New micro-sensor in medical instrument application solutions

1. preface

Today, with intelligent sensor technology picks up probe of respiratory medical monitors have heard, this is an ideal solution for all kinds of emotions and make substantial link between respiratory problems and can be used to the real record of a person's respiratory conditions and changes.

In this article on respiratory monitors with miniature sensor-Silicon Piezoresistive sensor (SPRT) and MAX1450 signal conditioner or MAX1457 formed new intelligent sensors and in function of the technical programme for an analysis and presentation.

2. respiratory medical monitors and smart sensors and technologies (its composition chart, see Figure 1 below)

2.1 respiratory medical monitor (see Figure 1, right)

　　

　　

The monitor is used to monitor the respiratory status and can give an approximate depth of breathing.

This monitor for monitoring some can be used to evaluate important parameters of anxiety: breathing frequency, breathe evenly and exhale and inhale the intermission between. Calm, positive emotion often leads to exhale is longer than the duration of inhalation, than parameters, can be from one side to reveal human anxiety. Relatively high levels of thoracic breathing (relative to the abdominal breathing) can also be instructions for anxiety. Monitor on chest breathing observation reflects the monitor has a large amount of Visual information.

2.2 intelligent sensor technology

Figure 1 respiratory monitors using silicon Piezoresistive sensor (SPRT — Silicon piezoresistance transducer) detect when you inhale and exhale corresponding pressure reduction and increased.

SPRT output is fed into a MAX1450 signal conditioner or MAX1457, chip or MAX1457 MAX1450 signal conditioner chip and on the incentive to drive SPRT (or current source), the inherent error was SPRT-infeed MAX1450 or after and correction MAX1457, then after compensation voltage signal fed with 12-bit analog-to-digital converter (ADC) MAX1202. ADC output (digital pressure signals) into a PC interface MAX3232, and converted to RS-232 level. Last signal is passed to the PC system, so that you can demonstrate respiratory waveform, and on the above mentioned parameters for analysis.

3. about Silicon Piezoresistive sensor (SPRT)

3.1 Silicon Piezoresistive sensor detection principle (SPRT)

Silicon Piezoresistive sensor typically configured as a close Wheatstone bridge.

When there is pressure sensitive to the SPRT bridge arm (as shown in Figure 2a), diagonal leg resistance value of R is the same as the direction, will take place the same amount of change, i.e. R + R and R-△ △ R. When a diagonal leg two resistance in the pressure increases, the other diagonal leg of resistance values, and vice versa. For SPRT half sensitive bridge (see fig. 2b), only half-arm resistance value changes. Whether it is a full-bridge or half-sensitive bridge sensor with high sensitivity (> 10mv/v), good linear, temperature stability and no signal hysteresis, etc., the measuring range can be from destructive limit.

　　

　　

3.2 Silicon Piezoresistive sensor applications

Today, thanks to the new IC technology has been able to accurately calibrate SPRT, Silicon Piezoresistive sensor applications range from the low precision inspection extends to the high-end segment.

Makes up for in the past, SPRT typically can only be used in low-precision detection and on high-end products to use expensive strain gauges, etc. But to apply Silicon Piezoresistive sensor must resolve error compensation-calibration technology to increase the precision. To do this, first and foremost with Silicon Piezoresistive sensor error for analysis.

4. about Silicon Piezoresistive sensor (SPRT) error

Calibration SPRT sensor main difficulty lies in the fact that their scope is very wide margin of error, a different process for the production of SPRT sensors have different types of errors and error range.

Even from the same manufacturer of same type sensor margin of error, there are slight differences. SPRT sensor error factors include: full partial signal with temperature variations strongly nonlinear (up to 1% oK); a large initial offset (up to 100 per cent of the full rate or higher) and offset drift over temperature strongly. To a certain extent, these defects can be compensated by the electronic circuit.

Temperature, Figure 2 (a) (b) as shown in the two types SPRT bridge resistance (between the Vcc and the equivalent resistance) to very wide pressure range remains fairly constant, however, with increasing temperature, bridge resistance increased significantly, driven by a constant current source if bridge, the bridge voltage (means the Vcc and ground between equivalent resistance and constant current source output current of the product) is incremented.

