What are the limitations to adding input filter resistors to high side current sensors such as the AD8210?
In fact there is a limitation to the Resistance you can put in series at the inputs of the AD8210. Because the internal resistors are 1Kohm precision trimmed to match each other within 0.01%, adding a resistor externally causes mismatches. Here is an example below of the filter implementation.
I would suggest that you do not add more than 30 ohm resistors at the inputs. The errors due to the resistor can be calculated using the following formulas. Rin for the AD8210 is 3.5kohm. Rf is the value of the resistor you choose. %Error is simply the accuracy of the external resistor.
This is applicable for all current sensors, and the formulas can be applied in the same manner. Look at the specification page for other current sensors such as AD8211/12/13/15 for the RIN resistance value and plug in the formula above to obtain the error due to the input filters.
Additionally, an article is available that obtains further information on this issue. It can be found via the link below.
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