The Input Impedance of this module is very high and is symmetric. This circuit can be used for strain gauges and for four wire measurements. If inputs are in mV use OP07. The merit is that it uses only 2 OpAmps yet has high differential Input Impedance.

Dual Differential Amp – Op-Amp Designs

The Outputs of Opamps are low impedance but still have limits they cannot drive more than a few mA of Current into the Load. If low ohmic value loads are to be applied use external transistors as amplifiers. If inputs Vn-Vp are floating, outputs may be random or oscillating, it is good to have a bias network of 10M resistors to a potential even zero or common, this enables some Vout when input floats.

Interactive Siulation below – Operate Sliders and Buttons

Slider drag-drop and buttons just hover mouse pointer

 Vout = (Vp – Vn) * (Rf + Ri) / Ri

The Variable Resistor or Sliding Potentiometer for Rf and Ri are present for Gain Setting. Slide them to vary Rf and Ri, Assume both Rf values vary in tandem, when you slide the Rf Pot. Like a Ganged Pot or Dual Pot. Even Ri is a Dual Pot that varies both Ri Simultaneously. Now set the mV of Vp and Vn using the mV Sources provided. Just Hover Mouse pointer over buttons, variation ramps up if mouse pointer is left on the buttons. No pressing Buttons here. Take the Vout readings, tabulate the Results.