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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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Sound-Level Meter Circuit Diagram

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The NE604`s signal-strength indicator section is used, based on an internal logarithmic converter. This enables a lineardecibel scale so that the moving-coil meter (shown in the diagram) can be replaced by a digital instrument. The signal source is assumed to be an electret microphone that converts ambient noise into an electrical signal. Because this type of microphone normally contains a buffer stage, R7, R8, and C13 have been included to provide the supply voltage for this stage. The NE604 delivers an output current (at pin 5) of 0 to 50, which causes a potential difference across R2 + R3 of 0 to 5 V. 

 Sound-Level Meter Circuit Diagram

Sound-Level Meter Circuit Diagram


The input and output signal range is equivalent to a sound range of 70 dB. lb compensate for the effects of temperature changes, the required resistance of 100 KOhmhm is formed by two resistors (R2 and R3) and a diode (Dl). Any ripple remaining on the output voltage is removed by R4/C9/C10 before the output is buffered by IC2. The indicating instrument, here a moving-coil meter, is connected to the output (pin 6) of IC2 via a series resistance, R^ + . 

The preset is adjusted to give full-scale deflection (FSD) for an output voltage of 4 V. Calibrating the meter is a little tricky, unless you have access to an already calibrated instrument. Otherwise, if you know the efficiency of your loudspeaker, that is, how many decibels for 1W at 1 m, you can use that as reference. The scale of the meter can then be marked with the (approximate) value. In any case, the meter deflection must at all times be seen as an indication, not as an absolute value: it was not thought to be worthwhile to add a filter to the circuit to enable absolute measurements to be made.

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