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AD515AJH bảng dữ liệu(PDF) 5 Page - Analog Devices |
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5 / 6 page AD515A –5– REV. A ELECTROMETER APPLICATION NOTES The AD515A offers subpicoampere input bias currents available in an integrated circuit package. This design will open up many new application opportunities for measurements from very high impedance and very low current sources. Performing accurate measurements of this sort requires careful attention to detail; the notes given here will aid the user in realizing the full measurement potential of the AD515A and perhaps extending its performance limits. 1. As with all junction FET input devices, the temperature of the FETs themselves is critical in determining the input bias currents. Over the operating temperature range, the input bias currents closely follow a characteristic of doubling every 10 °C; therefore, every effort should be made to minimize device operating temperature. 2. The heat dissipation can be reduced initially by careful investigation of the application. First, if it is possible to reduce the required power supplies, this should be done since internal power consumption contributes the largest component of self-heating. To minimize this effect, the quiescent current of the AD515A has been reduced to less than 1 mA. Figure 8 shows typical input bias current and quiescent current versus supply voltage. 3. Output loading effects, which are normally ignored, can cause a significant increase in chip temperature and therefore bias current. For example, a 2 k Ω load driven at 10 V at the output will cause at least an additional 25 mW dissipation in the output stage (and some in other stages) over the typical 24 mW, thereby at least doubling the effects of self-heating. The results of this form of additional power dissipation are demonstrated in Figure 9, which shows normalized input bias current versus additional power dissipated. Therefore, although many dc performance parameters are specified driving a 2 k Ω load, to reduce this additional dissipation, we recommend restricting the load resistance to be at least 10 k Ω. 4. Figure 10 shows the AD515A’s input current versus differen- tial input voltage. Input current at either terminal stays below a few hundred fA until one input terminal is forced higher than 1 V to 1.5 V above the other terminal. Input current limits at 30 µA under these conditions. Figure 8. Input Bias Current and Supply Current vs. Supply Voltage Figure 9. Input Bias Current vs. Additional Power Dissipation Figure 10. Input Bias Current vs. Differential Input Voltage AD515A CIRCUIT APPLICATION NOTES The AD515A is quite simple to apply to a wide variety of applications because of the pretrimmed offset voltage and internal compensation, which minimize required external components and eliminate the need for adjustments to the device itself. The major considerations in applying this device are the external problems of layout and heat control which have already been discussed. In circuit situations employing the use of very high value resistors, such as low level current to voltage converters, electrometer operational amplifiers can be destabi- lized by a pole created by the small capacitance at the negative input. If this occurs, a capacitor of 2 pF to 5 pF in parallel with the resistor will stabilize the loop. A much larger capacitor may be used if desired to limit bandwidth and thereby reduce wide- band noise. Selection of passive components employed in high impedance film or deposited ceramic oxide to obtain the best in low noise and high stability performance. The best packaging for high M Ω resistors is a glass body sprayed with silicone varnish to minimize humidity effects. These resistors must be handled very carefully to prevent surface contamination. Capacitors for any high impedance or long-term integration situation should be of a polystyrene formulation for optimum performance. Most other types have too low an insulation resistance, or high dielectric absorption. Unlike situations involving standard operational amplifiers with much higher bias currents, balancing the impedances seen at the input terminals of the AD515A is usually unnecessary and probably undesirable. At the large source impedances, where these effects matter, obtaining quality matched resistors will be difficult. More important, instead of a cancelling effect, as with bias current, the noise voltage of the additional resistor will add by root-sum-of-squares to that of the other resistor thus increasing |
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