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

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

Simple Photodiode Amplifier Circuit Diagram

This is a simple Photodiode Amplifier Circuit Diagram. In this simple circuit adding two passive components to a standard photodiode amplifier reduces noise. Without the modification, the shunt capacitance of the photodiode reacting with the relatively large feedback resistor of the trans impedance (current-to-voltage) amplifier, creates excessive noise gain. 

The improved circuit, Fig. 3-18b, adds a second pole, formed by R3 and C2. The modifications reduce noise by a factor of 3. Because the pole is within the feedback loop, the amplifier maintains its low output impedance. If you place the pole outside the feedback loop, you have to add an additional buffer, which would increase noise and de error. 

 Simple Photodiode Amplifier Circuit Diagram




The signal bandwidth of both circuits is 16kHz. In the standard circuit (Fig. 3-18a), the 1-pF stray capacitance in the feedback loop forms a single 16-kHz pole. The improved circuit has the same bandwidth as the first, but exhibits a 2-pole response.

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