Simplified Current-Sensing Circuits
by Dennis L. Feucht
Innovatia Laboratories
Non-isolated current sensing can often be accomplished with a sense resistor
followed by an amplifier that scales the output for desired volts/ampere.
This article addresses design tradeoffs between using a differential amplifier
and a single-ended, non-inverting op amp as the sense amplifier.
If the full-scale current is large the sense resistor is made small to minimize power dissipation and interaction with the sensed circuit. Small-valued resistors can have significant parasitic lead resistance. If it is in series with the accurate sense resistance, Rs, the sensed voltage will be larger by the parasitic resistance voltage drop. The usual solution to this problem is to use a 4-terminal resistor, where a pair of sense terminals connected across the desired resistance is brought out separately from the other pair, the drive terminals. The sense terminals become the inputs to a differential amplifier, which amplifies only the desired quantity, ii Rs. This is the optimal approach to sense-resistor circuit design. It is also unnecessary in some applications and more expensive than using a single-ended-input (non-differential) amplifier.
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