When the bridge voltage as the temperature increases, the SPRT sensitivity is increased. On the other hand, if you keep the bridge voltage constant, SPRT pressure sensitivity also increases with temperature. Therefore, the pressure sensitivity by SPRT two competing factors: temperature and the temperature of the bridge voltage. This bridge resistance or bridge voltage changes can he modern signal conditioning circuits IC utilized to correct SPRT sensitivity in the temperature range of error. It is worth mentioning that this type of signal conditioning circuits IC through bridge resistance changes to correct sensitivity with temperature changes. According to the main SPRT error analysis of the factors that will be targeted for correction and compensation. The following are currently used traditional correction and modern correction and simplify compensation method for introduction.

5. traditional correction method

　

Figure 3 circuit as legacy correction scheme

　　

　　

It can compensate for SPRT precision to the appropriate level, and can SPRT disorders, disorders of temperature drift (OFFSETTC) and SPRT sensitivity adjust temperature drift.

The sensitivity of the SPRT temperature drift is associated with a partial Elm (FSO) temperature drift, the two parameters (SPRT sensitivity temperature drift and full partial Yu-out temperature drift) is proportional to the temperature characteristics of interoperability, circuit nulling resistor Rjz to compensate sensor at room temperature for offset voltage, temperature sensitive resistor Rts and Rtz (or R ' tz) temperature error for correction. Mentioned earlier, the bridge resistance increases as the temperature rises, so that sensors both sides voltage Vo Vo +-() have also increased, this increased voltage △ Vo + Vo-makes the sensor's sensitivity is increased, that is, given the pressure it will output a higher voltage.

However, if you maintain a constant voltage sensor, the sensor's sensitivity is increased as the temperature of the lower (or negative sensitivity coefficient), but because the bridge resistance temperature impact increases, it is the sensitivity to variation coefficient is greater than negative sensitivity coefficients, so that full partial output (FSO) tend to increase as temperature increases.

Resistor Rts can temperature rise bypass off part of the bridge circuit current to offset these effects. Similarly, Rtz or R ' tz can offset calibration of drift. Select the Rts or circuit R ' tz depend on direction maladjusted temperature drift.

This traditional correction method has the advantage that the program is simple, cheap, but the main problem is the interaction between the compensation components, so that the calibration is very difficult, and limits can achieve the precision, the calibration nor easy to use electronic adjustment.

6, modern correction-compensation technology

Since the margin of error SPRT range is very wide, and therefore must be used by modern and streamlined compensation method to calibrate.

Current modern correction-compensation technology is a new type of signal conditioning MAX145 chips and combo, SPRT into intelligent sensors and technologies.

Use a signal conditioning MAXl457 IC to drive respiratory monitors Silicon Piezoresistive sensor (SPRT), and calibration sensor errors.

MAXl457 it with one used to drive sensors controlled current source and a bridge for sampling ADC voltage of the sensor (ADC).

MAXl457 also includes a programmable gain amplifier (PGA), is used to enlarge the sensor differential output; and 5 number DAC (digital-to-analog converter), used for the correction of various sensor errors.

Because the sensor output is a weak signal, the output voltage of PGA is not sufficient to drive the ADC. To do this, the built-in amplifier MAXl457 can be used to enhance the PGA's output to an appropriate level.

Because the bridge voltage increases with temperature, the temperature dependency can be used to compensate full partial output (output FSO-Full span) of temperature error.

For constant current source excitation current bridge, full partial output (FSO) decreases as the temperature to rise, resulting in a full partial output temperature related errors (FSOTC). However, if the bridge voltage with the temperature rise to a certain rate, just compensation off full partial sensitivity with temperature drop, full partial output (FSO) will remain constant.

How to use this 6.1MAXl457 method calibration temperature induced full partial output (FSO) errors (see Figure 4).

　　

　　

First of all, by the ADC to quantify the bridge voltage, depending on the quantitative results, identify an advance meter open okay four correction coefficient (has saved in EEPROM) give FSOTC DAC.

And DAC output voltage on bridge excitation current is adjusted, adjusted excitation current changed the bridge voltage, thus compensating the specific temperature, sensor sensitivity changes caused by full partial output (FSO) error. In order to achieve smooth correction, the bridge voltage is used as the reference input FSOTC DAC, in the adjacent two digital compensation points (from ADC to EEPROM) for simulation of compensation. The same method is used to compensate for misalignment of temperature drift (OFFSETTC), the difference is that the output voltage of OFFSETTC DAC are fed human PGA output sum node (instead of the current source MAX1457).

