Guidelines to use a current sense resistor

Current sense resistors are used in Embedded circuits for sensing the current flowing to the load. Current sensing can be done by different methods:

• using low value current sense resistor in series with the current path and there by reading the voltage developed. (current shunt resistor)
• using Hall element
• using Current Transducer

If we look at the datasheet of INA180 from Texas Instruments, they suggest how to use a current sense resistor in the circuit. you can see from the below image is that RSENSE is connected between the power rail and the load. This kind of sensing is called High side sensing.

While there are different techniques, it is important that design engineer follow some guidelines while using the current sensors. We shall list some of the guidelines to use a current sense resistor in circuits.

• Pull the current sense traces as close as possible from the current sense resistor. This eliminates any additional resistance added to the current sense resistor there by resulting in less accurate output. We can see in the below image that traces are routed from the pads itself (this is called kelvin connection)

• We know that higher the current sense resistor, more the voltage developed and better the output of current sense IC.  However, current sense resistor cannot be a huge resistor. A huge resistor shall cause voltage drop there by power supply cannot provide enough voltage for the load to operate. Typical sense resistors shall be from 1 milliohm to 5 ohms
• Higher current sense resistor also has problem of higher power dissipated across resistor which meant higher wattage resistor is required which also meant unnecessary power dissipation and higher space usage on the board and at the end all this for nothing.
• If I is the maximum current flowing to the load, Rs is the sense resistor, then power dissipated in the resistor is I*I*Rs. This must not exceed the maximum power allowed by sense resistor. Select/check the sense resistor specifications properly
• Select the current sense resistor such that there is less voltage drop due to the resistor. 
• The current sense resistor must be selected such that Input offset voltage error is overcome.
• The output of current sense IC like INA180 shown above is voltage. This voltage is input to the Analog to Digital converter (ADC) of Micro controller. 
• The current sensing IC is a operational amplifier based circuit. This meant there is some gain associated with the circuit. When selecting the gain, ensure that the output of operational Amplifier doesn't exceed the reference voltage of the ADC. Else the output shall be saturated and we cannot read the full scale reading of the current. 
• Like how we use RC filter on ADC interfaces, ensure to use RC filter to eliminate noise on those lines.

Design steps:

• Get to know the maximum current to be measured
• Get to know the voltage that is applied to the load. This is the common mode voltage for the current sense amplifier
• Select the sense resistor with appropriate power rating
• Select the Current sense amplifier with appropriate gain such that output of current sense doesn't exceed it's maximum voltage
• Adjust the current sense resistor value is required after the operational amplifier selection
• Add any protection at the input to avoid transients
• Add RC filter at the output of current sense amplifier to filter out noise