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CA3240 Datasheet(PDF) 7 Page - Intersil Corporation |
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CA3240 Datasheet(HTML) 7 Page - Intersil Corporation |
7 / 15 page 7 It is well known that MOSFET devices can exhibit slight changes in characteristics (for example, small changes in input offset voltage) due to the application of large differential input voltages that are sustained over long periods at elevated temperatures. Both applied voltage and temperature accelerate these changes. The process is reversible and offset voltage shifts of the opposite polarity reverse the offset. In typical linear applications, where the differential voltage is small and symmetrical, these incremental changes are of about the same magnitude as those encountered in an operational amplifier employing a bipolar transistor input stage. Offset-Voltage Nulling The input offset voltage of the CA3240AE1 and CA3240E1 can benulledbyconnectinga 10k Ω potentiometer between Terminals 3 and 14 or 5 and 8 and returning its wiper arm to Terminal 4, see Figure 5A. This technique, however, gives more adjustment range than required and therefore, a considerable portion of the potentiometer rotation is not fully utilized. Typical values of series resistors that may be placed at either end of the potentiometer, see Figure 5B, to optimize its utilization range are given in the table “Electrical Specifications for Equipment Design” shown on third page of this data sheetAn alternate system is shown in Figure 5C. This circuit uses only one additional resistor of approximately the value shown in the table. For potentiometers, in which the resistance does not drop to 0 Ω at either end of rotation, a value of resistance 10% lower than the values shown in the table should be used. Typical Applications On/Off Touch Switch The on/off touch switch shown in Figure 6 uses the CA3240E to sense small currents flowing between two contact points on a touch plate consisting of a PC board metallization “grid”. When the “on” plate is touched, current flows between the two halves of the grid causing a positive shift in the output voltage (Terminal 7) of the CA3240E. These positive transitions are fed into the CA3059, which is used as a latching circuit and zero-crossing TRIAC driver. When a positive pulse occurs at Terminal 7 of the CA3240E, the TRIAC is turned on and held on by the CA3059 and its associated positive feedback circuitry (51k Ω resistor and 36k Ω/42kΩ voltage divider). When the positive pulse occurs at Terminal 1 (CA3240E), the TRIAC is turned off and held off in a similar manner. Note that power for the CA3240E is supplied by the CA3059 internal power supply. The advantage of using the CA3240E in this circuit is that it can sense the small currents associated with skin conduction while allowing sufficiently high circuit impedance to provide protection against electrical shock. Dual Level Detector (Window Comparator) Figure 7 illustrates a simple dual liquid level detector using the CA3240E as the sensing amplifier. This circuit operates on the principle that most liquids contain enough ions in solution to sustain a small amount of current flow between two electrodes submersed in the liquid. The current, induced by an 0.5V potential applied between two halves of a PC board grid, is converted to a voltage level by the CA3240E in a circuit similar to that of the on/off touch switch shown in Figure 6. The changes in voltage for both the upper and lower level sensors are processed by the CA3140 to activate an LED whenever the liquid level is above the upper sensor or below the lower sensor. Constant-Voltage/Constant-Current Power Supply The constant-voltage/constant-current power supply shown in Figure 8 uses the CA3240E1 as a voltage-error and current-sensing amplifier. The CA3240E1 is ideal for this application because its input common-mode voltage range includes ground, allowing the supply to adjust from 20mV to 25V without requiring a negative supply voltage. Also, the ground reference capability of the CA3240E1 allows it to sense the voltage across the 1 Ω current-sensing resistor in the negative output lead of the power supply. The CA3086 transistor array functions as a reference for both constant- voltage and constant-current limiting. The 2N6385 power Darlington is used as the pass element and may be required to dissipate as much as 40W. Figure 9 shows the transient response of the supply during a 100mA to 1A load transition. Precision Differential Amplifier Figure 10 shows the CA3240E in the classical precision differential amplifier circuit. The CA3240E is ideally suited for biomedical applications because of its extremely high input impedance. To insure patient safety, an extremely high electrode series resistance is required to limit any current that might result in patient discomfort in the event of a fault condition. In this case, 10M Ω resistors have been used to limit the current to less than 2 µA without affecting the performance of the circuit. Figure 11 shows a typical electrocardiogram waveform obtained with this circuit. VS = ±15V TEMPERATURE (oC) -60 -40 -20 0 20 40 60 80 100 120 140 100 10 1K 10K FIGURE 4. INPUT CURRENT vs TEMPERATURE CA3240, CA3240A |
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