Traditional Chinese ECG machine calibrator signal source (Workshop Practice) — Circuit, Wiring & Debug Guide

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Cardiac EEG Calibrator Signal SourceThe core part of the electrocardiogram calibrating instrument is the signal source. In order to meet the needs of calibration, the signal source should be able to generate sine, square, triangle, differential and other waveforms. In addition to the above functions, the signal source introduced in this article can obtain various other waveforms by appropriately changing the software.The structural block diagram is shown in Figure 14-1; the circuit schematic diagram is shown in Figure 14-2.Figure 14-1(1) Voltage reference: Use LM336-5.0V, see VS in Figure 14-2. VD1~VD4 are temperature compensation. If R2 and R3 are replaced with potentiometers, the reference voltage of the reference source can be adjusted (fix it here).(2) Waveform formation: It is mainly composed of A1 digital-to-analog converter DAC1230 and operational amplifier A2. When working, send different codes to DI1~DI8 of A1, and the VRFE terminal can output the corresponding voltage. When the microprocessor sends codes to A1 through the data bus D6~D7 according to the function rules of the signal to be generated, the VRFE end will get the corresponding waveform. The waveform output by VRFE is then amplified and level shifted by R4, R5 and A2, and A2 outputs a VDZ~4095/4096VDZ bipolar signal. Its peak value is close to 2VDZ.Table 1Output nominal value (peak-peak value)AttenuationKL1KL2KL32V1/5arbitrary114V2/51018V4/50010.2V1/50arbitrary100.4V2/501000.8V4/50000(3) Step attenuator 1: It is composed of resistors R7~R11 and breakers KL1~KL3. Through the combination of different states of the three relays, different attenuations are obtained. The corresponding relationship is shown in Table 1. In Table 1, it is assumed that the attenuator input signal peak-peak value 2VDZ=10V, and the relay contact state drawn in Figure 14-2 is assumed to be a logic 1 state.(4) Following and active filtering: A non-phase input second-order active filter is composed of A3, R12~R15, RP1, C5 and C6, and the turning frequency is about 10Hz. Its functions: ① Take advantage of its high input impedance to make the error caused by changes in the input impedance of the precision attenuator so small that it can be ignored; ② Properly smooth the small steps in the signal generated by A1; ③ Fine-tune the amplitude through RP1. During adjustment, the amplification factor of A3 …

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