AD743 Datasheet, PDF (10/12 Pages) Analog Devices – Ultralow Noise BiFET Op Amp

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.
Figure 35. An Input Impedance Compensated
Sallen-Key Filter
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:
∆ 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).
Figure 36b. An Accelerometer Circuit Employing a
DC Servo Amplifier
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:
Time
Constant
≥ 10 R1^{}1 +
R2
R3

C1
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:
Time Constant
=
2π
1
× C_C × 100Ω
Figure 36a. A Basic Accelerometer Circuit
*pC = Picocoulombs
g = Earth's Gravitational Constant
–10–
Figure 37a. A Basic Hydrophone Amplifier
REV. C