AD745KRZ-16 Ver la hoja de datos (PDF) - Analog Devices
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AD745KRZ-16 Datasheet PDF : 12 Pages
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AD745
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
= ∆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
110M⍀
(5 ؋ 22M⍀)
R2
9k⍀
R3
1k⍀
B AND K
4370 OR
EQUIVALENT
AD745
OUTPUT
0.8mV/pC
Figure 14. A Basic Accelerometer Circuit
C1
1250pF
R1
110M⍀
(5 ؋ 22M⍀)
R2
9k⍀
R3 C2
1k⍀ 2.2F
R4
18M⍀
AD711
R5
18M⍀
C3
2.2F
B AND K
4370 OR
EQUIVALENT
AD745
OUTPUT
0.8mV/pC
low frequency performance, the time constant of the servo loop
(R4C2 = R5C3) should be:
Time
Constant
≥ 10
R1
1
+
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 CC is
used, the circuit will have a low frequency cutoff determined by
an RC time constant equal to:
Time
Constant
≥
10
R1 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 17 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.
R2
1900⍀
R3
100⍀
CC
R4*
C1*
B AND K TYPE 8100 HYDROPHONE AD745
OUTPUT
CT
R1
108⍀ 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 = CT)
in series with the inverting input of the AD745.
R3
100⍀
R4*
108⍀
R2
1900⍀
C1*
AD745
OUTPUT
R4
16M⍀
C2
0.27F
R1 R5
108⍀ 100k⍀
CT
R6
1M⍀
AD711K
16M⍀
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
DC OUTPUT 1mV FOR IB (AD745) 100nA
*OPTIONAL, SEE TEXT
Figure 17. A Hydrophone Amplifier Incorporating a DC
Servo Loop
–10–
REV. D
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