air,
A few things to check:
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Are the connections to the OP-Amp inputs short?
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Is there a resistor between the NI(+) input to ground? If so, you can try bypassing it with a capacitor to minimize positive feedback to the NI input.
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Do you have power supply bypass capacitors? If so, are the leads short? Some OP-Amps, such as the 741 are tolerant of high supply impedance; others, especially high-speed Op-Amps are not.
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The common to the input voltage source should connect to the NI input (or the common side of the resistor between NI input and ground) by as direct a path as possible to prevent ground loop problems.
Regards,
Kral