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CMOS magnetic sensor integrated circuit with sectorial MAGFET
Sensors and Actuators A 126 (2006) 154–158
CMOS magnetic sensor integrated circuit with sectorial MAGFET Guo Qing ∗ , Zhu Dazhong ∗∗ , Yao Yunruo Institute of Microelectronics Technology and System Design, Department of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027, PR China Received 17 January 2005; received in revised form 2 September 2005; accepted 4 October 2005 Available online 9 November 2005
Abstract In this paper, a CMOS magnetic sensor integrated circuit (IC) for a perpendicular magnetic field is introduced. The sensor integrated circuit is designed and fabricated in a 0.6 m digital CMOS process. It consists of a pair of common-source split-drain magnetic field-effect transistor (MAGFET), a pre-processing circuit with a switches array, a correlated double sampling (CDS) circuit and a digital controlling part. The measurements of perpendicular magnetic field as well as the noise shaping are realized on-chip by the two functioning modes of the sensor integrated circuit. The MAGFET is sectorial with a radius of 48 m and an angle of 90◦ , and the measured sensitivity SA (current-related sensitivity) is 3.61%/T. At the working frequency of 10 kHz, the sensitivity of the magnetic sensor is 2.62 V/T. © 2005 Elsevier B.V. All rights reserved. Keywords: CMOS; Sensor; MAGFET; Integrated circuit
1. Introduction
2. The sectorial MAGFET
CMOS compatible magnetic sensors are currently developed by many research groups. With the one-chip integration of the magnetic sensor and CMOS analog and digital integrated circuits, the magnetic sensor system-on-chip is achieved. The split-drain magnetic field-effect transistor (MAGFET) can be integrated with CMOS integrated circuits to enhance the performance of the magnetic sensor. Many research works have been reported on the integration of the MAGFET and CMOS integrated circuits, such as the magnetic operational amplifier (MOP) [1], the magnetic-to-digital converter [2] and the magnetic-to-frequency converter [3]. In this paper, the CMOS magnetic sensor integrated circuit with sectorial MAGFET is introduced. The design consists of two sectorial split-drain MAGFETs as the magnetic sensor cell, a pre-processing circuit with two functioning modes, a correlated double sampling (CDS) circuit and a digital part. In Section 2, the sectorial MAGFET is introduced. In Section 3, the design of the sensor integrated circuit is introduced, and measurement results of the sensor chip and discussions are given in Section 4.
When the drain of a MOS transistor is split into two, the transistor can act as a magnetic field sensor. When a magnetic field is applied perpendicularly to the sensor, the carriers are deflected under the action of the Lorenz’s force, leading to different currents between the two split drains. The current difference is proportional to the intensity of the perpendicular magnetic field, and this current signal acts as the magnetic sensor signal. From the paper by Johannes et al. [4] on rectangular MAGFET, the decrease of channel width leads to an increase of sensitivity of the magnetic sensor, and so it brings the idea of sectorial MAGFET. The proposed sectorial n-MAGFET is shown in Fig. 1. The n-MAGFET is chosen because n-MAGFET has less noise than p-MAGFET [5]. The current-related sensitivity SA is defined as:
∗ ∗∗
Corresponding author. Corresponding author. Tel.: +86 571 87951705; fax: +86 571 87952404. E-mail address: [email protected] (Z. Dazhong).
0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.10.004
SA =
I = f (R, L, α, d, µ, VGS , VDS ) I(0) · BZ
where BZ is the magnetic field perpendicular to the sensor surface; I, the current difference between split drains of the MAGFET in the presence of BZ and I(0) is the total drain current in the absence of BZ . The sensitivity SA is a function of geometry factors R, L, d, α and µ, and also the biasing conditions of VGS and VDS .
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Fig. 1. The proposed sectorial n-MAGFET.
