Datasheet

REV. D
AD745
–10–
TWO HIGH PERFORMANCE ACCELEROMETER
AMPLIFIERS
Two of the most popular charge-out transducers are hydrophones
and accelerometers. Precision accelerometers are typically cali-
brated for a charge output (pC/g).
* Figures 14 and 15 show two
ways in which to configure the AD745 as a low noise charge
amplifier for use with a wide variety of piezoelectric accelerom-
eters. The input sensitivity of these circuits will be determined
by the value of capacitor C1 and is equal to:
∆V
OUT
=
∆Q
OUT
C1
The ratio of capacitor C1 to the internal capacitance (C
T
) of the
transducer determines the noise gain of this circuit (1 + C
T
/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
R3
1k
R2
9k
R1
110M
(5  22M)
C1
1250pF
B AND K
4370 OR
EQUIVALENT
OUTPUT
0.8mV/pC
AD745
Figure 14. A Basic Accelerometer Circuit
R3
1k
R2
9k
R1
110M
(5  22M)
C1
1250pF
AD745
B AND K
4370 OR
EQUIVALENT
OUTPUT
0.8mV/pC
AD711
C2
2.2F
R4
18M
R5
18M
C3
2.2F
Figure 15. An Accelerometer Circuit Employing a DC
Servo Amplifier
A dc servo loop (Figure 15) 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 16 can be used to amplify the output of a
typical hydrophone. If the optional ac coupling capacitor C
C
is
used, the circuit will have a low frequency cutoff determined by
an RC time constant equal to:
Time Constant ≥
××
10 1
1
2 100
R
C
C
πΩ
where the dc gain is 1 and the gain above the low frequency
cutoff (1/(2π C
C
(100 Ω))) is equal to (1 + R2/R3). The circuit
of Figure 17 uses a dc servo loop to keep the dc output at 0 V
and to maintain full dynamic range for I
B
’s up to 100 nA. The
time constant of R7 and C1 should be larger than that of R1
and C
T
for a smooth low frequency response.
C1*
C
C
R3
100
R2
1900
R4*
C
T
R1
10
8

B AND K TYPE 8100 HYDROPHONE
AD745
OUTPUT
INPUT SENSITIVITY = –179dB RE. 1V/mPa**
*OPTIONAL DC BLOCKING CAPACITOR
**OPTIONAL, SEE TEXT
Figure 16. 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 = C
T
)
in series with the inverting input of the AD745.
C1*
R3
100
R2
1900
C
T
R4*
10
8

AD745
OUTPUT
AD711K
R1
10
8

16M
C2
0.27F
R5
100k
R4
16M
R6
1M
DC OUTPUT
1mV FOR IB (AD745) 100nA
*OPTIONAL, SEE TEXT
Figure 17. A Hydrophone Amplifier Incorporating a DC
Servo Loop