Simulation comparison of weak current/voltage conversion circuit in PSPICE and EWB

1 Introduction

Weak current signal detection has been widely used in signal processing, measurement technology, communication technology, state detection and general electronic circuit design. It has greatly promoted the development of related fields. It is very important to analyze and discuss weak current detection. Based on two simulation softwares, PSPICE and EWB, this paper analyzes and discusses the circuit simulation of converting weak current into 3-5V voltage.

2 Introduction and comparison of PSPICE and EWB software

1) Introduction to PSPICE software

PSPICE is a general electronic circuit analysis and simulation software. It is mainly used to simulate and analyze the circuit with a computer before the hardware of the analyzed circuit is implemented. It uses SPICE language as the core, and can add device models described in SPICE language obtained through various channels to the model library. Compared with traditional SPICE, PSPICE9.0 has six functional modules: circuit schematic design module Capture, core module PSPICE A/D, stimulus signal editing module Stimulus Editor, model parameter extraction module, simulation analysis and display module, and optimization module Optimizer. Its simulation functions are:

(1) DC analysis: Calculate the DC operating point of the circuit, DC small signal transfer function, and DC transfer characteristic curve.

(2) AC small signal analysis: frequency domain analysis (calculating the amplitude-frequency and phase-frequency characteristics of the circuit) and noise analysis (calculating the equivalent output noise of the specified output terminal and the equivalent input noise of the specified input terminal at each frequency point).

(3) Transient analysis: Perform transient characteristic analysis of the circuit within the user-specified interval; perform Fourier analysis on the output waveform under large signal sinusoidal excitation, and calculate the fundamental wave and 9th harmonic coefficients and distortion coefficients.

(4) Sensitivity analysis: Calculate the changes in circuit output caused by changes in circuit component parameters, including DC sensitivity analysis and AC small signal sensitivity analysis.

(5) Tolerance analysis: Calculate the impact of circuit component parameters deviating from the nominal value on the circuit output characteristics. This includes Monte Carlo analysis and worst-case analysis.

(6) Temperature analysis: According to the temperature specified by the user, analyze the circuit characteristics at different temperatures.

(7) Optimization design: Determine the optimal parameter combination of circuit components under given circuit topology and circuit performance constraints.

[page] 2) Introduction to EWB software

EWB software is a special electronic circuit simulation software launched by Interactive Image Technologies of Canada in the late 1980s. It can simulate the connection of circuits, the selection of component models and parameters on the computer, and then select the required virtual test instrument to simulate and test the circuit, and read the test results on the simulation instrument. The components required for drawing circuit diagrams and the test instruments required for circuit simulation can be directly captured from the screen, and the operating switches and buttons of the instruments are very similar to those of actual instruments. It is based on the SPICE3 language, and can read circuit netlist files in SPICE format for simulation analysis, and can also use circuit netlist files in OrCAD format. It has a huge component library, with a total of nearly 10,000 components, providing the necessary guarantee for the practical application of circuit simulation software. In addition to using virtual instruments, it can also perform a more detailed analysis of the circuit. The analysis results are presented in the form of graphs or reports. There are 13 analysis methods in total: DC operating point analysis, AC frequency analysis, transient analysis, Fourier analysis, noise analysis, distortion analysis, parameter sweep analysis, temperature sweep analysis, transfer function analysis, zero-pole analysis, DC and AC sensitivity analysis, Monte Carlo analysis and worst-case analysis.

3) Comparison

(1) EWB has an intuitive interface, is easy to operate and master. Compared with other simulation software, its price is extremely low and has a very high price-performance ratio.

(2) Compared with EWB, PSPICE has the following advantages: First, it has a wide variety of signal sources, especially signal sources for transient analysis; second, it has a powerful output waveform function. You only need to place a voltage (current) probe at the observed node to observe the result in the simulation diagram. It also integrates many mathematical operations and can edit the simulation result window. Although it is not as easy to simulate electronic circuits with PSPICE software as with EWB, it can complete many tasks that EWB cannot complete. Therefore, it is best to use these two software together for electronic circuit simulation.