Theoretical analysis and experimental measurements have shown that the sectorial MAGFET sensitivity is higher than the conventional rectangular MAGFETs [6]. The circuit for sensitivity measurement of separate MAGFET is shown in Fig. 2. In Fig. 2, R = 200 k is a balance resistance for zero magnetic sensor output as BZ equals zero. With L = 48 m, R = 10 m, α = 90◦ and d = 3 m, the measured SA is 3.61%, which is higher than the rectangle MAGFET sensitivity of 3.0% reported by Rubio et al. [7]. The advantage of sectorial MAGFET lies in the gradient reduction of channel width from the drain to the source. This structure of sectorial MAGFET causes the carrier deflection to increase between the split drains and then its sensitivity is increased.
Fig. 3. The two functioning modes of MAGFETs.
field. There are many kinds of approaches to achieve the magnetic field shielding, but the off-chip method will be not valuable and cost efficient. So, the two functioning modes of the magnetic field signal processing circuit is designed: in the first mode, which is called the sampling mode, the magnetic field signal and also the noise signal are sampled, and in the second mode, which is called the noise-shaping mode, only the noise signal is sampled and this noise signal will be used to achieve the noise shaping of the magnetic signal. 3.1. The design of the two functioning modes
3. The design of the sensor integrated circuit In the design of the sensor integrated circuit, we were concerned by the noise shaping of the magnetic sensor signal. The magnetic field of the earth is weak enough that it can be ignored. So in the design, two kinds of noise have to be taken to account. First, in the fabrication of the MAGFET, offset is introduced into the sensor signal because of the deviation of fabrication process. For example, the mismatch of the split drains will introduce offset both in the presence and in the absence of magnetic field. Secondly, in the signal processing circuits, such as the switch array, the current source and the switched capacitor circuit, the noise of the circuits will be introduced into the sensor signals. The noise mentioned above will be eliminated in the design. In order to achieve the noise shaping of the magnetic sensor signal, the noise must be extracted in the absence of magnetic
The two functioning modes achieve the on-chip measuring of the magnetic field in one mode, and the on-chip shielding of the magnetic field in another mode. Fig. 3 shows the MAGFETs in each functioning mode. In the sampling mode, when the switches k1 and k4 are on and the switches k2 and k3 are off, the sectorial MAGFETs are connected in parallel and the split drains current difference is doubled. In the noise-shaping mode, when the switches k2 and k3 are on and the switches k1 and k4 are off, the MAGFETs are connected in the way that the current deflection of four split drains are balanced and the MAGFET works as a common MOSFET, which is not sensitive to the magnetic field. So a switched capacitor circuit can easily sample the noise signal. This noise signal will be used to calibrate the magnetic sensor signal of the sampling mode. The change from one mode to another mode is under the control of the switches k1 , k2 , k3 and k4 , the switch states of which are controlled by the digital signals. 3.2. CDS implementation
Fig. 2. Circuit for MAGFET sensitivity measurement.
The correlated double sampling technique was introduced first by White et al. [8] to eliminate the KT/C noise and decrease the 1/f noise of CCD sensor. In this design, two switched capacitor circuits are used to sample and hold the sensor signal and the noise signal separately. Fig. 4 shows a schematic of the CDS system. At the beginning, the magnetic sensor works in the sampling mode, and the capacitor C1 is set by the “set” signal at the voltage of “Vset”. Then the capacitor C1 is discharged
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Fig. 4. The schematic of CDS system.
