VOICE OF THE ENGINEER - ElectronicsAndBooks

The Avago Advantage Technical NotesAlthough the application circuit is relatively simple, a fewrecommendations should be followed to ensure optimalperformance.Shunt Resistor SelectionA real-world motor current sense resistor calculation willshow what must be considered in selecting the currentsense resistor. First determine how much current theresistor will be sensing. The graph in Figure 3 shows the RMScurrent in each phase of a three phase induction motor asa function of average motor output horsepower and motordrive supply voltage. The maximum value of the senseresistor is determined by the current being measured andthe maximum recommended input voltage of the isolationamplifier. For example, if a motor has a maximum RMScurrent of 10 A and can experience up to 50% overloadsduring normal operation, then the peak current is 21.1 A(=10 x 1.414 x 1.5). With a maximum amplifier input voltageof 200 mV, the maximum sense resistance would be about10 mΩ. The maximum average power dissipation in thesense resistor, which is about 1 W in this example, shouldalso be checked by multiplying the sense resistance timesthe square of the maximum RMS current. If the powerdissipation is excessive, a lower resistance value can be used.MOTOR OUTPUT POWER - HORSEPOWER403530252015105005440 V380 V220 V120 V10 15 20 25 30MOTOR PHASE CURRENT - A (rms)35+Input–InputRaRbC5 VV DD1V IN+Figure 4. ACPL-C79x differential input connectionVoltage SensingThe ACPL-C79B/C79A/C790 can also be used to isolatesignals with amplitudes larger than its recommendedinput range with the use of a resistive voltage divider at itsinput. The only restrictions are that the impedance of thedivider be relatively small (less than 1 kΩ) so that the input affect the accuracy of the measurement. An input bypasscapacitor is still required, although the 10 Ω series dampingresistor is not (the resistance of the voltage divider providesthe same function). The low-pass filter formed by the dividerresistance and the input bypass capacitor may limit theachievable bandwidth.Evaluate the ACPL-C79XThe ACPL-C79X evaluation board demonstrates the highlinearity and low-offset capability of the ACPL-C79B/C79A/C790. It allows a designer to easily test the performance ofthe isolation amplifier in an actual application under reallifeoperating conditions. A surface mount shunt resistor isprovided along with the board.V IN–GND1ACPL-C79B/ACPL-C79A/ACPL-C790Figure 3. Horsepower vs. motor phase current/voltageDifferential Input ConnectionIn Figure 2, the isolation amplifier is connected in a singleendedinput mode. However, given the fully differentialinput structure, a differential input connection, shown inFigure 4, can be used for better performance. Any noiseinduced on one pin will be coupled to the other pin by thecapacitor C and creates only common mode noise which isrejected by the ACPL-C79x.SummaryFigure 5. ACPL-C79x evaluation boardAvago’s three new Miniature Precision Isolation Amplifiers,made possible by proprietary optical isolation technology,offer increased accuracy, speed, bandwidth andinsulation ratings. Additional information is available atwww.avagotech.comContact us for your design needs at: www.avagoresponsecenter.com/401Avago, Avago Technologies, the A logo and LaserStream are trademarks of Avago Technologies in the United States and othercountries. All other trademarks are the property of their respective companies.Data subject to change. Copyright © 2010 Avago Technologies