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High CMRR Instrumentation Amplifier (Schematic and Layout) design for biomedical applications

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Instrumentation amplifiers are intended to be used whenever acquisition of a useful signal is difficult. IA’s must have extremely high input impedances because source impedances may be high and/or unbalanced. bias and offset currents are low and relatively stable so that the source impedance need not be constant. Balanced differential inputs are provided so that the signal source may be referenced to any reasonable level independent of the IA output load reference. Common mode rejection, a measure of input balance, is very high so that noise pickup and ground drops, characteristic of remote sensor applications, are minimized.Care is taken to provide high, well characterized stability of critical parameters under varying conditions, such as changing temperatures and supply voltages. Finally, all components that are critical to the performance of the IA are internal to the device. The precision of an IA is provided at the expense of flexibility. By committing to the one specific task of
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Wheatstone bridge application using ATmega16A

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Abstract: In this project we are going to build a circuit that calculates the unknown resistance in a Wheatstone bridge using voltage subtractor and Internal ADC of a ATmega16A.

Wheatstone bridge is used to calculate the unknown resistance in a circuit. Here we are using OpAmp based subtractor circuit to calculate the voltage 'Vg', which is used to calculate the unknown resistance. The two buffer amplifiers are used to provide virtual isolation between the wheatstone bridge and the subtractor. Below circuit shows the working of a subtractor,



NOTE: In case of ATmega16A we don't need to use external subtractor because ATmega16A is having differential input mode.

Here we selected external ADC reference voltage 5V. So, the output of a ADC value is multiplied with its resolution to get input voltage, 5/1024. This voltage is given as 'Vg' in wheatstone bridge equation to calculate the resistance, remaining three resistors are set to 1k ohm.

Here's the program flow


  to download the code

Working images of the total circuit



Click here to see the working video.