by the current determined by the magnetic sensor signal and the noise signal till the end of the sampling mode. Then the noise-shaping mode begins. The capacitor C2 is set by the “set” signal at the same “Vset” voltage. Then the current determined by the noise signal discharges the capacitor C2. The two switched capacitor circuits share the same “set” signal and “Vset” voltage signal. In Fig. 5, the curves of “out1”, “read1”, “out2” and “read2” show how the two CDS circuits are controlled by the digital signals to implement the two functioning modes. Fig. 5 shows all the digital signals: “set”, “ck1 ”, “ck2 ”, “ck3 ”, “ck4 ”, “sr”, and “ss”. The “set” signal sets the capacitor at a certain voltage “Vset”. The “ck1 ”, “ck2 ”, “ck3 ”and “ck4 ” signals are used to switch between the two functioning modes. The “ss” and “sr” are used to keep the two CDS circuits to be on and off separately to sample and hold the required signals. 4. Results and discussions The CMOS magnetic sensor integrated circuit has been fabricated in a 0.6 m double metal double poly n-well CMOS process. The die photo of the magnetic sensor is shown in Fig. 6. The sectorial MAGFET has a channel length of 48 m, a source radius of 10 m, a source angle of 90◦ and a split-drain gap of 3 m. The total area of the die is 2.0 mm × 2.0 mm.
Fig. 6. The die photo of the CMOS magnetic sensor IC.
4.1. Measurement in both working modes During measurements, the MAGFETs were appropriately biased to get maximum sensitivity. In order to test each functioning mode, the sensor integrated circuit is measured separately as BZ is zero and BZ varies. When BZ is zero, the output voltage curves of sampling capacitors are shown in Fig. 7. The out1 and the out2 represent the outputs of the two CDS circuits. As shown in the curve of out1 in Fig. 7, from 0 to 10 s, the capacitor C1 is set to 2 V, and then the sensor works in the sampling mode from 10 to 50 s, when the capacitor C1 is discharged. As shown in the curves of out2 in Fig. 7, from 50 to 60 s, the capacitor C2 is set to 2 V, and from 60 to 100 s, the sensor works in the noise-shaping mode, when the capacitor is discharged. When BZ is zero, the capacitor is discharged by the noise signal. The sectorial MAGFETs work in two different states, as the four split drains are connected in two different ways depending on the functioning mode. So the noise signals in the two modes
Fig. 5. The digital controlling signals.
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157
Fig. 9. Discharging curves of capacitors at different frequencies.
Fig. 7. The curves of two functioning modes as BZ equals zero.
are different. But, in the design of MAGFETs, the large geometry (L = 48 m, R = 10 m, α = 90◦ and d = 3 m) MAGFETs are fabricated in a 0.6 m CMOS process, which will provide the sectorial MAGFETs with precise geometry dimensions. So the offset due to geometric and fabrication process deviations are not serious, not like the offset in small size devices in signal processing circuits. As it is proved by the curves of out1 and out2 in Fig. 7, as the BZ equals zero, the noise difference between both functioning modes is only a little fraction of the total sensor noise signal. Since the noise signal mainly depends on the signal processing circuits, the noise signals of MAGFETs in both modes are almost the same, as shown by experimental results. When BZ varies (0.11, 0.26 or 0.33 T), the output voltages of the sampling capacitors are shown in Fig. 8. As shown in the curve of out1 in Fig. 8, the capacitor C1 is set at first, and then is discharged till the end of the sampling mode, and the discharging current increases with the increase of the intensity of perpendicular magnetic field. As shown in the curve of out2 in Fig. 8, the capacitor C2 is set at first, and then is discharged by the noise signal. These curves are not changed with different values of BZ , for they are not sensitive with the magnetic field and
Fig. 10. The sensitivity of the magnetic sensor IC.
mainly represent the noise of the MAGFETs and the processing circuits. From Figs. 7 and 8, we can conclude that with the two functioning modes and the CDS circuits of the magnetic sensor integrated circuit, the magnetic field can be measured and the noise of the magnetic sensor signal can be eliminated efficiently. 4.2. The sensitivity of the sensor IC When the perpendicular magnetic field BZ becomes large, equals to 0.33 T, different working frequencies of the sensor lead to different slopes, as shown in Fig. 9. The working frequency of the magnetic sensor is 20, 10, 5 and 2 kHz. From Fig. 9, we see that the higher frequency brings the lower voltage drop, and on the other hand, the lower frequency pushes the discharging curve into non-linear area. So the 10 kHz working frequency is chosen. Fig. 10 shows the measured sensitivity of the magnetic sensor integrated circuit. The working frequency of the sensor is set to be 10 kHz. At the working frequency of 10 kHz, the sensitivity of the magnetic sensor is 2.62 V/T. 5. Conclusion
Fig. 8. The curves of two functioning modes as BZ vary.
