1 Introduction

In scientific research and engineering, data acquisition system has a wide range of applications. In view of the different voltage sensor output signal voltage value, this paper proposes a data acquisition system which can control the gain. The system takes FPGA as the logic control core, selects the instrument operational amplifier AD623 as the amplifier circuit and adg704 as the analog switch. By programming and configuring the FPGA, the analog switch is controlled to gate different resistors, and the gate resistor cooperates with AD623 to realize amplification. Simultaneous interpreting of the different signals from different sensors can be achieved by the system. The requirement of wide acquisition range is realized.

2 system design scheme

This design uses FPGA as the main control unit to realize the logic control of the whole system. The principle block diagram of the whole system is shown in Figure 1. From the point of view of the whole system, it can be divided into several parts: the whole operation module and the control module.

Design scheme of typical circuit using field programmable gate array device

The working process of the whole system is as follows: firstly, the input signal range of each channel is determined to determine the gain of each channel. Then, the FPGA is programmed and configured to make adg704 select the gain configuration resistor. After being amplified by AD623, the output is followed by the Precision Op Amp op113, and the analog signal is output to the AD converter through multiplexer switching, and the converted data is stored in flash through FPGA. Complete the data acquisition of the whole system.

3 typical circuit design

Figure 2 shows the circuit diagram of the operational amplifier module.

The core of the module is the instrument operational amplifier AD623. AD623 is an integrated single power supply instrument amplifier, which can provide full power amplitude output under single power supply (+ 3V – + 12V), and its gain setting range is 1 ~ 1000. AD623 maintains the minimum error by providing an excellent AC common mode rejection ratio (accmrr) that increases with gain. Line noise and harmonics will be suppressed because the CMRR remains constant up to 200 Hz. The gain can be set by the resistance between pin 1 and pin 8. The formula is as follows:

G is the magnification.To adjust the resistance.

In the figure, a first-order passive low-pass filter is added before the input signal to filter out the high-frequency components in the signal, and the signal end frequency can pass through

F = 1 / 2 Π RC, at the same time, a voltage divider filter is added at the output of AD623, which forms a second-order passive low-pass filter with the previous filter. For AD623, if the signal is directly amplified without common mode signal, the maximum output signal will be limited to 1.25V. As shown in the figure, the input signal is 0-20mv sine wave, and its gain is set to 100 by adjusting the resistance. At this time, the output should be 0-2v sine wave, but the actual waveform is as follows:

If 2.5V common mode voltage is applied at the input end, the circuit diagram is connected as shown in Fig. 2. Similarly, 0 ~ 20mV sine wave is applied at the input end, the gain is set to 100, and the output waveform is as follows:

In order to improve the driving ability, as shown in Figure 2, a follower op113 is added at the output end of AD623 as the driver, because the AD623 is designed to drive a load of 10K Ω or above. If the load is less than 10k Ω, a Precision Op Amp should be used as buffer to improve the driving ability. When the load is less than 600 Ω, the output swing of 0-4V can be obtained on the load.

In Fig. 2, adg704 is used as an analog switch, which is mainly used to switch a channel from S1 to S4 to connect with D. the selected channel cooperates with AD623 to realize gain control. The control of adg704 is realized by programmable logic device. Control enable and selection signals A1, A0, en are switched through truth table 1.

Conclusion

In this paper, the programmable logic device (PLD) is used to control the amplification factor of the system, which makes full use of the gain adjustable function of the operational amplifier AD623 and its advantages. Each thermocouple input signal can have multiple different magnification, which makes the acquisition of input signals with different amplitudes more convenient, reliable and fast. The selected 16 bit AD converter has the advantages of high acquisition accuracy, convenient control and fast conversion speed, which greatly optimizes the system.

The author’s Innovation: by programming and configuring FPGA, we can set the gain of voltage signals with different input voltage values of different channels, so that the output signals of each channel are in the range of signal needed by AD acquisition. The design has the advantages of simple structure, small volume, strong applicability, excellent performance, good controllability, and can meet the needs of most experiments and scientific research processes, and has a wide range of applicability.

Editor in charge: GT

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