Moteur de recherche de fiches techniques de composants électroniques |
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AD515A Fiches technique(PDF) 4 Page - Analog Devices |
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AD515A Fiches technique(HTML) 4 Page - Analog Devices |
4 / 6 page AD515A –4– REV. A COAXIAL CABLE AND CAPACITANCE EFFECTS If it is not possible to attach the AD515A virtually on top of the signal source, considerable care should be exercised in designing the connecting lines carrying the high impedance signal. Shielded coaxial cable must be used for noise reduction, but use of coaxial cables for high impedance work can add problems from cable leakage, noise and capacitance. Only the best polyethylene or virgin teflon (not reconstituted) should be used to obtain the highest possible insulation resistance. Cable systems should be made as rigid and vibration free as possible since cable movement can cause noise signals of three types, all significant in high impedance systems. Frictional movement of the shield over the insulation material generates a charge that is sensed by the signal line as a noise voltage. Low noise cable with graphite lubricant such as Amphenol 21-537 will reduce the noise, but short rigid lines are better. Cable movements will also make small changes in the internal cable capacitance and capacitance to other objects. Since the total charge on these capacitances cannot be instantly changed, a noise voltage results, as predicted from: ∆V = Q/∆C. Noise voltage is also generated by the motion of a conductor in a magnetic field. The conductor-to-shield capacitance of coaxial cable is usually about 30 pF/foot. Charging this capacitance can cause consider- able stretching of high impedance signal rise time, thus cancel- ling the low input capacitance feature of the AD515A. There are two ways to circumvent this problem. For inverting signals or low level current measurements, the signal is carried on the line connected to the inverting input and shielded (guarded) by the ground line as shown in Figure 2. Since the signal is always at virtual ground, no voltage change is required and no capaci- tances are charged. In many circumstances, this will destabilize the circuit; if so, capacitance from output to inverting input will stabilize the circuit. Noninverting and buffer situations are more critical since the signal line voltage and therefore charge will change, causing signal delay. This effect can be considerably reduced by connecting the cable shield to a guard potential instead of ground, an option shown in Figure 3. Since such a connection results in positive feedback to the input, the circuit may be destabilized and oscillate. If so, capacitance from positive input to ground must be added to make the net capacitance at Pin 3 positive. This technique can considerably reduce the effective capacitance that must be charged. Typical Performance Curves Figure 4. PSRR and CMRR vs. Frequency Figure 5. Open Loop Frequency Response Figure 6. Input Common-Mode Range vs. Supply Voltage Figure 7. Peak-to-Peak Input Noise Voltage vs. Source Impedance and Bandwidth OBSOLETE |
Numéro de pièce similaire - AD515A_15 |
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Description similaire - AD515A_15 |
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