Shunt resistors are a popular current-sensing sensor because of their low cost and good accuracy.Microhm Electronics' shunt resistor MMS2512, MMS3920 to MMS8420 are applicable to this function. The voltage drop across a known low value resistor is monitored in order to determine the current flowing through the load. If the resistor is small in magnitude, the voltage drop will be small and the measurement will not have a major effect on the motor circuit.


The selection criteria of a shunt current resistor requires the evaluation of several trade-offs, including:
• Increasing resistance voltage, which makes the voltage offset (VOS) and input bias current offset (IOS) amplifier errors less significant.
• A large resistance value causes a voltage loss and a reduction in the power efficiency due to the I2 x R loss of the resistor.
• A large resistance value will cause a voltage offset to the load in a low-side measurement that may impact the EMI characteristics and noise
sensitivity of the system.
• Special-purpose, low inductance resistors are required if the current has a high-frequency content.
• The power rating of RSENSE must be evaluated because the I2 x R power dissipation can produce self heating and a change in the nominal resistance of the shunt.

 
Special-purpose, shunt current measurement resistors are available from a number of vendors,like Microhm Electronics. If standard resistors are used, it is recommended that metal-film resistors be used rather than wire-wound resistors that have a relatively large inductance.
 
A shunt resistor can also be created from the trace resistance on a PCB. PCB shunt resistors offer a low cost alternative to discrete resistors. However, their accuracy over a wide temperature range is poor when compared to a discrete resistor. The temperature coefficient of a copper PCB trace shunt resistor is equal to approximately +0.39%/°C.