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Nowadays, smart sensor technology has been developed as a respiratory medical monitor with a sensor head. It can be used to conduct a lot of research on the connection between various emotions and breathing, and can be used to truly record a person's breathing status and its changes.

I. Respiratory medical monitor and smart sensor technology (see Figure 1)

1. Respiratory medical monitor

It is used to monitor breathing status and can give an approximate breathing depth. This monitor monitors some important parameters that can be used to evaluate anxiety: breathing rate, breathing uniformity, and the interval between exhalation and inhalation. Calm and positive emotions usually cause exhalation to be longer than inhalation, and the ratio of the two times reveals the level of anxiety in one aspect. Relatively high levels of chest breathing (relative to abdominal breathing) can also indicate the level of anxiety. Observation of chest breathing can increase the visual information of the monitor.

2. Smart sensor technology

The monitor in Figure 1 uses a silicon piezoresistive sensor (PRT) to detect the decrease and increase in pressure corresponding to inhalation and exhalation. The output of the PRT is fed into a MAX1450 signal conditioning IC to correct the inherent error of the PRT, and then the compensated voltage signal is sent to the 12-bit analog-to-digital converter ADC. The ADC output (digital pressure signal) then enters a PC interface and is converted to RS-232 level. Finally, the signal is transmitted to the PC, so that the breathing waveform can be displayed and the above parameters can be analyzed.

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II. PRT Sensor

1. PRT Detection Principle

PRT is generally configured as a compact Wheatstone bridge. When pressure is applied to the sensitive bridge of the PRT (see Figure 2a), the resistance value of the diagonal bridge arm will change in the same direction and the same magnitude. When the two resistance values on one diagonal bridge arm increase under the action of pressure, the resistance value of the other diagonal bridge arm decreases, and vice versa. For a semi-sensitive bridge PRT (see Figure 2b), only the resistance value of half of the bridge arm changes. Whether it is a full-bridge or semi-sensitive bridge PRT sensor, it has the advantages of high sensitivity (>10mv/v), good linearity and temperature stability, no signal hysteresis, etc., and its measurement range can reach the destructive limit.

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2. Application of PRT

Today, as new IC technology can accurately calibrate PRT sensors, its application range has been expanded from medium and low precision detection to high-end fields. It solves the shortcomings that PRT sensors can only be used in medium and low precision detection in high-end products, such as expensive strain gauges.

3. Signal conditioning IC and modern and simplified correction scheme

Since the error amplitude range of PRT sensors is very wide, they must be calibrated by modern and simplified compensation methods. There are two latest signal conditioning ICs used to calibrate PRT sensor errors, one is MAX1457, and the other is MAX1450.

1. Modern and correction scheme

MAX1457 has a controlled current source for driving the sensor and an ADC for sampling the sensor bridge voltage, which is the product of the current output current and the temperature-related bridge resistance. MAX1457 contains EEPROM, etc. Four correction coefficients are calculated by the application software designed for MAX1457: full-scale output (FSO), temperature (FSOTC), offset (Offset), temperature offset (OffsetTC). To correct the error.

Since the accuracy provided by MAX1457 is much higher than the requirements of a respiratory monitor, there is no need to use a 16-bit resolution DAC for correction, but the temperature error compensation capability of MAX1457 is also necessary for small temperature changes: a change of 10℃ usually causes a 3% change in the full-scale output (FSO) of the PRT. Because MAX1457 enables the monitor to work in a wide temperature range, the combination of PRT and MAX1457 can obtain better accuracy than 100%, so it can be used in space exploration and diving respirators.

2. Simplified Correction Scheme

The MAX1450 signal conditioner (see Figure 3) is essentially the same as the MAX1457, except that resistors are used instead of DACs for error correction. Because the MAX1450 uses far fewer calibration points than the MAX1457, its accuracy is 1%. It is often used in a hybrid solution that combines the MAX1450 with laser trimming resistors (all external resistors) to provide a low-cost solution, that is, using the MAX1450 signal conditioner with external laser trimming resistors to provide 1% accuracy. Adjusting the RFSOA resistor sets the initial (FSO) sensitivity, and the temperature deviation is adjusted by feedback sensor drive voltage (from the BDRIVE pin), and the PGA programmable amplifier completes the deviation compensation.

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Now let's introduce the pin functions of MAX1450:

INP sensor positive signal input;

SOTC temperature offset signal input;

A0 programmable amplifier (PGA) gain setting lowest bit input;

A1 programmable amplifier gain setting;

A2 programmable amplifier (PGA) gain setting highest bit input;

OFFTC offset temperature correction;

OFFSET offset adjustment input;

BBUF buffered bridge voltage input;

FSOTRIM bridge drive current setting input;

OUT programmable amplifier output voltage;

ISRC current source reference;

BDRIVE sensor excitation current input;

INM sensor negative signal input;

VDD power supply voltage;

VSS ground.