I am using an AD8217 high side current sensor to measure currents up to 20A. It works as expected when testing with lower currents and sense resistors of 1\$ \Omega \$ and 0.3\$ \Omega \$ and currents of 660mA and 130mA.
For the eventual 20A, I will need to use a 5m\$ \Omega \$ sense resistor. The output of the AD8217 is not what I expect using this resistor and the 660mA or 130mA currents.
At 130mA, I expect an output of 13mV but I measure 34mV. At 660mA, I expect 66mV but measure 115mV.
Directly measuring the differential voltage across the sense resistor looks right (e.g., 3.3mV with 660mA which should produce an output of 20 x 3.3mV = 66mV).
The application note (https://www.analog.com/en/analog-dialogue/articles/high-side-current-sensing-wide-dynamic-range.html) says that a differential voltage across the sense resistor of at least 1mV is fine. I don't think that the AD8217 offset voltage or bias current can account for what I am seeing.
Thanks for any ideas.
This is the circuit I am using with a 12.5V battery, either 18\$ \Omega \$ or 100\$ \Omega \$ load resistors and 0.3\$ \Omega \$, 1\$ \Omega \$ or 0.005\$ \Omega \$ sense resistors.