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AD515ALH Fiches technique(PDF) 3 Page - Analog Devices |
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AD515ALH Fiches technique(HTML) 3 Page - Analog Devices |
3 / 6 page –3– REV. A AD515A LAYOUT AND CONNECTIONS CONSIDERATIONS The design of very high impedance measurement systems in- troduces a new level of problems associated with the reduction of leakage paths and noise pickup. 1. A primary consideration in high impedance system designs is to attempt to place the measuring device as near to the signal source as possible. This will minimize current leakage paths, noise pickup and capacitive loading. The AD515A, with its combination of low offset voltage (normally eliminating the need for trimming), low quiescent current (minimal source heating, possible battery operation), internal compensation and small physical size lends itself to installation at the signal source or inside a probe. As a result of the high load capaci- tance rating, the AD515A can comfortably drive a long signal cable. 2. The use of guarding techniques is essential to realizing the capability of the ultralow input currents of the AD515A. Guarding is achieved by applying a low impedance bootstrap potential to the outside of the insulation material surround- ing the high impedance signal line. This bootstrap potential is held at the same level as that of the high impedance line; therefore, there is no voltage drop across the insulation and, hence, no leakage. The guard will also act as a shield to reduce noise pickup and serves an additional function of reducing the effective capacitance to the input line. The case of the AD515A is brought out separately to Pin 8 so it can also be connected to the guard potential. This technique virtually eliminates potential leakage paths across the package insulation, provides a noise shield for the sensitive circuitry and reduces common-mode input capacitance to about 0.8 pF. Figure 1 shows a proper printed circuit board layout for input guarding and connecting the case guard. Figures 2 and 3 show guarding connections for typical inverting and noninverting applications. If Pin 8 is not used for guarding, it should be connected to ground or a power supply to reduce noise. Figure 1. Board Layout for Guarding Inputs with Guarded TO-99 Package 3. Printed circuit board layout and construction is critical for achieving the ultimate in low leakage performance that the AD515A can deliver. The best performance will be realized by using a teflon IC socket for the AD515A; at a minimum a teflon standoff should be used for the high impedance lead. If this is not feasible, the input guarding scheme shown in Figure 1 will minimize leakage as much as possible; the guard ring should be applied to both sides of the board. The guard ring is connected to a low impedance potential at the same level as the inputs. High impedance signal lines should not be extended for any unnecessary length on a printed circuit; to minimize noise and leakage, they must be carried in rigid, shielded cables. 4. Another important concern for achieving and maintaining low leakage currents is complete cleanliness of circuit boards and components. Completed assemblies should be washed thoroughly in a low residue solvent such as TMC Freon or high purity methanol, followed by a rinse with deionized water and nitrogen drying. If service is anticipated in a high contaminant or high humidity environment, a high dielectric conformal coating is recommended. All insulation materials except Kel-F or teflon will show rapid degradation of surface leakage at high humidities. Figure 2. Picoampere Current-to-Voltage Converter Inverting Configuration Figure 3. Very High Impedance Noninverting Amplifier INPUT PROTECTION The AD515A is guaranteed for a maximum safe input potential equal to the power supply potential. Many instrumentation situations, such as flame detectors in gas chromatographs, involve measurement of low level currents from high voltage sources. In such applications, a sensor fault condition may apply a very high potential to the input of the current-to-voltage converting amplifier. This possibility necessi- tates some form of input protection. Many electrometer type devices, especially CMOS designs, can require elaborate Zener protection schemes that often compromise overall performance. The AD515A requires input protection only if the source is not current limited and, as such, is similar to many JFET-input designs. The failure mode would be overheating from excess current rather than voltage breakdown. If the source is not current limited, all that is required is a resistor in series with the affected input terminal so that the maximum overload current is 0.1 mA (for example, 1 M Ω for a 100 V overload). This simple scheme will cause no significant reduction in performance and give complete overload protection. Figures 2 and 3 show proper connections. |
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