[Changshu Institute of Technology] Differential Amplification Circuit Blue Cat

Differential amplifier circuit tester

Summary

   Briefly describe the design, simulation and testing methods of a typical differential input differential output amplifier circuit, and discuss its design principles and problems that need to be solved. It focuses on the design and calculation of differential filters, points out the differences between the design and testing of single-ended amplifier circuits, and combines practical work experience with a simple case for testing DC signals and AC signals. .

   Compared with ordinary single-ended amplifiers, differential amplifiers can effectively suppress the impact of common-mode noise in the input signal and ground-level voltage fluctuations on the circuit. Therefore, they are widely favored in industrial applications. Among differential amplifiers, instrumentation amplifiers are the most widely used. With the development of technology, more and more ADCs and MCUs support differential input . Because differential transmission can better suppress common-mode interference and enable longer signal transmission distances, differential transmission will be used in more and more situations. However, typical instrumentation amplifiers only support single-ended output.

   Therefore , a differential input differential output amplifier circuit was built using dual operational amplifiers. Compared with ordinary single-ended amplifier circuits, differential amplifier circuits have many differences in design, analysis, simulation and testing, and this knowledge is rarely introduced in general analog circuit textbooks.

Keywords: amplifier circuit common mode operational amplifier differential amplifier

1. Plan demonstration

There are two design options: use a single-chip microcomputer for DA output to obtain a sine wave, and then further obtain differential signals and common-mode signals; another option is to connect variable resistors, capacitors, and inverters through perforated boards, and the 51 single-chip microcomputer output can Tuned sine wave.

The design of Scheme 1 is mainly divided into two parts: a differential signal source and a triode differential amplification circuit. The differential signal source is composed of a 51 microcontroller coupled with a timer, DAC output, buttons, and digital tubes. The timer is used to control the frequency of the sine wave. The DAC converts the digital quantity into an analog quantity and outputs a sine wave whose amplitude and frequency are adjusted ; the other part is a differential amplifier circuit built by the triode S9013 , which measures the parameters of the differential signal.

Scheme 2 mainly uses a 51 microcontroller to output a PWM square wave under the control of a timer , and then divides the voltage by a variable resistor and filters the capacitor to form a sine wave, and then forms a differential signal or common mode signal through an inverter.

Comparison: Considering that the sine wave obtained by capacitor filtering may not reach the accuracy required for the experiment, it is more convenient to control the amplitude and frequency of the waveform by comparing the DAC output, so this experiment adopted option one.

Amplification circuit: On the test end, J1 , J2 , J3 , and J4 are led out using pin headers, which are convenient for using alligator clips and power supply. The test end uses female headers to facilitate the connection of the positive and negative poles of the power supply. Also reserve a ground terminal. Use 25mil for the power line in the circuit PCB board and 15mil for the connections between components .

2. Theoretical analysis

Related formulas:

3.  Circuit and Programming

51 microcontroller = >> DAC output

Multisim simulation

programming:

Write the following program on keil :

1 ) Calculate and obtain the sine wave data table;

2) Calculate the timer trigger interval based on the number of points in the period and period of the sine wave data table;

3) Initialize the DAC output channel and initialize the DAC working mode;

4) Configure the timer used to trigger the DAC ;

After the configuration is completed, the signal output can be detected in the D/A port pin.

4. Test plan and test results

Data measurement plan:

Use the control variable method: ( 1 ) At the same frequency, increase the amplitude from 100mv to 100mv      , observe its saturation value range, and at what amplitude it will be distorted;

(2) Change the frequency at the same amplitude and proceed by a multiple of 10. Take the frequency from       100HZ to 1MHZ to get the amplitude-frequency characteristic curve, and then take 70% of its amplitude to get the          corresponding bandwidth.

Test Results:

The test found that the oscilloscope forms a straight line through MATH+ operation when the common mode input is input ;

    The oscilloscope forms a sine wave through MATH- operation during differential mode input .

Conclusion: The differential amplifier circuit can suppress the temperature drift, but due to the component process, certain errors will occur, so it is sufficient within the allowable error range, indicating that the differential circuit can suppress the zero point drift.