In this paper, a magnetic sensor integrated circuit with sectorial split-drain MAGFETs is designed and fabricated in a 0.6 m double poly double metal CMOS technology. Comparing with rectangular split-drain MAGFET, the sensitivity of sectorial
158
G. Qing et al. / Sensors and Actuators A 126 (2006) 154–158
MAGFET is optimized to be 3.61%/T. In the design of the magnetic sensor integrated circuit, the pre-processing circuit with a switches array and a CDS circuit are used to suppress the noise of the magnetic sensor signals. At the working frequency of 10 kHz, the sensitivity of the magnetic sensor is 2.62 V/T.
[7] C. Rubio, S. Bota, J.G. Macias, J. Samitier. Monolithic integrated magnetic sensor in a digital CMOS technology using a switched current interface system, in: Instrumentation and Measurement Technology Conference, 2000. IMTC 2000. Proceedings of the 17th IEEE, vol. 1, 1–4 May 2000, pp. 69–73. [8] M.H. White, et al., Characterization of surface channel CCD image arrays at low light levels, IEEE J. Solid-State Circuits 9 (1) (1974) 1–13.
Acknowledgement Biographies This work is supported by the project of National Natural Science Foundation of China (NSFC):90307009. References [1] K. Maenaka, H. Okada, T. Nakamura, Universal magneto-operational amplifier (MOP), Sens. Actuators A 21–23 (1990) 807–811. [2] C.H. Kuo, S.L. Chen, L.A. Ho, S.I. Liu, CMOS oversampling magnetic to digital converters, IEEE J. Solid-State Circuits 36 (2001) 1582–1586. [3] S.-L. Chen, C.-H. Kuo, S.-I. Liu, CMOS magnetic field to frequency converter, IEEE Sens. J. 3 (2) (2003) 241–245. [4] J.W.A. von Kluge, W.A. Langheinrich, An analytical model of MAGFET sensitivity including secondary effects using a continuous description of the geometric correction factor G, IEEE Trans. Electron Device 46 (1) (1999). [5] D. Killnat, J.V. Kluge, F. Umbach, et al., Measurements and modeling of sensitivity and noise of MOS magnetic field-effect transistors, Sens. Actuators A 61 (1997) 346–351. [6] Y. Yunruo, Z. Dazhong, G. Qing, Sector split-drain magnetic field-effect transistor based on standard CMOS technology, Sens. Actuators: A Phys. 121 (2) (2005) 347–351.
Guo Qing was born in Shandong Province, PR China, 1979. In 2002, he received the BE degree from Department of Information and Electronics Engineering in Zhejiang University. Now he is PhD candidate in Microelectronics Technology and System Design Institute, Zhejiang University. His current research interests are integrated microsensor systems and the design of analog integrated circuits. Zhu Dazhong was born in Shanghai, PR China, 1945. He graduated from Semiconductor Physics Specialty, Physics Department, Nanjing University, 1967. He received MS degree in Semiconductor Physics and Device Specialty, Department of Radio Engineering, Zhejiang University, 1981. Now he is a professor and doctorate supervisor of Information and Electronics Engineering Department in Zhejiang University. His research focuses on mixed-signal integrated circuit design, sensor integrated circuit design and microwave and MEMS integrated electronics. Yao Yunruo was born in Shanghai, PR China, 1980. He graduated from Department of Information and Electronics Engineering in Zhejiang University and received the BE degree there, 2002. Now he is studying toward MS degree in Microelectronics Technology and System Design Institute, Zhejiang University. His current research interests are mixed-signal IC design and integrated microsensor.
