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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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Precision Audio Millivoltmeter

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Parts:

R1_____909K    1/2W 1% Metal Oxide Resistor
R2______90K9   1/2W 1% Metal Oxide Resistor
R3_______9K09  1/2W 1% Metal Oxide Resistor
R4_______1K01  1/2W 1% Metal Oxide Resistor
R5_____100K    1/4W Resistor
R6_______2M2   1/4W Resistor
R7______82K    1/4W Resistor
R8______12K    1/4W Resistor
R9_______1K2   1/4W Resistor
R10______3K3   1/4W Resistor
R11____200R    1/2W Trimmer Cermet

C1_____330nF   63V Polyester Capacitor
C2,C3__100µF   25V Electrolytic Capacitor
C4_____220µF   25V Electrolytic Capacitor
C5______33pF   63V Polystyrene Capacitor
C6_______2µ2   63V Electrolytic Capacitor

D1-D4___1N4148 75V 150mA Diodes

IC1_____CA3140 Op-amp
IC2_____CA3130 Op-amp

SW1_____2 poles 5 ways rotary switch
SW2_____SPDT switch

J1______RCA audio input socket
J2,J3___4mm. output sockets

B1______9V PP3 Battery

Clip for PP3 Battery 
 

Notes:

  • Connect J2 and J3 to an Avo-meter set to 50µA range:
  • Switching SW2 the four input ranges will be multiplied by 5
  • Total fsd ranges are: 10mV, 50mV, 100mV, 500mV, 1V, 5V, 10V, 50V
  • Set R11 to read 1V in the 1V range, with a sine wave input of 1V @ 1KHz
  • Compare the reading with that of another known precision Millivoltmeter or with an oscilloscope.
  • The oscilloscope reading must be a sinewave of 2.828V peak to peak amplitude
  • Frequency response is flat in the 20Hz-20KHz range
  • If you have difficulties in finding resistor values for R1, R2, R3 & R4, you can use the following trick:
    R1 = 10M + 1M in parallel
    R2 = 1M + 100K in parallel
    R3 = 100K + 10K in parallel
    R4 = 1K2 + 6K8 in parallel
    All resistors 1/4W 1% tolerance