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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

RL and RC Time Constants.



    Recall that in our lamp and cap circuit, the lamp lights up for a second, but then goes out. What determines how long the lamp stays light? The lamp is not a perfect device which generates light from electricity. It has losses, and shows electrical friction (read: resistance). Any time that you have a resistor and a capacitor in a circuit, we call it an RC circuit. Likewise, if there is a resistor and a coil in a circuit, we call it an RL circuit.


In this circuit, the moment the battery is applied, current starts to flow through the resistor, and a voltage begins to build up across the plates of the capacitor. The amount of time that it takes for the voltage across the capacitor plates to reach the voltage of the battery, is a result of the values of the capacitor and the resistor. The larger these two values are, the longer it will take for the entire voltage to appear across the cap. This relationship is expressed by a formula, and is called "Time Constant". The formula is:

T = RC

where T= Time in seconds, R=resistance in ohms, and C=capacitance in farads.

A time constant also exists between coils and resistors, however, because of the nature of how a coil is made (it is essentially a piece of wire), time constants in RL circuits are much smaller. Recall that earlier in the course, we discussed that inductors and capacitors are almost exact opposites. Here is another formula which will seem re-hashed.
T = L / R

where T= Time in seconds, R=resistance in ohms, and L=inductance in henries.

Remember this formula?
XL = 2pfL



Well, you are going to have to start using XL and XC again. Because we are going to begin using capacitors and coils in the same circuit. Now the real fun of electronics begins.