BP01 pressure sensor and its application in portable electronic sphygmomanometer

1 Overview

BP01 pressure sensor is specially designed for monitoring blood pressure, mainly used in portable electronic sphygmomanometers. It adopts precision thick film ceramic chip and nylon plastic package, with high linearity, low noise and small external stress. It adopts internal calibration and temperature compensation to improve the accuracy, stability and repeatability of measurement. Within the full range, the accuracy is ±1%, and the zero offset is no more than ±300μV.

2 Main performance parameters of BP01

The internal equivalent circuit and package of BP01 are shown in Figure 1; Table 1 lists the main performance parameters of BP01 when the power supply voltage Vs is 5.0V and the ambient temperature TA is 25℃.

The limit parameters of BP01 are as follows:

●Maximum operating voltage: 20VDC;

●Most

Maximum pressure resistance: 1500 mmHg;

●Work

Operating temperature range: 0～70℃;

●

Foot welding temperature (maximum): 250℃ (2~4 seconds).

3Electronic blood pressure monitor based on BP01

3.1 Working principle

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The principle circuit of the portable electronic sphygmomanometer constructed with BP01 is shown in Figure 2. It consists of a bias power supply circuit (A1, A2), a pre-processing circuit (A3-A6), a display circuit (A7) and a pressure sensor (BP01). The blood pressure measurement range of the sphygmomanometer is 0-200 mmHg, the resolution is 0.1 mmHg, and the working power supply is a 9V laminated battery. The working principles of the main circuits in the sphygmomanometer are described as follows:

a. Bias power supply circuit

The power supply circuit is composed of a dual op amp LM10 with a built-in reference voltage. A1 constitutes a common-mode amplifier and A2 constitutes a follower. Their function is to amplify the built-in reference voltage and use it as the bias voltage Vs of the pressure sensor BP01. The value of Vs is determined by the following formula:

Vs＝Vref（1＋R2／R3）

Where: Vref is the built-in reference voltage of LM10. Its value is 200mV. Substituting this value together with the values ​​of R2 and R3 in the circuit into the above formula, the value of the bias voltage Vs is 5V.

b. Pre-processing circuit

The pre-processing circuit is composed of four operational amplifiers A3 to A6, of which A3 constitutes an offset bias circuit to compensate for the circuit offset; A5 constitutes a follower for isolating and buffering the output signal of the pressure sensor BP01; A4 and A6 constitute an amplifier circuit, and its gain AV is determined by the following formula:

OFF＝1＋（R1／RT）

If the offset is ignored, the output voltage Vout of the pre-processing circuit is:

Vout＝2（1＋R1／RT）VIN

Where: VIN is the output voltage of the pressure sensor BP01.

c. Display circuit

The display circuit uses a three-and-a-half-digit display driver. When working, the output of the pressure sensor BP01 is amplified by the pre-processing circuit, and then the display driver circuit drives the LCD to read the measured blood pressure value.

3.2 Debugging methods

a. Zero-pressure output adjustment

When the zero-pressure output is reached, adjust the offset potentiometer RP1. When the displayed value of the sphygmomanometer is 000.0, the zero-pressure output adjustment is considered completed.

b. Adjustment of preamplifier circuit gain

The full-scale output of the pressure sensor BP01 has a certain relationship with the bias voltage. When biased at 5V, the output at a pressure of 200mmHg is 10mV, and the corresponding input of the display driver circuit is 200mV. Therefore, the gain AV of the preamplifier circuit is 200mV/10mV. In this way, the value of the gain resistor RT can be inferred using the previous calculation formula of Av.

If the selected resistor R1 is 10kΩ, the gain resistor RT should be 1.1kΩ. When debugging, you can first use the potentiometer to adjust the output value, then use a multimeter to measure the resistance of the potentiometer, and finally replace it with a fixed resistor.

c. Full scale adjustment

When adjusting the full scale, first add 200mV voltage to the input end of the display circuit, and then adjust the potentiometer RP2 so that the reading is 199.9mmHg.

After the above adjustment is completed, it should generally be repeated several times to ensure that the displayed value reliably meets the accuracy requirements.

3.3 Component selection

In order to ensure the measurement accuracy, the selection of peripheral components of the above circuit is also an important link that cannot be ignored. Generally, potentiometers RP1 and RP2 should use 1% precision metal film multi-turn potentiometers; resistors should use 1% precision metal film resistors; capacitors generally use polyester film or mica capacitors.

4 Conclusion

When using the pressure sensor BP01 and other devices to design a portable electronic blood pressure meter, it should be noted that the output is different for different bias voltages, so the gain of the pre-processing circuit should be adjusted accordingly to meet different full-scale requirements.