Auto-range function

A project log for Luke - Open Framework Multimeter/Logger

DMM dedicated to PC/smartphone with cross platform SDK provides user-friendly measurement and automation system

MaxMax 01/28/2017 at 15:290 Comments

Auto-range is feature to select suitable measurement range automatically. This function helps you when target signals have wide dynamic range. Especially, it works for software engineers who aren't good at measurement. For example, they sometimes fail to measure current because system reset happens due to voltage drop caused by impedance selected measurement range has.

The important thing to avoid this issue is to change current measurement range to suitable one automatically and quickly. Luke has a tricky solution using a critical alert pin of the current monitor IC, INA3221. When ADC result is higher than threshold set by a register, it asserts a critical alert pin, and then turns on FET that bypasses shunt resistors. As result, impedance becomes lower because there is only high current sense resistor on the path.

However, if you are measuring negative current, that is ADC result is a negative value, this auto-range function doesn't work at all. This is a limitation of this hardware solution using a critical alert function of INA3221.