Two Op-Amp Differential Amplifier
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 - Interactive Simulation
Above page has the Interactive Tutorials using Virtual Electronic Components of this example.
Interactive Tutor
This has Virtual Meters and Pots, shows how a Operational Amplifier works. Slide the Potentiometers and vary the mV Source, See Output DVM. Slide the Potentiometers just like you would operate a Sliding Control. Drag the Knob on Pot to increase or decrease the resistance. The Resistance is shown is blue letters and dynamically alters value as you slide the pot. The mV Source is varied by just moving or hovering mouse pointer over the two buttons, no clicking. The mV buttons are special, the variation picks up speed if you let the mouse pointer remain on the button. This is Ramp-up and Ramp-down. This enables you to set it faster with just two buttons.
Differential Amplifier using only Two Op-Amps, Interactive Tutorial Gadget. Instrumentation Amplifier. This is used in the front end of Signal Conditioners for Load, Strain and Pressure Measurement. High Z and Gain, Easy to configure Zero and Span.
Vout = (Vp - Vn) * (Rf+Ri)/Ri
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 COM this enables Vout when input floats.
More at Analog Op-Amp Circuits