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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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1KHz Sine wave Generator

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Parts:

R1____________5K6  1/4W Resistor
R2____________1K8  1/4W Resistor
R3,R4________15K   1/4W Resistors
R5__________500R   1/2W Trimmer Cermet
R6__________330R   1/4W Resistor
R7__________470R   Linear Potentiometer

C1,C2________10nF  63V Polyester Capacitors
C3__________100µF  25V Electrolytic Capacitor
C4__________470nF  63V Polyester Capacitor

Q1,Q2_______BC238  25V 100mA NPN Transistors

LP1___________12V  40mA Filament Lamp Bulb (See Notes)

J1__________Phono chassis Socket

SW1__________SPST  Slider Switch

B1_____________9V  PP3

Clip for 9V PP3 Battery
 
 

Circuit description:

This circuit generates a good 1KHz sinewave adopting the inverted Wien bridge configuration (C1-R3 & C2-R4). It features a variable output, low distortion and low output impedance in order to obtain good overload capability. A small filament bulb ensures a stable long term output amplitude waveform.

Notes:

  • The bulb must be a low current type (12V 40-50mA or 6V 50mA) in order to obtain good long term stability and low distortion.
  • Distortion @ 1V RMS output is 0.15% using a 12V 40mA bulb, raising to 0.5% with a 12V 100mA one.
  • Using a bulb differing from specifications may require a change of R6 value to 220 or 150 Ohms to ensure proper circuit's oscillation.
  • Set R5 to read 1V RMS on an Audio Millivoltmeter connected to the output with R7 rotated fully clockwise, or to view a sinewave of 2.828V Peak-to-Peak amplitude on the oscilloscope.
  • With C1, C2 = 100nF the frequency generated is 100Hz and with C1, C2 = 1nF frequency is 10KHz but R5 requires adjustment.
  • High gain transistors are preferred for better performance.

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