CMOS magnetic sensor integrated circuit with sectorial MAGFET Guo Qing ∗ , Zhu Dazhong ∗∗ , Yao Yunruo Institute of Microelectronics Technology and System Design, Department of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027, PR China Received 17 January 2005; received in revised form 2 September 2005; accepted 4 October 2005 Available online 9 November 2005
Abstract In this paper, a CMOS magnetic sensor integrated circuit (IC) for a perpendicular magnetic field is introduced. The sensor integrated circuit is designed and fabricated in a 0.6 m digital CMOS process. It consists of a pair of common-source split-drain magnetic field-effect transistor (MAGFET), a pre-processing circuit with a switches array, a correlated double sampling (CDS) circuit and a digital controlling part. The measurements of perpendicular magnetic field as well as the noise shaping are realized on-chip by the two functioning modes of the sensor integrated circuit. The MAGFET is sectorial with a radius of 48 m and an angle of 90◦ , and the measured sensitivity SA (current-related sensitivity) is 3.61%/T. At the working frequency of 10 kHz, the sensitivity of the magnetic sensor is 2.62 V/T. © 2005 Elsevier B.V. All rights reserved. Keywords: CMOS; Sensor; MAGFET; Integrated circuit
1. Introduction
2. The sectorial MAGFET
CMOS compatible magnetic sensors are currently developed by many research groups. With the one-chip integration of the magnetic sensor and CMOS analog and digital integrated circuits, the magnetic sensor system-on-chip is achieved. The split-drain magnetic field-effect transistor (MAGFET) can be integrated with CMOS integrated circuits to enhance the performance of the magnetic sensor. Many research works have been reported on the integration of the MAGFET and CMOS integrated circuits, such as the magnetic operational amplifier (MOP) [1], the magnetic-to-digital converter [2] and the magnetic-to-frequency converter [3]. In this paper, the CMOS magnetic sensor integrated circuit with sectorial MAGFET is introduced. The design consists of two sectorial split-drain MAGFETs as the magnetic sensor cell, a pre-processing circuit with two functioning modes, a correlated double sampling (CDS) circuit and a digital part. In Section 2, the sectorial MAGFET is introduced. In Section 3, the design of the sensor integrated circuit is introduced, and measurement results of the sensor chip and discussions are given in Section 4.
When the drain of a MOS transistor is split into two, the transistor can act as a magnetic field sensor. When a magnetic field is applied perpendicularly to the sensor, the carriers are deflected under the action of the Lorenz’s force, leading to different currents between the two split drains. The current difference is proportional to the intensity of the perpendicular magnetic field, and this current signal acts as the magnetic sensor signal. From the paper by Johannes et al. [4] on rectangular MAGFET, the decrease of channel width leads to an increase of sensitivity of the magnetic sensor, and so it brings the idea of sectorial MAGFET. The proposed sectorial n-MAGFET is shown in Fig. 1. The n-MAGFET is chosen because n-MAGFET has less noise than p-MAGFET [5]. The current-related sensitivity SA is defined as:
∗ ∗∗
Corresponding author. Corresponding author. Tel.: +86 571 87951705; fax: +86 571 87952404. E-mail address: [email protected] (Z. Dazhong).
0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.10.004
SA =
I = f (R, L, α, d, µ, VGS , VDS ) I(0) · BZ
where BZ is the magnetic field perpendicular to the sensor surface; I, the current difference between split drains of the MAGFET in the presence of BZ and I(0) is the total drain current in the absence of BZ . The sensitivity SA is a function of geometry factors R, L, d, α and µ, and also the biasing conditions of VGS and VDS .
G. Qing et al. / Sensors and Actuators A 126 (2006) 154–158
155
Fig. 1. The proposed sectorial n-MAGFET.
