The Impedance Measurement Box (IMB) measures impedance by exciting the battery with current that is a simultaneous sum of sine waves. On current IMB systems parallel current drivers work together to send a current that captures battery voltage response and processes that signal into impedance. While the data returned is good indication of the actual impedance measurements from the battery, the measurement quality can be improved by moving the ground of the current drivers from outside the box to in between the two current driver circuits (inside the box). This creates a floating ground (not connected to earth) and helps eliminate any noise that may accompany the current going into the battery when conducting test.
This technique of moving the ground between the current drivers is referred to as Push Pull. The name comes from what is taking place for the two current drivers. One current driver will push the current, meanwhile, the second current driver will pull the current and in between these two drivers will be the floating ground.
In order for this technique to work appropriately the current between the current drivers must match, otherwise mismatch will occur and results will suffer, achieving nothing but useless data. In order to get the currents to match, two control circuits are created: the bias current trim, and the gain trim. Bias trim will allow the use of trim pots to eliminate any bias current we do not want; meanwhile the gain trim will also use trim pots to make sure the two current driver gains are the same.
Ford, Randy, "Validation of Push Pull Current" (2016). Proceedings of the Annual Montana Tech Electrical and General Engineering Symposium. 12.