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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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One Condition Trimming Circuit Diagram

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One Condition Trimming Circuit Diagram. This relatively simple, inexpensive circuit requiring one trimming operation can multiply or divide with a consistent accuracy of greater than 1 part in 1,000. An inexpensive CMOS version of standard 555 timer chip T, in conjunction with low-drift LMll error amplifier A3, an inexpensive analog chopper switch SW, form a unique voltage-to-duty-cycle converter to produce the difficult transfer function necessary for accurate conversion. Read: Use 555 Build Spaceship Alarm

 One Condition Trimming Circuit Diagram


One Condition Trimming Circuit Diagram


An unknown multiplicand voltage applied to the A3 error op amp circuit`s Y input controls the duty cycle of the timer through its pin 5 modulation input. The network between the sink-and-source output of the timer, pin 3, and the state trigger inputs, pins 2 and 6, cause the timer to oscillate. An error feedback signal from the timer`s discharge output, pin 7, represents the duty cycle. Integrating this duty-cycle signal with voltage reference REF representing full scale, and applying the result to the inverting input of A3, closes the feedback loop and insures high accuracy. Read: Rf Probe Circuit Diagram For vtvm

Multiplier X feeds into another LMll op amp, A1, which acts as a input buffer and scaler. A third LMll, A2, filters and buffers the Z output. Between A1 and A2, the timer`s duty-cycle output modulates the analog switches of a CD4066 to achieve the desired multiplier output. To perform division instead of multiplication, reconfigure the op amp A1 circuit with the use of jumpers. Amplifier A2 isn`t required in the division configuration. To calibrate the circuit, connect the X andY inputs together and apply 10 V. Read: DC to AC Inverter by IC 555

Then adjust the 10-turn span potentiometer to achieve a 10-V output at Z for multiplication, or 1 V for the division configuration. Also check for zero output at a zero multiplier input. The circuit is scaled for 0 -10 V inputs and outputs with a small overrage capability, but other scalings are possible. Star grounding or a heavy ground bus should be used to reduce offset problems that are unavoidable in this design.


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