Theoretical analysis and experimental measurements have shown that the sectorial MAGFET sensitivity is higher than the conventional rectangular MAGFETs [6]. The circuit for sensitivity measurement of separate MAGFET is shown in Fig. 2. In Fig. 2, R = 200 k is a balance resistance for zero magnetic sensor output as BZ equals zero. With L = 48 m, R = 10 m, α = 90◦ and d = 3 m, the measured SA is 3.61%, which is higher than the rectangle MAGFET sensitivity of 3.0% reported by Rubio et al. [7]. The advantage of sectorial MAGFET lies in the gradient reduction of channel width from the drain to the source. This structure of sectorial MAGFET causes the carrier deflection to increase between the split drains and then its sensitivity is increased.
Fig. 3. The two functioning modes of MAGFETs.
field. There are many kinds of approaches to achieve the magnetic field shielding, but the off-chip method will be not valuable and cost efficient. So, the two functioning modes of the magnetic field signal processing circuit is designed: in the first mode, which is called the sampling mode, the magnetic field signal and also the noise signal are sampled, and in the second mode, which is called the noise-shaping mode, only the noise signal is sampled and this noise signal will be used to achieve the noise shaping of the magnetic signal. 3.1. The design of the two functioning modes
3. The design of the sensor integrated circuit In the design of the sensor integrated circuit, we were concerned by the noise shaping of the magnetic sensor signal. The magnetic field of the earth is weak enough that it can be ignored. So in the design, two kinds of noise have to be taken to account. First, in the fabrication of the MAGFET, offset is introduced into the sensor signal because of the deviation of fabrication process. For example, the mismatch of the split drains will introduce offset both in the presence and in the absence of magnetic field. Secondly, in the signal processing circuits, such as the switch array, the current source and the switched capacitor circuit, the noise of the circuits will be introduced into the sensor signals. The noise mentioned above will be eliminated in the design. In order to achieve the noise shaping of the magnetic sensor signal, the noise must be extracted in the absence of magnetic
The two functioning modes achieve the on-chip measuring of the magnetic field in one mode, and the on-chip shielding of the magnetic field in another mode. Fig. 3 shows the MAGFETs in each functioning mode. In the sampling mode, when the switches k1 and k4 are on and the switches k2 and k3 are off, the sectorial MAGFETs are connected in parallel and the split drains current difference is doubled. In the noise-shaping mode, when the switches k2 and k3 are on and the switches k1 and k4 are off, the MAGFETs are connected in the way that the current deflection of four split drains are balanced and the MAGFET works as a common MOSFET, which is not sensitive to the magnetic field. So a switched capacitor circuit can easily sample the noise signal. This noise signal will be used to calibrate the magnetic sensor signal of the sampling mode. The change from one mode to another mode is under the control of the switches k1 , k2 , k3 and k4 , the switch states of which are controlled by the digital signals. 3.2. CDS implementation
Fig. 2. Circuit for MAGFET sensitivity measurement.
The correlated double sampling technique was introduced first by White et al. [8] to eliminate the KT/C noise and decrease the 1/f noise of CCD sensor. In this design, two switched capacitor circuits are used to sample and hold the sensor signal and the noise signal separately. Fig. 4 shows a schematic of the CDS system. At the beginning, the magnetic sensor works in the sampling mode, and the capacitor C1 is set by the “set” signal at the voltage of “Vset”. Then the capacitor C1 is discharged
G. Qing et al. / Sensors and Actuators A 126 (2006) 154–158
156
Fig. 4. The schematic of CDS system.
