I was wondering:
what is the function of the 2 resistors(R4 and R5) in series with the input pins of the given Instrumentation amplifier?
Thank you!
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asked Nov 2, 2016 at 20:38
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\$\begingroup\$ Related: electronics.stackexchange.com/questions/32096/… \$\endgroup\$– Bence KaulicsCommented Nov 2, 2016 at 20:44
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1 Answer
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Short answer : Safety current limit for CM input range input Vcb forward bias current from 0.22Ω* current pulse as the source during OFF commutation or unintended loose wire contact when running.
Also ground shift from poor wiring ESL. where Vemf=Ldi/dt for sense wiring L
Normally the shunt R 0.22Ω is designed for 50mV drop at max rated current to keep I^2R losses low. But as we know Motor DCR cause start currents >8x the Imax rated current, the negative voltage on the current shunt may exceed -100mV rating for CM input range on this INA.
If it exceeds -600mV then the input Vbe of the PNP base collector begins to conduct and will blow the front end. So a wise choice of R value is to limit the current with some experience.
- Negative voltages occur from back EMF when the motor is shunted to stop, then it acts as a generator and V = L dI/dt depends on both conduction current and rate of change of current (negative) switching when motor is turned Off also resulting in a negative current spike.
