Hall effect and Nonlinear Correction

Hall effect and Nonlinear Correction

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Source: Today's electronics/21IC Author: Tian Yaping

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Hall effect

When an electric metal or semiconductor wafer is placed vertically in a magnetic field, the two ends of the wafer produce a potential difference, which is called Hall effect. The potential difference between the two ends is called Hall potential, and its expression is
UH = K · Ia · B/d

K is the Hall coefficient, Ia is the current passed in the thin sheet, B is the magnetic induction intensity of the external magnetic field (Lorenz force Lorrentz), and d is the thickness of the thin sheet. It can be seen that the sensitivity of Hall effect is proportional to the magnetic induction intensity of the applied magnetic field. Among them, the Hall effect of metal plates in the magnetic field, the Hall coefficient of pure metal is relatively small, and the Hall coefficient of semiconductor materials is several orders of magnitude larger, therefore, semiconductor materials such as silicon, GE, indium arsenic, and indium antimony are generally used as Hall Elements.

Figure 1 S-5711ANDL-I4T1G pin layout diagram

Among them, no-contact sensing will be the trend of the times. In non-contact sensors, Hall Effect Sensor has won wide application space in the automotive field thanks to its advantages such as high reliability. The advantage of Hall current sensor is that the circuit is simple and the cost is relatively low. The disadvantage is accuracy, poor linearity, slow response time, and large temperature drift. In this paper, the nonlinear problem of Hall current sensor is discussed to reduce the nonlinear degree.

Correction Method Analysis

The correction method is described by using the Integrated S-5711ANDL-I4T1G Hall sensor as an example.

1 S-5711A Introduction

The S-5711A series is a hall IC (magnetic switch IC) with high sensitivity and low current consumption developed using CMOS technology ). The intensity of the magnetic bundle density can be detected to change the output voltage. By combining with the magnet, you can perform on/off detection for various devices.

① S-5711ANDL-I4T1G pin column 1.

② Working Characteristics of S-5711A

The operating characteristic curves of S-5711A series Hall integrated sensors are shown in 2. If the magnet is close to the sensor IC, when the magnetic bundle density in the vertical direction exceeds BOPN or BOPS, VOUT is changed from "H" to "L "; if the magnet is removed from this IC, VOUT is changed from "L" to "H" when the density of the magnetic bundle is lower than BRPN or BRPS ".

According to the general operating characteristics of Hall Switch Integrated sensor, the hysteresis BH is very beneficial to the reliability of switch operation. The operating characteristic curve of the Hall Switch Integrated Sensor reflects the relationship between the external magnetic field and the sensor output level. When the external magnetic induction intensity is greater than the "on" magnetic induction intensity, the output level changes from high to low, and the sensor is on. When the added magnetic induction intensity is less than the released point's "off" magnetic induction intensity, the output level changes from low to high, and the sensor is in the off state.

The advantage of this type of sensor is that it is not affected by the polarity of the magnet, which can reduce human errors in the production assembly; from mechanical switch to IC switch, its low current consumption is also an advantage. Its application environment is also very extensive, mainly including mobile phones (slide cover, open and close cover/flip cover) and other occasions where the system or module switch function of the entire machine must be realized by using the induction opening and close components.

2 Correction Method

Figure 2 Working Characteristics of S-5711A

S-5711ANDL-I4T1G Integrated Sensor with high sensitivity, wide operating temperature range (-40 ~ + 85 ℃) and other features. Generally, VOUT of Hall voltage has a nonlinear relationship with B of magnetic induction intensity, and its absolute linearity is 29%. The unbalanced voltage UHe exists. Improper use will definitely affect the accuracy of the detection system. S-5711ANDL-I4T1G chip internal circuit principle 3 shows.

Figure 3 internal circuit principle of S-5711ANDL-I4T1G Chip

The output voltage is determined by the following formula:
UH = KHIHBcos θ + KeIH
Formula KH -- Hall sensitivity
IH-Hall sensor drive current;
B-magnetic induction intensity;
Cos θ: the angle between the element plane normal and B;
Ke --- imbalance coefficient.

KeIH = UHe is called unbalanced voltage, and (UHe/UH) × 100% is called imbalance rate and set as Re. Generally, the Re of the Hall element is about 10%. KH and B produced by the checked current are non-linear factors. Circuit 4 shows how to eliminate unbalanced voltage and improve non-linearity.

Figure 4 circuit for eliminating unbalanced voltage and improving non-linearity

In Figure 4, the current I is equivalent to the detected current, and the chip is not used at three ends. Changing Current I is changing the magnetic induction intensity B. The test curve 5 is shown.

Figure 5 test curve

In Figure 5, curve 1: R14 = 0; curve 2: R14 → ∞; curve 3: R14 = 100 Ω. Line 4 is called a theoretical Linear Fit Curve. It is a line between the origin (0, 0) and the limit point Q (3.8, 2.4. Curve 1 does not have a calibration potentiometer RP1. In this case, the unbalanced voltage UHe = 0.3 V and the imbalance rate is Re = (UHe/UH) × 100% = (0.3/2.4) × 100% = 12.5%.
The linearity is analyzed as follows.
Linearity is to measure the closeness of the static characteristics of the system to the selected fit line y = B + kx.
Delta L = (| Delta m |/Ym) × 100%
Medium △m-maximum fitting deviation between static characteristics and selected fitting lines;
Ym -- the limit value of y.
The Method for Determining the fitting line is different, and the Delta L is also different. In this paper, the absolute linearity method is used. The obtained value is generally greater than that of the least square method.

For curve 1: | △m1 | = 0.7 V in figure 5, the limited value Ym = UHm = 2.4 V is derived from Delta L = (| △m |/Ym) * 100% shows that the linearity Delta L1 = 29%, that is, when the Hall sensor is not added with a correction circuit, the absolute linearity is 29%, relatively large. After the circuit is corrected, the optimal working state is curve 3 in figure 5, with | △m3 | = 0.13 V. Similarly, Delta L3 = 7.6%, the linearity is greatly improved. Curve 2 (R14 → ∞) in Figure 5: The unbalanced voltage is eliminated and a good linearity is obtained. But at the same time, the output voltage is decreased under the same magnetic induction intensity, that is, the sensitivity of the sensor is decreased, but this deficiency can be compensated by the Post-amplifier.

Conclusion

Through nonlinear correction, the Hall current sensor has high accuracy, good linearity, and high practicability. Through experimental and theoretical analysis, the circuit designed in this paper can find out the Optimal Linear working state of Hall sensor and eliminate unbalanced voltage.