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AD745JN Datasheet PDF : 12 Pages
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AD745
TWO HIGH PERFORMANCE ACCELEROMETER
AMPLIFIERS
Two of the most popular charge-out transducers are hydro-
phones and accelerometers. Precision accelerometers are typi-
cally calibrated for a charge output (pC/g).* Figures 35a and
35b show two ways in which to configure the AD745 as a low
noise charge amplifier for use with a wide variety of piezoelectric
accelerometers. The input sensitivity of these circuits will be de-
termined by the value of capacitor C1 and is equal to:
∆V OUT
= ∆QOUT
C1
The ratio of capacitor C1 to the internal capacitance (CT) of the
transducer determines the noise gain of this circuit (1 + CT/C1).
The amplifiers voltage noise will appear at its output amplified
by this amount. The low frequency bandwidth of these circuits
will be dependent on the value of resistor R1. If a “T” network
is used, the effective value is: R1 (1 + R2/R3).
*pC = Picocoulombs
g = Earth’s Gravitational Constant
C1 1250pF
R1
R2
110MΩ
(5x22MΩ)
R3
9kΩ
1kΩ
B&K MODEL
4370 OR
EQUIVALENT
AD745
OUTPUT
0.8mV/pC
Figure 35a. A Basic Accelerometer Circuit
C1 1250pF
R1
R2
110MΩ
9kΩ
(5x22MΩ) R3 1kΩ
C2
2.2µF
18MΩ
AD711
R4
R5
18MΩ
C3 2.2µF
B&K MODEL
4370 OR
EQUIVALENT
AD745
OUTPUT = 0.8mV/pC
*pC = PICOCOULOMBS
g = EARTH'S GRAVITATIONAL CONSTANT
Figure 35b. An Accelerometer Circuit Employing a DC
Servo Amplifier
A dc servo loop (Figure 35b) can be used to assure a dc output
<10 mV, without the need for a large compensating resistor
when dealing with bias currents as large as 100 nA. For optimal
low frequency performance, the time constant of the servo loop
(R4C2 = R5C3) should be:
Time
Constant
≥ 10
R11
+
R2
R3
C1
A LOW NOISE HYDROPHONE AMPLIFIER
Hydrophones are usually calibrated in the voltage-out mode.
The circuit of Figures 36a can be used to amplify the output of
a typical hydrophone. If the optional ac coupling capacitor CC is
used, the circuit will have a low frequency cutoff determined by
an RC time constant equal to:
Time
Constant
=
2π
×
1
CC ×
100
Ω
where the dc gain is 1 and the gain above the low frequency
cutoff (1/(2π CC(100 Ω))) is equal to (1 + R2/R3). The circuit
of Figure 36b uses a dc servo loop to keep the dc output at 0 V
and to maintain full dynamic range for IB’s up to 100 nA. The
time constant of R7 and C1 should be larger than that of R1
and CT for a smooth low frequency response.
1900Ω
R3
100Ω
R4*
CC
R2
C1*
B&K TYPE 8100 HYDROPHONE AD745
OUTPUT
CT
10 8 Ω
R1 INPUT SENSITIVITY = –179dB RE. 1V/µPa**
*OPTIONAL, SEE TEXT
** 1 VOLT PER MICROPASCAL
Figure 36a. A Low Noise Hydrophone Amplifier
The transducer shown has a source capacitance of 7500 pF. For
smaller transducer capacitances (≤300 pF), lowest noise can be
achieved by adding a parallel RC network (R4 = R1, C1 = CT)
in series with the inverting input of the AD745.
R3
100Ω
10 8Ω
R4*
1900Ω
R2
C1*
OUTPUT
B&K TYPE
8100
HYDROPHONE
CT
AD745
R1 10 8Ω
R5
100kΩ
R6
1MΩ
16MΩ
R4
0.27µF
C2
AD711K
16MΩ
DC OUTPUT ≤ 1mV FOR IB (AD745) ≤ 100nA
*OPTIONAL, SEE TEXT
Figure 36b. A Hydrophone Amplifier Incorporating a DC
Servo Loop
–10–
REV. C
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