Datasheet AD743 (Analog Devices) - 10

AD743 AN INPUT IMPEDANCE COMPENSATED, SALLEN-KEY C1 1250pF FILTER
The simple high-pass filter of Figure 13 has an important source
R1
of error which is often overlooked. Even 5 pF of input capacitance
110M
⍀
R2 (5
ⴛ
22M
⍀
) 9k
⍀ in amplifier A will contribute an additional 1% of pass-band ampli- tude error, as well as distortion, proportional to the C/V characteristics
R3 C2
of the input junction capacitance. The addition of the network
1k
⍀
2.2
␮
F
designated Z will balance the source impedance—as seen by
R4
A—and thus eliminate these errors.
18M
⍀
AD711 +V Z S R5 500k
⍀
18M
⍀
C3 A 2.2
␮
F 1000pF 1000pF 500k
⍀
–VS 1000pF OUTPUT 500k
⍀
AD743 0.8mV/pC Z B AND K MODEL 500k
⍀
4370 OR 1000pF EQUIVALENT
Figure 13. Input Impedance Compensated Figure 14b. Accelerometer Circuit Using a DC Sallen-Key Filter Servo Amplifier A dc servo loop (Figure 14b) can be used to assure a dc output
TWO HIGH PERFORMANCE ACCELEROMETER
which is <10 mV, without the need for a large compensating
AMPLIFIERS
resistor when dealing with bias currents as large as 100 nA. For Two of the most popular charge-out transducers are hydrophones optimal low frequency performance, the time constant of the and accelerometers. Precision accelerometers are typically cali- servo loop (R4C2 = R5C3) should be 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  R2  Time Constant ≥ 10 R1 1 + C1 amplifier for use with a wide variety of piezoelectric accelerom-  R3 eters. The input sensitivity of these circuits will be determined by the value of capacitor C1 and is equal to
LOW NOISE HYDROPHONE AMPLIFIER
Hydrophones are usually calibrated in the voltage out mode. ∆ ∆Q V OUT = The circuits of Figures 15a and 15b can be used to amplify the OUT C1 output of a typical hydrophone. Figure 15a shows a typical The ratio of capacitor C1 to the internal capacitance (CT) of the dc-coupled circuit. The optional resistor and capacitor serve transducer determines the noise gain of this circuit (1 + CT/C1). to counteract the dc offset caused by bias currents flowing through The amplifier’s voltage noise will appear at its output amplified resistor R1. Figure 15b, a variation of the original circuit, has a by this amount. The low frequency bandwidth of these circuits low frequency cutoff determined by an RC time constant equal to will be dependent on the value of resistor R1. If a T network is 1 used, the effective value is R1(1 + R2/R3). Time Cons t tan = 2 π × × Ω C 100 C
C1 1250pF R3 R2 100
⍀
1900
⍀
R1 110M
⍀
R2 C1* (5
ⴛ
22M
⍀
) 9k
⍀
R3 R4* 1k
⍀
108
⍀
B AND K TYPE 8100 AD743 OUTPUT HYDROPHONE AD743 OUTPUT 0.8mV/pC* R1 B AND K MODEL CT 108
⍀
4370 OR EQUIVALENT *pC = PICOCOULOMBS INPUT SENSITIVITY = –179 dB re. 1V/
␮
Pa** g = EARTH’S GRAVITATIONAL CONSTANT *OPTIONAL, SEE TEXT
Figure 14a. Basic Accelerometer Circuit
**1V PER MICROPASCAL
Figure 15a. Basic Hydrophone Amplifier –10– REV. E Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PRODUCT HIGHLIGHTS CONNECTION DIAGRAMS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD SUSCEPTIBILITY ORDERING GUIDE Typical Performance Characteristics OP AMP PERFORMANCE: JFET VS. BIPOLAR DESIGNING CIRCUITS FOR LOW NOISE LOW NOISE CHARGE AMPLIFIERS HOW CHIP PACKAGE TYPE AND POWER DISSIPATION AFFECT INPUT BIAS CURRENT REDUCED POWER SUPPLY OPERATION FOR LOWER IB AN INPUT IMPEDANCE COMPENSATED, SALLEN-KEY FILTER TWO HIGH PERFORMANCE ACCELEROMETER AMPLIFIERS LOW NOISE HYDROPHONE AMPLIFIER BALANCING SOURCE IMPEDANCES OUTLINE DIMENSIONS Revision History