Training Course - Precision Instrument Amplifier

Project Introduction:
This project is the first lesson in the Lichuang training program for first-year university students. Building upon the basic knowledge of operational amplifiers taught in the previous lesson, it further explains the circuitry of precision instrumentation amplifiers and guides students in designing their application circuits, enabling them to apply their knowledge in practice.
Project Functions:
This precision instrumentation amplifier features ultra-low cost, ease of use, and wide applicability. Its basic function is to serve as the analog front-end for simple oscilloscopes and multimeters. It features high input impedance, differential input, high-precision gain, and adjustable bias output. Furthermore, the use of dual potentiometers allows for both coarse and fine adjustment, enabling convenient and precise gain control. Compared to traditional three-op-amp instrumentation amplifiers, an additional op-amp is used as a reference voltage buffer because this conveniently provides the bias voltage required by common sampling circuits such as subsequent ADCs.
Project Parameters
Core IC: TL084 Operational Amplifier: This is a classic, nationally used teaching-grade op-amp for electrical and electronic experiments. It is a high-input-impedance, high-slew-rate op-amp based on FET input-stage technology, with a very low offset voltage of only 3mV (datasheet data). It has almost no input compensation current and is unity-gain stable. One chip contains four op-amps, making it ideal for use as a three-op-amp instrumentation amplifier plus a reference voltage buffer.
Peripheral Circuit: A basic three-op-amp instrumentation amplifier topology. The gain adjustment uses a 200-ohm gain-limiting resistor plus two 3386 single-turn potentiometers, which can be easily adjusted with a screwdriver. The resistor values ​​are 1kΩ and 10kΩ, therefore, the lower gain limit is 2.8, and the upper gain is close to 100, providing a wide adjustable range while allowing for easy and precise adjustment.
Principle Analysis (Hardware Description)
: The principle of the three op-amp instrumentation amplifier is as follows: First, two op-amps form a fully differential amplifier. The amplification factor is easy to calculate: First, set a virtual short equation for each op-amp, and you can get
Vo+-Vo-=(1+20kΩ/R3)*(Vi+-Vi-).
Then, the last op-amp is a differential-to-single-ended circuit.
Precautions:
If you want to test the waveform of mains power, you should add at least a 40dB attenuator in the preamp stage. This can be achieved simply by using a 1MΩ resistor and a 10kΩ resistor for voltage division. Because this part of the circuit involves high voltage, it should be connected to an external board to prevent interference, and copper should not be laid (you can directly solder a resistor to the point).
Additionally, if you want to modify it into a current measurement module, you should add a 0.1Ω precision sampling resistor at the high-impedance input terminal, connected across the differential input terminal.
The assembly process
begins by applying solder paste to the surface mount pads, followed by the surface mount resistors and capacitors, which are then soldered using a hot air gun. Next, the through-hole IC sockets, pin headers, and potentiometers are soldered. Finally, the TL084 is inserted (note the orientation). Powering on and testing is then possible. First, connect the power supply and check for overcurrent (to prevent short circuits). If no overcurrent occurs, apply a test signal (typically a 1kHz, 100mVpp sine wave) to check the output waveform for normality. Adjust the gain to see if it's flexible and the bias is easy to adjust.
Afterward, place it in a non-common-ground circuit with a large common-mode voltage to check for a high common-mode rejection ratio. [
Image of the actual component]