
2
3
4
5
6
7
8
Q1
MRF904
R3
562
R6
562
Q2
MM4049
GAIN-CONTROL
1 F
AD600 or AD602
1 F
VOLTAGE
INPUT
GROUND
V
G
A2
A1
REF
15
14
13
12
10
16
11
9
C1HI
A1CM
A1OP
VPOS
VNEG
A2OP
A2CM
C2HI
0.1 F
0.1 F
+5V
–5V
GROUND
OUTPUT
100
100
100
R
IN
OF X AMP
V
OUT
50
–9–
An inexpensive circuit, using complementary transistor types
chosen for their low r
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
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 V
rection, Q1 conducts more heavily and its r
while that of Q2 increases. Conversely, more negative values of
V
The design is chosen such that the net emitter resistance is es-
sentially independent of the instantaneous value of V
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.
Measurement
Gain (f = 30 MHz)
Bandwidth (–3 dB)
Input Signal for
Distortion
System Input Noise
Input Resistance
Input Capacitance
Input Bias Current
Power Supply Voltage
Quiescent Current
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
resistor R1, while a time constant of 5 ns is formed by C1 and
the 50
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.
IN
1 dB Compression
V
V
Spectral Density (NSD)
(Preamp plus X-AMP)
IN
IN
result in the r
= 200 mV p-p
= 500 mV p-p
Table I. Measured Preamplifier Performance
of net resistance at the input of A2. This has the dual
e
Of Q2 decreasing, while that of Q1 increases.
bb
, is shown in Figure 14. The gain is de-
HD2
HD3
HD2
HD3
IN
increases in a positive di-
AD600/AD602
Value
6
250
1
0.27
0.14
0.44
0.58
1.03
1.4
15
15
load, assumed to be
150
5
e
becomes lower
IN
, result-
Unit
dB
MHz
V p-p
%
%
%
%
nV/ Hz
k
pF
V
mA
A