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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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25-W Audio Power Amplifier Circuit Diagram

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This audio power amplifier project is based on LM1875 amplifier module from National Semiconductor. It can deliver up to 30W of power using an 8 ohm load & dual 30V DC power supplies. It is designed to operate with maximum outside parts with current limit & thermal shutdown protection features . Other features include high gain, quick slew rate, wide power supply range, giant output voltage swing & high current capability.

Summary of the audio amply-fire features:

  • Low distortion: 0.015%, 1 kHz, 20 W
  • Wide power bandwidth: 70 kHz
  • Wide supply range 16V-60V
  • Up to 30 watts output power
  • Internal output protection diodes
  • Protection for AC & DC short circuits to ground
  • 94 dB ripple rejection
  • Plastic power package TO-220
25V Power Supply

The schematic below shows how the +25V DC & -25V DC are obtained. In order to provide power supply for two stereo amplifiers, a power transformer rating of 80VA with 240V/36V middle tapped secondary winding is used. The secondary output of the transformer is rectified by using 1N5401 diodes together with four electrolytic capacitors to smoother the ripple voltage. A fuse & a varistor are connected at the primary input to protect the circuit against power surge.

25-W Audio Power Amplifier Circuit Diagram



Audio Amplifier Module

The +25V & -25V DC power supply are connected to the audio amplifier module through a 2A fuse with the peripheral devices shown in the schematic below. The audio input signal to be amplified is coupled to pin one of LM1875 through the resistor R1 and electrolytic capacitor E5.

The output signal at pin four of LM1875 can be used to directly drive a 8 ohm loudspeaker. Resistor R6 and capacitor C5 prevent-the capacitance developed at the long speaker leads from driving the amplifier in to High Frequency Oscillation.

A heat-sink with a thermal resistance rating of one.4 Cecilius/Watt or better must be used or else the amplifier module will-be cut-off from operation due to the heat that will build up in the coursework of the operation of the amplifier. Take note that the heat sink tab on the IC module is internally connected to the -25V power supply hence it must be isolated from the heat sink by the use of an insulating washer. If this is not done, the negative rail will be shorted to ground.





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