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AD743JR Scheda tecnica(PDF) 10 Page - Analog Devices |
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AD743JR Scheda tecnica(HTML) 10 Page - Analog Devices |
10 / 12 page REV. E –10– AD743 AN INPUT IMPEDANCE COMPENSATED, SALLEN-KEY FILTER The simple high-pass filter of Figure 13 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 pass-band ampli- tude 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. A 500k 500k 1000pF 1000pF +VS –VS Z 1000pF 1000pF 500k 500k Z Figure 13. 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 cali- brated for a charge output (pC/g). * Figures 14a and 14b show two ways in which to configure the AD743 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 Q C OUT OUT = 1 The ratio of capacitor C1 to the internal capacitance (CT) of the transducer determines the noise gain of this circuit (1 + CT/C1). The amplifier’s 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). AD743 R2 9k R1 110M (5 22M ) OUTPUT 0.8mV/pC* C1 1250pF R3 1k B AND K MODEL 4370 OR EQUIVALENT *pC = PICOCOULOMBS g = EARTH’S GRAVITATIONAL CONSTANT Figure 14a. Basic Accelerometer Circuit AD743 R3 1k R2 9k R4 18M R1 110M (5 22M ) R5 18M OUTPUT 0.8mV/pC C1 1250pF C3 2.2 F C2 2.2 F B AND K MODEL 4370 OR EQUIVALENT AD711 Figure 14b. Accelerometer Circuit Using a DC Servo Amplifier A dc servo loop (Figure 14b) 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 Cons R R R C tant ≥+ 10 1 1 2 3 1 LOW NOISE HYDROPHONE AMPLIFIER Hydrophones are usually calibrated in the voltage out mode. The circuits of Figures 15a and 15b can be used to amplify the output of a typical hydrophone. Figure 15a 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 15b, a variation of the original circuit, has a low frequency cutoff determined by an RC time constant equal to Time Cons t C C tan = ×× 1 2 100 πΩ R2 1900 R4* 108 R1 108 R3 100 OUTPUT AD743 *OPTIONAL, SEE TEXT INPUT SENSITIVITY = –179 dB re. 1V/ Pa** **1V PER MICROPASCAL B AND K TYPE 8100 HYDROPHONE CT C1* Figure 15a. Basic Hydrophone Amplifier |
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