How do I calculate the input resistance of a feedback picoammeter?

How do I calculate the input resistance of a feedback picoammeter?

The input resistance of a feedback picoammeter on a particular range can be determined from the following equation: Input Resistance < Voltage Burden Specification / Full Scale Current on Particular Range In other words, the input resistance on a particular range of the feedback picoammeter is less than the ratio of the voltage burden specification to the full scale current on the range.