Datasheet AD8212 (Analog Devices) - 9

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Data Sheet AD8212 THEORY OF OPERATION NORMAL OPERATION ILOAD BATTERY RSHUNT (7 V TO 65 V SUPPLY (V+) RANGE) 1 8
In typical applications, the AD8212 measures a small
AD8212
differential input voltage generated by a load current
R1 R2
flowing through a shunt resistor.
D
The operational amplifier (A1) is connected across the shunt
LOA
resistor (R
A1
SHUNT) 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
Q1
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
OUTPUT
circuitry.
CURRENT COMPENSATION BIAS
A load current flowing through the external shunt resistor
CIRCUIT
produces a voltage at the input terminals of the AD8212.
5 2 3 6
Amplifier A1 responds by causing Transistor Q1 to conduct the
VOUT IOUT
necessary current through Resistor R1 to equalize the potential
R
at both the inverting and noninverting inputs of Amplifier A1.
OUT
003 The current through the emitter of Transistor Q1 (I 05942- OUT) is proportional to the input voltage (V Figure 22. Typical Connection (7 V to 65 V Supply (Pin V+) Range) SENSE), and, therefore, the load current (ILOAD) through the shunt resistor (RSHUNT). The When using the AD8212 as described, the battery/supply output current (IOUT) is converted to a voltage by using an voltage in the system must be between 7 V to 65 V. The 7 V external resistor, the value of which is dependent on the input minimum supply range is necessary to turn on the internal to output gain equation desired in the application. regulator (shown as a Zener diode in Figure 22). This regulated The transfer function for the AD8212 is voltage then remains a constant 5 V, regardless of the supply (V+) voltage. The 65 V maximum limit in this mode of IOUT = (gm × VSENSE) operation is due to the breakdown voltage limitation of the VSENSE = ILOAD × RSHUNT AD8212 process. VOUT = IOUT × ROUT Typically, a 1% resistor can be used to convert the output V current to a voltage. Table 4 provides suggested R OUT = (VSENSE × ROUT)/1000 OUT values. where:
Table 4. Suggested ROUT Values
gm = 1000 µA/V.
Gain (V/V) ROUT (kΩ)
In normal voltage operation mode, the bias circuit is connected 1 1 to GND, as shown in Figure 22. In this mode, I 10 10 BIAS is typically 185 µA throughout the 7 V to 65 V (V+) range. 20 20 50 49.9 100 100 Rev. C | Page 9 of 13 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION NORMAL OPERATION (7 V TO 65 V SUPPLY (V+) RANGE) HIGH VOLTAGE OPERATION USING AN EXTERNAL PNP TRANSISTOR OUTPUT CURRENT COMPENSATION CIRCUIT APPLICATIONS INFORMATION GENERAL HIGH-SIDE CURRENT SENSING MOTOR CONTROL 500 V CURRENT MONITOR BIDIRECTIONAL CURRENT SENSING OUTLINE DIMENSIONS ORDERING GUIDE