A small shunt that reads high by a fixed fraction

Tags: Current sensing Measurement

Symptom

Currents read through a shunt of a few milliohms are high by the same fraction at every current: a gain error, not an offset. The error is larger the smaller the shunt.

Why

With a shunt of a few milliohms or less, the resistance of the solder becomes a substantial part of the sense element’s resistance [O'Sullivan 2012]. If the amplifier’s sense connections leave from the far side of the pads, the amplifier sees the current through the shunt, the pads and the solder together, and the reading is high by their resistance over the shunt’s at every current (pad error). For example, pads and solder of 100 µΩ in the sense path of a 1 mΩ shunt make every reading 0.1 high.

How to confirm

Pass a known current through the shunt and measure the voltage twice: at the ends of its pads, and at the shunt’s own terminals (or its Kelvin pads). The difference over the current is the pads’ and the solder’s resistance; the ratio of the two voltages, minus one, is the reading’s fractional error.

Fix

Take the pads out of the sense path with a Kelvin connection: a four-terminal shunt, or a two-terminal one on a four-pad (split-pad) layout [O'Sullivan 2012]. The LTC6101 data sheet recommends Kelvin connections to the sense resistor in all but the lowest-power applications [Analog Devices, LTC6101/LTC6101HV, Applications Information]. With a Kelvin connection, the pad resistance in the sense chain tool is zero.

References

• [O'Sullivan 2012]: the solder’s resistance with milliohm shunts; Kelvin and split-pad sensing.
• [Analog Devices, LTC6101/LTC6101HV, Applications Information]: Kelvin connections to the sense resistor.
• Current sensing: the pad error, worked through, and a bench exercise.