Dual Differential Amp - Interactive Simulation
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.
Vout = (Vp - Vn) * (Rf+Ri)/RiRelated Reading
Precision Instrumentation Amplifiers