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AD8212 Scheda tecnica(PDF) 9 Page - Analog Devices |
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AD8212 Scheda tecnica(HTML) 9 Page - Analog Devices |
9 / 16 page AD8212 Rev. 0 | Page 9 of 16 THEORY OF OPERATION NORMAL OPERATION (7 V TO 65 V SUPPLY (V+) RANGE) In typical applications, the AD8212 measures a small differential input voltage generated by a load current flowing through a shunt resistor. The operational amplifier (A1) is connected across the shunt resistor (RSHUNT) with its inverting input connected to the battery/supply side, and the noninverting input connected to the load side of the system. Amplifier A1 is powered via an internal series regulator (depicted as a Zener diode in Figure 22). This regulator maintains a constant 5 V between the battery/supply terminal of the AD8212 and COM (Pin 2), which represents the lowest common point of the internal circuitry. A load current flowing through the external shunt resistor produces a voltage at the input terminals of the AD8212. Amplifier A1 responds by causing Transistor Q1 to conduct the necessary current through Resistor R1 to equalize the potential at both the inverting and noninverting inputs of Amplifier A1. The current through the emitter of Transistor Q1 (IOUT) is proportional to the input voltage (VSENSE), and, therefore, the load current (ILOAD) through the shunt resistor (RSHUNT). The output current (IOUT) is converted to a voltage by using an external resistor, the value of which is dependent on the input to output gain equation desired in the application. The transfer function for the AD8212 is IOUT = (gm × VSENSE) VSENSE = ILOAD × RSHUNT VOUT = IOUT × ROUT VOUT = (VSENSE × ROUT)/1000 where: gm = 1000 µA/V. In normal voltage operation mode, the bias circuit is connected to GND, as shown in Figure 22. In this mode, IBIAS is typically 185 μA throughout the 7 V to 65 V (V+) range. IOUT ILOAD ROUT AD8212 BATTERY RSHUNT 1 8 5 2 3 6 OUTPUT CURRENT COMPENSATION R1 R2 A1 Q1 VOUT BIAS CIRCUIT Figure 22. Typical Connection (7 V to 65 V Supply (Pin V+) Range) When using the AD8212 as described, the battery/supply voltage in the system must be between 7 V to 65 V. The 7 V minimum supply range is necessary to turn on the internal regulator (shown as a Zener diode in Figure 22). This regulated voltage then remains a constant 5 V, regardless of the supply (V+) voltage. The 65 V maximum limit in this mode of operation is due to the breakdown voltage limitation of the AD8212 process. Typically, a 1% resistor can be used to convert the output current to a voltage. Table 4 provides suggested ROUT values. Table 4. Suggested ROUT Values Gain (V/V) ROUT (kΩ) 1 1 10 10 20 20 50 49.9 100 100 |
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