3 Principle and simulation analysis of micro-current/voltage conversion circuit diagram

1 Circuit diagram principle

Since the useful signals encountered in practice are very small and the common-mode interference is very high, a three-stage operational amplifier differential amplification detection circuit with high gain, high common-mode rejection ratio and high input impedance is selected. Its schematic diagram is shown in Figure (1):

Figure 1 Three-stage operational amplifier differential amplification detection circuit

In the figure, op amps A1 and A2 are connected in a proportional operational circuit, and both use the same-phase input to make the circuit input impedance high. The circuit structure adopts a symmetrical form, and the peripheral resistors use high-precision resistors, so that drift, noise, offset voltage and offset current offset each other to improve the measurement accuracy of the circuit, otherwise the circuit's amplification factor, common-mode rejection ratio and accuracy will be significantly reduced. According to the virtual short and virtual open principles of the integrated op amp, the input-output relationship can be obtained as follows:

[page] 2. Apply EWB simulation

(a) Draw the circuit schematic according to Figure (1):

(1) Create a new circuit diagram

Click File->New to create a new blank circuit diagram.

(2) Drawing settings

Click Circuit->Schematic Options to make corresponding settings for the newly created circuit diagram.

(3) Place the required components

Grab the required components from the component cabinet

(4) Component editing, component layout adjustment, and wiring.

(5) Run the simulation and observe the output or electrical parameters (current, voltage, power, etc.) of certain nodes from the virtual instrument.

(II) Take the input of 15uA current as an example. First select the component. Right-click the mouse and select Component Properties from the pop-up menu to edit the component accordingly. If the component is an integrated operational amplifier, you can also select Edit from Model to make corresponding changes to the operational amplifier model, such as changes to parameters such as bias current, open-loop gain, and input impedance. After selecting the components and setting the parameters as required, arranging the components reasonably and laying out the wiring, the weak current/voltage conversion circuit diagram is shown in Figure (2).

The operational amplifier is LF356, and the virtual instrument uses an oscilloscope to observe the final output voltage waveform.

(5) Simulation analysis

Click the Run button in the upper right corner of the desktop. The circuit diagram will run according to time. If you want to observe whether the output voltage meets the requirements, double-click the oscilloscope, set the oscilloscope accordingly (just like operating an actual oscilloscope), and then observe the output waveform from the oscilloscope as shown in Figure 3. It can be seen from the figure that the output voltage is 3.3204V, which meets the requirements. When R1 is changed, different output voltages are obtained. The simulation result list (1) is as follows:

Table 1 Comparison of EWB simulation results

(6) Similarly, setting the value of the current element to other values ​​can also meet the requirements.

[page] 3. Apply PSPICE simulation

(I) Steps to draw a circuit diagram

(1) Create a circuit diagram file

Enter Capture to create a new Project, and select the design project type as Analog or Mixed Signal Circuit.

(2) Loading the component simulation library

(3) Calling simulation components

In the circuit diagram editing window, start the [Place/Part] command and select the required components; select the menu command [Place/wire] to enable the wiring mode to complete the wiring; select the menu command [Place/Net] to enable the node setting to complete the node setting. Nodes with the same name will be connected together during simulation. This is conducive to the simplification of complex circuit diagrams.

(4) Edit the components

Double-click to activate the component, set the parameters of the component from the pop-up menu, and click Apply. Select Circuit Diagram Page from the Windows command menu to return to the circuit diagram design desktop.

(5) Run simulation analysis

(ii) Taking 15uA current as an example, select the parameters and its circuit diagram is shown in Figure (4).

Figure 4 PSPICE simulation schematic diagram

The op amp is uA741. After DC sweep analysis and transient analysis, the output voltage is 4.006V, which meets the requirements. The output waveform is shown in Figure 5. It can be seen from the simulation result that as the input current (0~10uA) changes, the output voltage also changes. However, there are certain requirements for the input current range. If it exceeds this range, the change of the output voltage cannot be detected.

4 Conclusion

This paper gives the circuit schematic diagram for converting weak current into detectable voltage circuit. PSPICE and EWB software were used for simulation analysis, and the simulation results were compared. The circuit principle is to connect the sampling resistor in series with the circuit under test to directly measure the voltage across the resistor. Its advantage is that the measurement is simple. However, when the measured current changes greatly or the sampling resistor is large or small, it will seriously affect the measurement precision and accuracy, and there are disadvantages such as small measurement range and large measurement error. For example, when the measured current exceeds more than ten microamperes when simulated by PSPICE software, the circuit diagram fails. The use of new integrated current/voltage conversion chips such as MAX472 can overcome the shortcomings of the above conventional detection methods and achieve high-precision measurement of step current.