Datasheet AD745 (Analog Devices) - 10

AD745 TWO HIGH PERFORMANCE ACCELEROMETER
low frequency performance, the time constant of the servo loop
AMPLIFIERS
(R4C2 = R5C3) should be: Two of the most popular charge-out transducers are hydrophones   and accelerometers. Precision accelerometers are typically cali- Time Constant ≥10 R1 1+ R2  R3 C1 brated for a charge output (pC/g).* Figures 14 and 15 show two ways in which to configure the AD745 as a low noise charge amplifier for use with a wide variety of piezoelectric accelerom-
A LOW NOISE HYDROPHONE AMPLIFIER
eters. The input sensitivity of these circuits will be determined Hydrophones are usually calibrated in the voltage-out mode. by the value of capacitor C1 and is equal to: The circuit of Figures 16 can be used to amplify the output of a typical hydrophone. If the optional ac coupling capacitor CC is ∆ used, the circuit will have a low frequency cutoff determined by ∆ Q V = OUT OUT an RC time constant equal to: C1 1 The ratio of capacitor C1 to the internal capacitance (C Time Constant ≥ 10 1 R T) of the 2π × C Ω C × 100 transducer determines the noise gain of this circuit (1 + C T/C1). The amplifiers voltage noise will appear at its output amplified where the dc gain is 1 and the gain above the low frequency by this amount. The low frequency bandwidth of these circuits cutoff (1/(2π CC(100 Ω))) is equal to (1 + R2/R3). The circuit will be dependent on the value of resistor R1. If a “T” network of Figure 17 uses a dc servo loop to keep the dc output at 0 V is used, the effective value is: R1 (1 + R2/R3). and to maintain full dynamic range for IB’s up to 100 nA. The time constant of R7 and C1 should be larger than that of R1 *pC = Picocoulombs and CT for a smooth low frequency response. g = Earth’s Gravitational Constant
R2 C1 1900 1250pF R3 R1 100 R4* C1* 110M R2 CC (5 22M ) 9k R3 B AND K TYPE 8100 HYDROPHONE AD745 OUTPUT 1k C R1 T 108 INPUT SENSITIVITY = –179dB RE. 1V/mPa** OUTPUT AD745 B AND K 0.8mV/pC 4370 OR *OPTIONAL DC BLOCKING CAPACITOR EQUIVALENT **OPTIONAL, SEE TEXT
Figure 16. A Low Noise Hydrophone Amplifier The transducer shown has a source capacitance of 7500 pF. For Figure 14. A Basic Accelerometer Circuit smaller transducer capacitances (≤300 pF), lowest noise can be
C1
achieved by adding a parallel RC network (R4 = R1, C1 = CT)
1250pF
in series with the inverting input of the AD745.
R1 110M R2 R2 (5 22M ) 9k 1900 R3 R3 C2 100 R4* 1k 2.2 F C1* 108 OUTPUT R4 R4 18M 16M AD745 R5 AD711 C2 18M 0.27 F C3 2.2 F R1 R5 108 100k AD745 OUTPUT B AND K 0.8mV/pC AD711K 4370 OR C R6 EQUIVALENT T 1M 16M
Figure 15. An Accelerometer Circuit Employing a DC
DC OUTPUT 1mV FOR IB (AD745) 100nA
Servo Amplifier
*OPTIONAL, SEE TEXT
A dc servo loop (Figure 15) can be used to assure a dc output Figure 17. A Hydrophone Amplifier Incorporating a DC <10 mV, without the need for a large compensating resistor Servo Loop when dealing with bias currents as large as 100 nA. For optimal –10– REV. D