AD745AN Analog Devices, AD745AN Datasheet - Page 10

• Manufacturer Part Number: AD745AN
• Description: Ultralow Noise/ High Speed/ BiFET Op Amp
• Manufacturer: Analog Devices

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:
The ratio of capacitor C1 to the internal capacitance (C
transducer determines the noise gain of this circuit (1 + C
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
Figure 35b. An Accelerometer Circuit Employing a DC
Servo Amplifier
EQUIVALENT
B&K MODEL
EQUIVALENT
Figure 35a. A Basic Accelerometer Circuit
4370 OR
B&K MODEL
4370 OR
V
OUT
(5x22M )
110M
(5x22M )
110M
R1
Q
AD745
C1
R1
OUT
*pC = PICOCOULOMBS
g = EARTH'S GRAVITATIONAL CONSTANT
AD745
R3
AD711
C1
R3
C1
1k
C2
1250pF
C3
1250pF
2.2 F
1k
9k
2.2 F
R2
18M
18M
R4
R2
R5
9k
OUTPUT = 0.8mV/pC
0.8mV/pC
OUTPUT
T
) of the
T
/C1).
–10–
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:
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 C
used, the circuit will have a low frequency cutoff determined by
an RC time constant equal to:
where the dc gain is 1 and the gain above the low frequency
cutoff (1/(2 C
of Figure 36b uses a dc servo loop to keep the dc output at 0 V
and to maintain full dynamic range for I
time constant of R7 and C1 should be larger than that of R1
and C
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
in series with the inverting input of the AD745.
Figure 36b. A Hydrophone Amplifier Incorporating a DC
Servo Loop
T
Figure 36a. A Low Noise Hydrophone Amplifier
B&K TYPE 8100 HYDROPHONE
100
HYDROPHONE
for a smooth low frequency response.
R3
B&K TYPE
100
8100
R3
C
C
Time Constant 10 R1 1
Time Constant
T
C
10
T
R4*
(100 ))) is equal to (1 + R2/R3). The circuit
DC OUTPUT
C
8
C
R4*
10
R1
8
R1
*OPTIONAL, SEE TEXT
10
1M
INPUT SENSITIVITY = –179dB RE. 1V/ Pa**
C1*
R6
8
1900
100k
1mV FOR I (AD745)
*OPTIONAL, SEE TEXT
** 1 VOLT PER MICROPASCAL
C1*
R5
R2
AD745
1900
R2
AD745
2
B
C
0.27 F
C
AD711K
B
1
’s up to 100 nA. The
R2
R3
100
100nA
16M
C2
R4
C1
OUTPUT
OUTPUT
16M
REV. C
T
C
)
is