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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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Design problem: How can you convert an XOR gate into a buffer or an inverter?

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Figure 1 above shows the truth table of a 2-input XOR gate. It has two inputs A, B and an output OUT. On looking closely, we observe that:

  • If one of the inputs (say B) is 0, OUT is equal to the other input. For instance, when B = 0,
    • OUT = 0 when A = 0
    • OUT = 1 when A = 1
  • Similarly, if one of the inputs is 1, OUT is equal to invert of the other input. For instance, when B = 1,
    • OUT = 1 when A = 0
    • OUT = 0 when A = 1
Using the above information, XOR gate can be easily converted to a buffer and an inverter.

Buffer using XOR gate: Simply connecting one of the inputs to logic '0' will convert XOR gate into a buffer. As shown in the truth table below, OUT is following the value of B.




Inverter design using XOR gate: Similarly, we can realize an inverter using an XOR gate by connecting one of the inputs to logic '1'. As shown in the truth table below, OUT is opposite to the value of B.


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