Walk on the high side

Most analogue ics are designed to handle voltage. When it comes to current, designers have fewer options and more headaches. That’s unfortunate, since monitoring and measuring current directly can bring advantages. What’s needed is a circuit that can sense current precisely and convert this into a voltage that can be amplified, conditioned and measured. Although a resistor can translate current to voltage, this is not a complete solution. The most common approach is to use a sense resistor in series with the current and an amplifier to isolate and condition the voltage across this resistor (Vsense). Placing a resistor in series with a ground contact may seem the most straightforward approach. This technique – low side current sensing – requires that no ground paths exist that could allow current to be diverted around the sense resistor or that could contribute current from an adjacent circuit. If a mechanical frame establishes the system ground, it may be impractical to insert this sense resistor. Also, since grounds are not perfect conductors, ground voltage can vary at different points in the system, necessitating the use of a differential amplifier for precision measurements. There is a more serious problem with low side current sensing. A resistor in the ground path means the load’s ‘ground’ will change with current. This can induce common mode errors and presents a problem for interfacing to other systems requiring the same ground potential. Since measurement resolution is enhanced by the magnitude of Vsense, the designer must trade ‘ground noise’ for resolution. The problem of ground variation can be avoided by placing the current sense resistor between the power supply and the load – high side current sensing.