AD743BQ Analog Devices, AD743BQ Datasheet - Page 10

• Manufacturer Part Number: AD743BQ
• Description: Ultralow Noise BiFET Op Amp
• Manufacturer: Analog Devices

AD743
AN INPUT-IMPEDANCE-COMPENSATED,
SALLEN-KEY FILTER
The simple high pass filter of Figure 35 has an important source
of error which is often overlooked. Even 5 pF of input capacitance
in amplifier “A” will contribute an additional 1% of passband
amplitude error, as well as distortion, proportional to the C/V
characteristics of the input junction capacitance. The addition
of the network designated “Z” will balance the source
impedance–as seen by “A”–and thus eliminate these errors.
TWO HIGH PERFORMANCE
ACCELEROMETER AMPLIFIERS
Two of the most popular charge-out transducers are hydrophones
and accelerometers. Precision accelerometers are typically
calibrated for a charge output (pC/g).* Figures 36a and 36b
show two ways in which to configure the AD743 as a low noise
charge amplifier for use with a wide variety of piezoelectric
accelerometers. The input sensitivity of these circuits will be
determined 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 35. An Input Impedance Compensated
Sallen-Key Filter
Figure 36a. A Basic Accelerometer Circuit
V
OUT
Q
C1
OUT
T
) of the
T
/C1).
–10–
A dc servo-loop (Figure 36b) can be used to assure a dc output
which is <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 circuits of Figures 37a and 37b can be used to amplify the
output of a typical hydrophone. Figure 37a shows a typical dc
coupled circuit. The optional resistor and capacitor serve to
counteract the dc offset caused by bias currents flowing through
resistor R1. Figure 37b, a variation of the original circuit, has a
low frequency cutoff determined by an RC time constant equal
to:
Figure 36b. An Accelerometer Circuit Employing a
DC Servo Amplifier
Figure 37a. A Basic Hydrophone Amplifier
Time Constant
Time Constant
10 R1 1
2
C
C
1
R2
R3
100
C1
REV. C