by the current determined by the magnetic sensor signal and the noise signal till the end of the sampling mode. Then the noise-shaping mode begins. The capacitor C2 is set by the “set” signal at the same “Vset” voltage. Then the current determined by the noise signal discharges the capacitor C2. The two switched capacitor circuits share the same “set” signal and “Vset” voltage signal. In Fig. 5, the curves of “out1”, “read1”, “out2” and “read2” show how the two CDS circuits are controlled by the digital signals to implement the two functioning modes. Fig. 5 shows all the digital signals: “set”, “ck1 ”, “ck2 ”, “ck3 ”, “ck4 ”, “sr”, and “ss”. The “set” signal sets the capacitor at a certain voltage “Vset”. The “ck1 ”, “ck2 ”, “ck3 ”and “ck4 ” signals are used to switch between the two functioning modes. The “ss” and “sr” are used to keep the two CDS circuits to be on and off separately to sample and hold the required signals. 4. Results and discussions The CMOS magnetic sensor integrated circuit has been fabricated in a 0.6 m double metal double poly n-well CMOS process. The die photo of the magnetic sensor is shown in Fig. 6. The sectorial MAGFET has a channel length of 48 m, a source radius of 10 m, a source angle of 90◦ and a split-drain gap of 3 m. The total area of the die is 2.0 mm × 2.0 mm.
Fig. 6. The die photo of the CMOS magnetic sensor IC.
4.1. Measurement in both working modes During measurements, the MAGFETs were appropriately biased to get maximum sensitivity. In order to test each functioning mode, the sensor integrated circuit is measured separately as BZ is zero and BZ varies. When BZ is zero, the output voltage curves of sampling capacitors are shown in Fig. 7. The out1 and the out2 represent the outputs of the two CDS circuits. As shown in the curve of out1 in Fig. 7, from 0 to 10 s, the capacitor C1 is set to 2 V, and then the sensor works in the sampling mode from 10 to 50 s, when the capacitor C1 is discharged. As shown in the curves of out2 in Fig. 7, from 50 to 60 s, the capacitor C2 is set to 2 V, and from 60 to 100 s, the sensor works in the noise-shaping mode, when the capacitor is discharged. When BZ is zero, the capacitor is discharged by the noise signal. The sectorial MAGFETs work in two different states, as the four split drains are connected in two different ways depending on the functioning mode. So the noise signals in the two modes
Fig. 5. The digital controlling signals.
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157
Fig. 9. Discharging curves of capacitors at different frequencies.
Fig. 7. The curves of two functioning modes as BZ equals zero.
are different. But, in the design of MAGFETs, the large geometry (L = 48 m, R = 10 m, α = 90◦ and d = 3 m) MAGFETs are fabricated in a 0.6 m CMOS process, which will provide the sectorial MAGFETs with precise geometry dimensions. So the offset due to geometric and fabrication process deviations are not serious, not like the offset in small size devices in signal processing circuits. As it is proved by the curves of out1 and out2 in Fig. 7, as the BZ equals zero, the noise difference between both functioning modes is only a little fraction of the total sensor noise signal. Since the noise signal mainly depends on the signal processing circuits, the noise signals of MAGFETs in both modes are almost the same, as shown by experimental results. When BZ varies (0.11, 0.26 or 0.33 T), the output voltages of the sampling capacitors are shown in Fig. 8. As shown in the curve of out1 in Fig. 8, the capacitor C1 is set at first, and then is discharged till the end of the sampling mode, and the discharging current increases with the increase of the intensity of perpendicular magnetic field. As shown in the curve of out2 in Fig. 8, the capacitor C2 is set at first, and then is discharged by the noise signal. These curves are not changed with different values of BZ , for they are not sensitive with the magnetic field and
Fig. 10. The sensitivity of the magnetic sensor IC.
mainly represent the noise of the MAGFETs and the processing circuits. From Figs. 7 and 8, we can conclude that with the two functioning modes and the CDS circuits of the magnetic sensor integrated circuit, the magnetic field can be measured and the noise of the magnetic sensor signal can be eliminated efficiently. 4.2. The sensitivity of the sensor IC When the perpendicular magnetic field BZ becomes large, equals to 0.33 T, different working frequencies of the sensor lead to different slopes, as shown in Fig. 9. The working frequency of the magnetic sensor is 20, 10, 5 and 2 kHz. From Fig. 9, we see that the higher frequency brings the lower voltage drop, and on the other hand, the lower frequency pushes the discharging curve into non-linear area. So the 10 kHz working frequency is chosen. Fig. 10 shows the measured sensitivity of the magnetic sensor integrated circuit. The working frequency of the sensor is set to be 10 kHz. At the working frequency of 10 kHz, the sensitivity of the magnetic sensor is 2.62 V/T. 5. Conclusion
Fig. 8. The curves of two functioning modes as BZ vary.
