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Wednesday, September 18, 2019

Three Opamp Differential Instrumentation

This is the best Instrumentation OpAmp, Great CMRR, ensure supply has no ripple and keep analog and digital grounds separate. Ri can be replaced with a trimpot and resistor to alter gain. Connect a preset ends to pins 1 and 8 and preset wiper to VCC for Offset Null when high gains are configured.

3 Op-Amp Differential Instrumentation Amp

The Input zeners and diodes form a protective clamp for all voltages above VCC-VDD. If supply is changed to +12 -12 change zeners to 12V zeners. Use similar Zeners at output to protect Output from being zapped by overvoltages or high energy - voltage*frequency transients. Add plastic capacitors across Rf for damping AC operation or ripple. Also avoid floating inputs by providing a bias.

3 Op-Amp Differential Instrumentation Amp

There is also listed  below all the Interactive Tutorials using Virtual Electronic Components. Instrumentation Differential Amplifier using Three Op-Amps, Slide the Potentiometers and vary the mV Source, See Output DVM. This is close to the Ideal Instrumentation Amp, High Z and Excellent CMRR.

Vout = (Vp - Vn) * (2Rf+Ri)/Ri

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Buffer or Unity Gain Op-Amp

If output impedance of a point is a high value then connecting another circuit at that point will load it resulting in malfunction or error. Buffers are used as interface between circuits. Low impedance of an output means it can source sink lot of current, when you need 2 opamps use LF353 or TL072 which are dual opamps.

Buffer or Unity Gain Op-Amp 

There is an Interactive Tutor in the above linked page. it has Virtual Meters and Pots, shows how a Unity Gain Buffer Operational Amplifier works. When no gain is required and you need to match impedance this can be use. Front end amps, Sensor amps. Polarity Inversion in Analog Computing or Signal Conditioning.

Buffer or Unity Gain Op-Amp

Vout = -(Vin) for inverting

Vout = Vin for non-inverting

A non-inv FET input is the best buffer, for inverting buffer use high R values Using very high R values like 2.2M or higher requires a glass epoxy PCB and guard rings around pin 2, 3 to prevent leakage currents on the PCB reaching the PINs. Also moisture and dust has to be prevented by using RTV coating or Varnish. Use 78L05 79L05 for the dual supply required by this circuit.

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    Differential Amplifier - Op-Amp Circuits

    This amplifies the difference between two inputs Vp and Vn the low impedance of this configuration is a drawback, but can be used in analog computing. Optimum VCC VDD can be +12/-12. AC signals common to Vp and Vn are canceled by this configuration.

    Differential Amplifier - OpAmp Circuits

    Use a capacitor like 10nF plastic from pin 2 to 3 or across R2 to make circuit stable. For AC applications use LF351 TLO71 as they have good slew rate and also are FET inputs. For AC applications use a capacitor (1uF) in series with Ri to block DC Components. The Inputs have asymmetrical input impedance this affects CMRR, also use 1% tolerance MFR resistors for Rf and Ri.

    Differential Amplifier - Op-Amp Circuits

    Vout = (Vp - Vn) * (Rf/Ri)

    Differential Amplifier is used in Analog Computation for the Minus Operation or Subtraction. It also has a better noise rejection and CMRR, but this one amp differential circuit has low impedance as the resistors network load the signal source. It is good only for an intermediate stage

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