Current Sense Amplifier
It is the circuit that brings the millivolt-level difference across the shunt resistor up to the volt level the converter can read. The 30 mV produced by a 1 mΩ shunt at 30 A rises to 1.5 V with a gain of 50. The amplifier's offset and temperature drift directly determine the accuracy of the driver's current threshold.
The difference between the two ends of the shunt is small, while the common-mode voltage on it can be as large as the supply. That is why the measurement stage is designed to amplify only the difference and reject the common mode.
- Gain selection: the gain is chosen so that the highest current to be measured comes close to the converter's full scale. Too much gain saturates the signal, too little wastes resolution.
- Offset: deviation of the output from zero at zero current means a proportionally large error in the low-current region. In bidirectional measurement the output is usually offset to the middle of the scale.
- Bandwidth: to be able to capture the starting surge and the peak current within the PWM, the measurement stage's response must be faster than the switching period.
In the field, the cause of current protection triggering early or late is not always the threshold setting; the offset, filter and sampling instant of the measurement chain are equally decisive. So before changing the threshold, verify the real current with an independent instrument.
Context where this term is used: DC motor and driver fault diagnosis matrix