Pressure sensor circuit design for automotive electronics - circuit diagrams read every day (311)

Automotive sensors are the foundation and key to automotive electronics and intelligence. Among them, pressure sensors are the most used and fastest growing. Automobile pressure sensors are used in many systems of automobiles, such as electronic detection systems, security anti-collision systems, etc. The purpose of its application in tire pressure is to minimize or eliminate early tire damage caused by high-pressure punctures and low-pressure rolling, so that the tires can always maintain standard air pressure, extend the life of the tires, reduce tire consumption, and improve economic benefits. It is reported that micro pressure sensors are embedded in car tires to measure the air pressure in order to control the amount of tire inflation to avoid over and under inflation, thereby saving 10% of gasoline.

A simple signal conditioning circuit should allow the output of the amplifier to be independent of the sensor used, provide interchangeability and high level output and be low cost. Laser-trimmed resistors on the sensor compensation board adjust the gain of the external amplifier to normalize it to changes in pressure sensitivity.

The signal conditioning circuit shown in Figure 1 provides a precision constant current source for sensor excitation and an instrumentation amplifier whose gain is controlled by the feedback resistor R in the sensor.

The current source is controlled by the ±1% bandgap reference diode VR, and the reference current I0 is defined by the following formula: I0=(E0-e0)/R2

Among them: E0-diode reference voltage: 1.235V±1% (LM385)eo---offset of amplifier A1; R2---feedback resistor value

By selecting amplifier A1 with an offset voltage less than 1mV and resistor R2 with a tolerance of ±1%, a current I0=0.996MA can be generated, with a typical accuracy of ±1.4%. The gain adjustment resistor r is adjusted to R3=R4=100K and a 2V differential output.

If zero adjustment is required, use OP227 instead of LT1013 amplifier and add zero potentiometer P1. The zero voltage is ±4mV relative to an input with a differential bias of less than 0.5mV, which leaves about ±3.5mV of zero to compensate for typical sensor offsets (less than ±1mV).

The amplifier provides additional amplification R8/R5 and converts the different drift voltages of the first stage into a single-ended output voltage. The entire output voltage equation is: Vout=2&TImesA&TImesR8/R5=5.000V@A=I. A is the ratio of the actual excitation current I0 to the specified current.

The overall accuracy of the output amplitude is affected by the accuracy of the feedback resistors R3 to R8. For resistors with ±0.1% accuracy such as Mepco/Electra5063Z, the typical gain error is about ±0.24%. If you use a well-matched thin film resistor, such as Bechman694-3-A, the accuracy error can be further reduced. Without any debugging and pressure testing, the amplitude comprehensive error of the entire signal conditioning circuit at a certain reference temperature is Typical value is 1.1%, this error will be superimposed on the sensor's accuracy of ±1%.

If there is no pressure source, the following steps can be used to reduce the gain error of the amplifier. By using the following method, the resistors R2 to R8 do not need precision resistors, and the sensor amplitude error of 1% can also be achieved.

Calibration steps:

1) Replace the resistor r with a 7.50K resistor with an accuracy of 0.1%

2) Check the gain K of the amplifier and calculate the gain ratio X (compared to the ideal gain K0=69.028V/V), where X=K/K0

3) Set the current I0=0.996/X (A) by adjusting the potentiometer.

Assuming that the maximum value of the bridge resistance is 6.4KΩ (50℃), the bridge resistance is 0.996mA, and the diode reference voltage is 1.2V, so the maximum output voltage of amplifier A1 is 7.4V, and for the LTC1051 amplifier, at 1A The forward saturation voltage drop when outputting current is 0.5V, so the minimum excitation voltage associated with the current source and amplifier LTC1051 used is 7.9V (7.4V+0.5V). For the LT1490, the maximum excitation voltage should be 7.6V. The maximum excitation voltage is controlled by the voltage regulation characteristics of the given amplifier.