6.2 on these four temperature correction coefficient calculation step description

First of all, the sensor and the minimum temperature gets MAXl457 at different pressures of sensor data, and then, place the sensors and the highest temperature MAXl457 gets different pressures of sensor data.

Use these limiting temperature point data, designed for MAXl457 designed application software to calculate the four correction factor: full partial output (FSO), full partial output temperature related errors (FSOTC), offset-offset (Offset), disorders of temperature offset (OFFSETTC), these four factors can amend an order error SPRT.

To get the precision of 0.1% MAXl457 allows specific temperature compensation, you only need to provide temperature calculation FSOTC and calibration by the user decides to OFFSETTC point number (up to 120 points).

If the sensor error with good repeatability, SPRT and MAXl457 combination can get better than 0.1% accuracy.

MAXl457 compensation technology relative to figure 3 for the traditional approach has clear advantages.

MAXl457 eliminates compensation interaction between components, which benefited from the independent disorders and full-scale adjustment: disorder in PGA output for compensation,But FSO amendment implementation through the current source. Another benefit is that since for different temperature point for specific amendments, for increased accuracy possible. This approach is essentially better than using external resistors, since the latter failed to point at a specific temperature sensor for precise compensation.

Because the MAX1457 accuracy can be much higher than a respiratory monitors, chose it, mainly because of its internal also contains an additional amplifier, you can breath monitoring instrument sensors to zoom in on low level signal.

Because the monitor can MAX1457 work in a wide temperature range, SPRT and MAX1457 combination applied for superior accuracy, it can be applied to space exploration and diving apparatus, etc.

7. practical simplify correction scheme

Because of the precision MAX1457 can be much higher than a respiratory monitors, so there is no need to use 16-bit resolution DAC.

Usually adopts MAX1450 signal conditioner for simplified school compensation programme (see Figure 5).

　　

　　

7.1MAX1450 signal conditioner functions the same as the nature and the MAX1457 is replaced with resistance to error correction DAC.

Than by MAX1450 MAX1457 considerably less precision calibration point, which is 1%.

Is usually used for a mixed programme, the programme will MAX1450 and laser trimming resistor (Figure 5 shows the external trimming resistor) combination provides a low cost solution, which uses MAX1450 signal conditioner with external laser trimming resistor provides 1% accuracy. Adjusted Figure 5 RFSOA trimming resistor is used to set the initial (FSO) sensitivity and offset temperature drift compensation (OFFSETTC) is through ROFFA and fine tune the resistance adjustment ROTCA.

7.2MAX1450 pin function introduction £ º

A0 programmable gain amplifier (PGA) gain settings the lowest bit input; programmable amplifier gain set A1; A2 programmable gain amplifier (PGA) gain settings the MSB input; SOTC programmable gain amplifier current overcurrent control switches; SOFF programmable gain amplifier power off; OFFTC temperature offset correction input; OFFSET offset input; BBUF belt buffer bridge voltage output; FSOTRIM bridge driving current set input; OUT output voltage-controlled amplifier; ISRC current source base; BDRIVE excitation current output; INM sensor negative signal output; INP sensor is signal output; VDD power supply voltage; and VSS.

8. concluding remarks

Due to the new signal conditioner IC technology, have been able to accurately calibrate SPRT error, Piezoresistive sensor in addition to the application in the medical field, you can expand into other areas of industrial safety as well as military and other high-end products.

This from the above several designs that, by applying Silicon Piezoresistive sensor (SPRT) and MAX1450, MAX1457, signal conditioner chip consisting of smart sensors enable research and development of new types of respiratory monitors with high accuracy, low cost, sophisticated features, also demonstrated in the high end without requiring expensive strain gage's vision is achievable.

According to initial use, the new silicon Piezoresistive sensor (SPRT) and MAX1450, MAX1457, signal conditioner chip consisting of smart sensors enable research and development of new types of respiratory monitors after medical institutions and integrity index (mental status changes trigger physiological change detection) verification instrument in recruitment and assessment of the trial, the effect is good.

Especially in the diagnosis and treatment in the hospital so that doctors on patient's respiratory and cardiopulmonary status of testing, not only features strong and indicators have greatly improved accuracy, according to numerous trial, its qualitative (PC monitor) and quantitative (digital recorder) measurement results for doctors determine symptoms have completely more scientific basis.