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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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Digital Low Input Capacitance Buffer Circuit Diagram

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uses Ql and Q2 constitute a simple, high-speed FET input buffer.·Ql functions as a source follower, with the Q2 current-source load setting the drain-source channel current. The LT1010 buffer provides output drive capability for cables or whatever load is required. 

 Digital Low Input Capacitance Buffer Circuit Diagram


Digital Low Input Capacitance Buffer Circuit Diagram


The LTC1052 stabilizes the circuit by comparing the filtered circuit output to a similarly filtered version of the input signal. The amplified difference between these signals is used to set Q2`s bias, and hence Ql`s channel current. This forces Ql`s V0 s to whatever voltage is required to match the circuit`s input and output potentials. The diode in Ql`s source line ensures that the gate never forward biases and the 2000-pF capacitor at Al provides stable loop compensation. 

The rc network in Al `s output prevents it from seeing high-speed edges coupled through Q2`s collector-base ]unction. A2`s output is also fed back to the shield around Ql `s gate lead, bootstrapping the circuit`s effective input capacitance to less than 1 pF.

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