Single power supply I/F conversion circuit design

Abstract: This paper introduces the design method and function of the single-power supply I/F conversion circuit. The single-power supply I/F conversion circuit composed of OPA2335AID, TS5A23166, hysteresis voltage comparator, CMOS switch and other devices can convert 4-20 mA current signal or 1-5 V voltage signal into 10-50 kHz square wave signal. It adopts 3-5 V single power supply, and the output linearity can reach +0.1% FS. The circuit structure is simple, the stability is high, and the linearity is good.
Keywords: OPA2335AID; voltage-frequency conversion; current-frequency conversion; TS5A23166

With the widespread use of digital instruments, converting the standard signal of 1-5 V/4-20 mA of type III instruments into a digital signal has become a key problem that many instruments need to solve. At present, the conversion circuit is mainly used, most of which are dual-power supply, with complex structure and poor stability. This circuit design adopts single power supply voltage, with simple structure, good stability, and can improve the efficiency of power supply.
The I/F conversion circuit mainly includes two parts: voltage/frequency conversion circuit (VFC) and current-frequency conversion circuit (IFC). The function of the voltage/frequency conversion circuit (VFC) is to convert the input DC voltage of 1~5 V into an output voltage of 0.5~2.5 V through a voltage follower. After the integrator, a stable triangle wave is output, which is then input into the voltage comparator, and finally a stable rectangular wave is output, so that the output frequency is 10~50 kHz. The function of the current/frequency conversion circuit is to amplify the input DC current of 4~20 mA through gain, output a constant voltage of 0.5~2.5 V, and finally complete the frequency conversion and output a stable rectangular wave. This circuit has the function of converting I/F and V/F circuits into frequency. It only uses a single power supply of 3~5 V, with a simple circuit structure, high output linearity and perfect functions.

1 Circuit design
The main part of the single power supply I/F conversion circuit is composed of an operational amplifier, an integrator, a comparator, and a CMOS switch circuit. The rationality of its design is directly related to the success of the circuit output waveform. The designed circuit principle block diagram is shown in Figure 1.

(1) Operational amplifier circuit.
The operational amplifier uses LM358, which plays the role of amplifying the input signal and can also form an absolute value voltage follower to adjust the input voltage. Although the function of this circuit can be realized, the LM358 chip has a slow amplification rate, a small signal supply voltage range of only 4 to 5 V, is not stable enough, and has a small characteristic temperature range of 0 to 70°C, which is easily affected by temperature. The operational amplifier uses OPA2335, which is powered by a single power supply and has an operating voltage of +2.7 < +5.5 V. It has two packaging forms, MSOP-8 and SO-8, and contains two operational amplifiers. The amplification rate is high and relatively stable, and the characteristic temperature range is large, from -40 to 125°C, and is not easily affected by temperature. Therefore, OPA2335 is selected.
(2) CMOS switch circuit.
The CMOS switch uses the chip CD4016, but the chip CD4016 has a large internal resistance, which can reach a maximum of 100 Ω, and is not stable enough and will change with the change of the supply voltage. The CMOS switch uses the TS5A23166 chip. When the input is high level, the switch is turned on; when the input is low level, the switch is turned off. The internal resistance of the TS5A23166 chip is very small, only 0.5 Ω, which is relatively stable and will not change with the change of voltage. Therefore, the TS5A23166 chip is selected.

2 Implementation of single power supply I/F conversion circuit
The single power supply I/F conversion circuit is mainly composed of operational amplifier, absolute value voltage follower, integrator, reference voltage, hysteresis voltage comparator, CMOS switch circuit design and other parts.
The single power supply I/F conversion circuit is mainly divided into three levels. The first level is to process the input voltage or current through the absolute value voltage follower and operational amplifier, and finally output a constant voltage. The design of the circuit is mainly to directly convert the input current of the current/frequency conversion circuit (IFC) 4~20 mA into 0.2~1 V voltage, and process it through the absolute value voltage follower and operational amplifier, and output 0.5~2.5 V voltage as the second level input voltage. Similarly, the voltage/frequency conversion circuit (VFC) converts the input voltage 1~5 V and also outputs 0.5~2.5 V voltage as the second level input voltage. The second level is to charge and discharge the constant voltage through the integrator, and finally output a stable triangle wave. The third stage is to input a stable triangle wave into the comparator, where the voltage at the positive input of the comparator is provided with a stable voltage by a three-terminal fixed voltage regulator circuit, so that a stable square wave can be output, and the output square wave has good linearity.
(1) Analysis of the first stage operational amplifier circuit (chip OPA2335AID).
The input current signal in the range of 4 to 20 mA is converted into a voltage of 0.2 to 1 V through the sampling resistor R1, amplified 2.5 times by the amplifier, and outputs a constant voltage of 0.5 to 2.5 V; the input voltage signal in the range of 1 to 5 V is divided by a resistor and a voltage follower, and finally outputs a constant voltage of 0.5 to 2.5 V.

The outstanding feature of this part of the circuit is that it can input two signals, current and voltage, and the final output voltage is a constant voltage of 0.5 to 2.5 V, completing the functions of the two parts. The circuit is shown in Figure 2.

(2) Analysis of the second-level circuit (chip OPA2335AID, TS5A23166).
Input a voltage of 0.5 to 2.5 V, through the integrator, the capacitor C1 is charged, and after a certain period of time, the charging is completed and the voltage reaches the maximum value. When the CMOS switch is turned on, the capacitor begins to discharge, and the capacitor is repeatedly charged and discharged, and this cycle is repeated, and finally a stable triangle wave is output. And using OPA2335AID as an integrator shortens the period, thereby increasing the frequency. The circuit is shown in Figure 3.

(3) Analysis of the third-level circuit (chips OPA2335AID, TS5A23166, REF3020).
A stable triangle wave is input to the negative input terminal of the voltage comparator, while the positive input voltage is provided by the reference voltage. Using REF3020, a constant 2 V voltage can be output. When the comparator output is high, the COMS switch is turned on, and a high level is input to the positive input terminal. Otherwise, a low level is input. After this repeated cycle, the circuit finally outputs a stable square wave. The circuit is shown in Figure 4.

3 Design Results
The single-power supply I/F conversion circuit designed in this paper can convert current signals and voltage signals into stable square wave signals. After testing, it has good linearity. The test method and test data are recorded as follows:
(1) Input the output voltage of the signal 5 V, 4.5 V, 4 V, 3.5 V, 3 V, 2.5 V, 2 V, 1.5 V, 1 V, respectively, measure the corresponding output frequency value, calculate the linearity, and realize V/F conversion.
(2) Input the signal 20 mA, 18 mA, 16 mA, 14 mA, 12 mA, 10 mA, 8 mA, 6 mA, 4 mA respectively, observe the output waveform, measure the corresponding output frequency value, calculate the linearity, and realize I/F conversion.
Test result analysis: After analysis, the circuit can realize I/F and V/F conversion, and the calculated linearity is +0.1% FS (5 Hz). The results are shown in Figures 5 and 6.

4 Conclusion
The circuit design in this paper can convert current and voltage signals into stable square wave signals. In the design process, it fully utilizes the characteristics of single power supply, simple circuit structure, high stability, good linearity, and can meet the accuracy requirements of process instruments. It is convenient and concise, and the instrument design has a high reference value.