The Strain Gauge/Instrument Amplifier shield (SGS from here on further) are intended for precise amplification of measurements specifically for bridge amplifiers, medical instrumentation and industrial process control. Instrumentation signals of small values, in the order of mV (millivolts) or less, need to be amplified to generate information of meaning to be processed. Prototyping boards like Arduino have a 10 bit ADC which means that the resolution of input/outputs are 4.9mV for a 5V (will be different for 3.3V or other referencing voltages) supply. Therefore changes less than 4.9mV cannot be easily recognised by the process board without the necessary amplification and possible noise filtering.

The AD8426 was selected to be used as the amplifier for the shield. The gain produced by the AD8426 amplifier ranges from 1 to 1000, AD8426 Datasheet Analog Devices Rev.0, depending on the RGAIN resistor value. Another valuable feature is the voltage reference adjustment on board to adjust the output voltage reference in order to be used with a single power supply ADC, which the Arduino makes use of.

For filtering, a second order Bessel low pass filter for 1k Hz was designed to help smooth out unwanted interference. The filter frequency can easily be changed by replacing the resistors and capacitors for a low pass filter. It is also possible to alter the filter to become a high-pass filter.

Strain gauges are essentially sensors which produce a very small change in resistance for sensing strain. In basic terms a strain gauge is an instrumentation sensor within the correct setup dynamics of a circuit. The following paragraph’s goal is to explain how strain gauges work. Strain gauges can be substituted with output characteristics of different sensors per specific application if there is a need.

Strain measurements are the actual strain experienced by a material when a force is applied to it whether it is a linear, axial, compression or expansion force.

The Wheatstone bridge is the principal layout for certain sensors in general to use the electrical properties of the circuit to extract meaningful information. The SGS without the Wheatstone bridge will only be a differential amplifier with a high sensitivity for weak signals.

There are different setups of Wheatstone bridges depending on the sensitivity and magnitude of the signal produced by the sensing element/s, in this case the strain gauge. It could however be any sensing element that replace the strain gauge/s. The Wheatstone bridge can also adjust for certain initial offset imbalances or for outside factors that influence the sensing element like temperature.