AD743

REV. C

–10–

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

∆Q

OUT

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 36a. A Basic Accelerometer Circuit

*pC = Picocoulombs

g = Earth's Gravitational Constant

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 R11+ 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

=

1

2

π× C

Figure 37a. A Basic Hydrophone Amplifier