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OPA454: Torque motor control

Part Number: OPA454

When I am design the improved Howland current pump circuit, Iwould like to conduct the worst case analysis considering the bias current, offset voltage of amplifier and the components tolerance. 

By chance I have found the slide 'The Improved Howland Current Pump' which is written by Tim Green (PA- Linear Applications), here I am puzzled by the formula which is explained on page 5.

Question : Set RX=RF and RZ=RI.  it looks like IL does not fully match with the equation when the RX and RZ are replaced and calculated.

could someone help me to better understand the original formula on page 5? or what the formula will be if the bias current,offset voltage of amplifier are considered into the worst case analysis?

Thanks a lot for the kind help.

Improved Howland Current Pump.ppt