What Are Some Useful Considerations in Designing A Resistor Based Current Sensing System?

Ohm’s law serves as the simplest method to measure electric current and also monitor it. It states that the voltage across a resistor is directly proportional to the current passing through it. This resistor based current sensing phenomena is quite easy to apply across a variety of electrical equipment due to several reasons:

• Resistor based current sensing is of low cost.
• It has linear behaviour.
• It does not require any calibration.

Practical Issues Related to Resistor based Current Sensing Systems:
There are also certain practical issues that must be assessed carefully when using a resistor based current sensing technique. For example all resistors dissipate power when current passes through them. This happens due to the fact that heat is produced by energy losses due to current flow through the resistor, so it is always recommended to handle such systems with due care. From Ohm’s law, a large current sensing resistor will obviously measure current with greater accuracy, but in turn it will also dissipate more power. Similarly, if we choose a current sensing resistor of small value, it will sense only a low level voltage across it. There is a useful strategy to cope with this issue – use of an amplifier. An amplifier is a great device to amplify this low level voltage to a suitable level according to the external circuit. They’re used in set-ups such as analog to digital converters (ADC) or comparators, in conjunction with capacitors.

Expected Measurement Error:

One major issue associated with such low sensing voltages is that of greater chances of measurement errors in them. These measurement errors arise by the inherent bias current in the amplifier and its input offset voltage. If the input offset voltage of an amplifier is 5 mV then the measurement error can reach up to 10% at full scale current value. It can get worse for lower value currents.

How Can You Eliminate Measurement Error?

For the above mentioned problem, the solution is to use lower values for both input offset voltage and the input bias current of the current-sensing amplifier. Take the example of a dedicated high-side current sensing amplifier, in which a current sensing resistor is placed between the voltage source and the load. This arrangement is extremely helpful as it simplifies and improves circuit performance. It also helps to avoid extraneous resistance in the ground plane of the circuit.