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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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RESISTANCE TYPE SENSORS

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These work on the principle that the electrical resistance of a conductor change with temperature. If a constant
voltage is applied to the conductor then the current flowing through it will change with temperature. The resistivity
of the conductor change with temperature. This usually means the resistance gets bigger as the conductor gets
hotter. The following law relates the resistance and temperature.
R = Ro(1 + aq)
a is the temperature coefficient of resistance. Ro is the resistance at 0oC. Sometimes the equation is given as
R = Ro(1 aq - bq2)
A basic temperature sensor is made by winding a thin resistance wire into a small sensor head. The resistance of
the wire then represents the temperature. This has an advantage over a thermocouple in that it is unaffected by the
temperature of the gauge end. The main type of wire used is PLATINUM. The sensors are usually manufactured
to have a resistance of 100 W at 0oC and the value of a is 0.00385 to 0.00390. A typical operating range is -
200 to 400oC.
A special type of resistance sensor is called a THERMISTOR. They are made from a small piece of semiconductor
material. The material is special because the resistance changes a lot for a small change in temperature
and so can be made into a small sensor and it costs less than platinum wire. The temperature range is limited.
They are only used for a typical range of -20 to 120oC and are commonly used in small hand held thermometers
for every day use. The relationship between resistance and temperature is of the form R = AeB/q