In this paper, a magnetic sensor integrated circuit with sectorial split-drain MAGFETs is designed and fabricated in a 0.6 m double poly double metal CMOS technology. Comparing with rectangular split-drain MAGFET, the sensitivity of sectorial
158
G. Qing et al. / Sensors and Actuators A 126 (2006) 154–158
MAGFET is optimized to be 3.61%/T. In the design of the magnetic sensor integrated circuit, the pre-processing circuit with a switches array and a CDS circuit are used to suppress the noise of the magnetic sensor signals. At the working frequency of 10 kHz, the sensitivity of the magnetic sensor is 2.62 V/T.
[7] C. Rubio, S. Bota, J.G. Macias, J. Samitier. Monolithic integrated magnetic sensor in a digital CMOS technology using a switched current interface system, in: Instrumentation and Measurement Technology Conference, 2000. IMTC 2000. Proceedings of the 17th IEEE, vol. 1, 1–4 May 2000, pp. 69–73. [8] M.H. White, et al., Characterization of surface channel CCD image arrays at low light levels, IEEE J. Solid-State Circuits 9 (1) (1974) 1–13.
Acknowledgement Biographies This work is supported by the project of National Natural Science Foundation of China (NSFC):90307009. References [1] K. Maenaka, H. Okada, T. Nakamura, Universal magneto-operational amplifier (MOP), Sens. Actuators A 21–23 (1990) 807–811. [2] C.H. Kuo, S.L. Chen, L.A. Ho, S.I. Liu, CMOS oversampling magnetic to digital converters, IEEE J. Solid-State Circuits 36 (2001) 1582–1586. [3] S.-L. Chen, C.-H. Kuo, S.-I. Liu, CMOS magnetic field to frequency converter, IEEE Sens. J. 3 (2) (2003) 241–245. [4] J.W.A. von Kluge, W.A. Langheinrich, An analytical model of MAGFET sensitivity including secondary effects using a continuous description of the geometric correction factor G, IEEE Trans. Electron Device 46 (1) (1999). [5] D. Killnat, J.V. Kluge, F. Umbach, et al., Measurements and modeling of sensitivity and noise of MOS magnetic field-effect transistors, Sens. Actuators A 61 (1997) 346–351. [6] Y. Yunruo, Z. Dazhong, G. Qing, Sector split-drain magnetic field-effect transistor based on standard CMOS technology, Sens. Actuators: A Phys. 121 (2) (2005) 347–351.
Guo Qing was born in Shandong Province, PR China, 1979. In 2002, he received the BE degree from Department of Information and Electronics Engineering in Zhejiang University. Now he is PhD candidate in Microelectronics Technology and System Design Institute, Zhejiang University. His current research interests are integrated microsensor systems and the design of analog integrated circuits. Zhu Dazhong was born in Shanghai, PR China, 1945. He graduated from Semiconductor Physics Specialty, Physics Department, Nanjing University, 1967. He received MS degree in Semiconductor Physics and Device Specialty, Department of Radio Engineering, Zhejiang University, 1981. Now he is a professor and doctorate supervisor of Information and Electronics Engineering Department in Zhejiang University. His research focuses on mixed-signal integrated circuit design, sensor integrated circuit design and microwave and MEMS integrated electronics. Yao Yunruo was born in Shanghai, PR China, 1980. He graduated from Department of Information and Electronics Engineering in Zhejiang University and received the BE degree there, 2002. Now he is studying toward MS degree in Microelectronics Technology and System Design Institute, Zhejiang University. His current research interests are mixed-signal IC design and integrated microsensor.