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Instrumentation Amp

This is the best Instrumentation OpAmp, Great CMRR, ensure supply has no ripple and keep analog and digital grounds separate. Ri can be replaced with a trimpot and resistor to alter gain. Connect a preset ends to pins 1 and 8 and preset wiper to VCC for Offset Null when high gains are configured.

The Input zeners and diodes form a protective clamp for all voltages above VCC-VDD. If supply is changed to +12 -12 change zeners to 12V zeners. Use similar Zeners at output to protect Output from being zapped by over-voltages or high energy - voltage or frequency transients. Add plastic capacitors across Rf for damping AC operation or ripple. Also avoid floating inputs by providing a bias, till you connect it to a sensor; or you will see electrometer like effects.

Three OpAmp Instrumentation Amp xml code

Vout = ( Vp - Vn ) * ( 2Rf + Ri) / Ri

Slide the Pots by Drag-Drop. Hover the mouse pointer on the mV Source buttons for changing values. The readings will ramp up or down faster if mouse pointer is left on button. No pressing buttons.

This configuration is revered by people making weighing scales. It may also be called strain gauge measurements. From tiny Kitchen Scales to Weigh Bridges. A Cantilever having a small sticker like film epoxied/glued onto it, which happens to be a sensor. A metal film or strip whose resistance varies when stretched. Stress and strain are the words they use. In the early days, this technology was a closely guarded professional secret, Not anymore. Read more about it here, but remember only the toughest designers can design such systems. - Strain Gage Technical Data

Four wire measurement, sounds unnecessary, looks intriguing. Thought i would make a writeup, but i found a good one here, read and learn - Resistance Measurements with a DMM - Understand how a DMM performs 2,4 and 6 wire measurements.

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