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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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Precision Instrumentation Amplifier Circuit Diagram

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This is a Digital Ultra Precision Instrumentation Amplifier Circuit Diagram. This circuit will run from a single 5 V power supply. The LTC1043 switched-capacitor instrumentation building block provides a differential-to-single-ended transition using a flying-capacitor technique. Cl alternately samples the differential input signal and charges ground referred C2 with this information. 

Precision Instrumentation Amplifier Circuit Diagram


 
 The LTC1052 measures the voltage across C2 and provides the circuit`s output. Gain is set by the ratio of the amplifier`s feedback resistors. Normally, the LTC1052`s output stage can swing within 15 mV of ground. If operation all the way to zero is required, the circuit shown in dashed lines can be employed. This configuration uses the remaining LTC1043 section to generate a small negative voltage by inverting the diode drop. 

This potential drives the 10-KO, pull-down resistor, forcing the LTC1052`s output into class A operation for voltages near zero. Note that the circuit`s switched-capacitor front-end forms a sampled-data filter allowing the common-mode rejection ratio to remain high, even with increasing frequency. The 0.0047uF unit sets front-end switching frequency at a few hundred Hz.

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