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A Low-Voltage Current-Mode Wheatstone Bridge Using CMOS Transistors

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A Low-Voltage Current-Mode Wheatstone Bridge Using CMOS Transistors World Academy of Science, Engineering and Technology International Journal of Electronics and Communication Engineering Vol:5, No:10, 2011 A Low-Voltage Current-Mode Wheatstone Bridge using CMOS Transistors Ebrahim Farshidi However, the need for low-voltage/low-power CMOS analog Abstract—This paper presents a new circuit arrangement for a circuits is growing due to the downscaling of CMOS current-mode Wheatstone bridge that is suitable for low-voltage processes and also, in many situations, particularly in the integrated circuits implementation. Compared to the other proposed mixed analog-digital systems, it is desirable to implement the circuits, this circuit features severe reduction of the elements number, circuits in the MOS technology [12]. low supply voltage (1V) and low power consumption (<350uW). In addition, the circuit has favorable nonlinearity error (<0.35%), In this paper, a new CMWB design to overcome the above operate with multiple sensors and works by single supply voltage. problems is presented [13]. The circuit is designed in CMOS The circuit employs MOSFET transistors, so it can be used for technology, which exhibits flexible characteristics with standard CMOS fabrication. Simulation results by HSPICE show respect to other CM or VM devices, Its main features are high performance of the circuit and confirm the validity of the reducing circuit elements, area efficient implementation, proposed design technique. operation in single supply voltage, current output signal and possibility of low-voltage and low-power due to the fact that Keywords—Wheatstone bridge, current-mode, low-voltage, MOS transistors are employed. MOS. The paper is organized as follows. In section 2, basic I. INTRODUCTION principle of Wheatstone bridges is presented. In section 3, the circuit design of the proposed CMWB is proposed. Section 4 HEATSTONE bridge has found widespread use in the explains the new linearization technique. Simulation results instrumentation and measurement systems for sensing W are presented and discussed in section 5 and concluding strain, temperature, pressure, force, displacement and remarks are provided in section 6. humidity [1-4]. Voltage-mode Wheatstone bridge (VMWB) offers a good method for measuring small resistance changes II. BASIC PRINCIPLE WHEATSTONE BRIDGES accurately. Recently, a method based on the circuit duality concept [5] has been proposed to develop a current-mode Wheatstone bridge (CMWB) [6]. It is based on the use of A. Voltage-mode Wheatstone bridge current-mode as an alternative to conventional voltage-mode Traditionally, the voltage-driven Wheatstone bridge Wheatstone bridge. It has the following advantages: It uses configuration is used for the measurement of small resistance the principle of superposition without adding any signal changes. It consists of four resistors connected in a conditioning circuitry. Also, it has a smaller area because it quadrilateral form in addition of an excitation voltage reduces the sensing passive elements, and uses only two connected across one diagonal of the bridge. The output resistors without degradation in the performance. The new voltage of the bridge is measured differentially between the CMWB has a much-improved common-mode cancellation; voltage divider outputs connected across the other diagonal. furthermore, it has a high accuracy [6]. Following this The deviation of one or more resistors in the bridge from a technique Some CMWB designs based on the second- nominal value is measured as an indication of change in the generation current conveyors (CCII) [7] and operational measured physical variable, and the output voltage across the floating current conveyor (OFCC) [8] to implement a CMWB bridge indicates the resistance change. The bridge can have have been proposed [6, 9, 10 , 11]. The main drawbacks of one, two, or four resistors, whose values are deviated with the International Science Index, Electronics and Communication Engineering Vol:5, No:10, 2011 waset.org/Publication/4743 theses proposed circuits are as follows: firstly, the extra applied physical variable, as shown in Fig. 1. Typically, in needed circuits lead to a large number of transistors and high sensor applications, the nominal values of four resistors are power consumption; secondly, most of them are area- chosen to be equal. The differential output voltage and the intensive, due to the use of more circuitry and additional end-point linearity error of the bridges in Fig. 1 are resistors for balance networks and also for linearization; summarized in Table 1, where V is the excitation voltage to Thirdly, they need two separate supply voltage; and finally, ref the bridge. The linearity error is calculated as the maximum the proposed circuits are based on the bipolar transistors. error in percentage full scale from a straight line that connects the origin and the end point at full scale. Table 1 shows that Ebrahim Farshidi is with the Electrical Engineering Department, University inherent linearity between the resistance variation and the of Shahid Chamran , Ahvaz, Iran, 61357-8313 (e-mail: [email protected]). International Scholarly and Scientific Research & Innovation 5(10) 2011 1368 scholar.waset.org/1307-6892/4743 World Academy of Science, Engineering and Technology International Journal of Electronics and Communication Engineering Vol:5, No:10, 2011 output voltage variation can be obtained with the all-element and two-element varying configurations in Fig. 1b and Fig. 1c, respectively. However, linearity error is not critical I 1 R ΔR because it can easily be compensated by using software in I ref R ± ΔR m digital systems [14]. More importantly, to reduce offset and increase the sensitivity of the sensor, the bridge should have accurate resistance matching among resistors and equal I R ± ΔR absolute resistance variation. These requirements are difficult R m ΔR 2 to achieve in the all-element and two-element varying bridges, not to mention the drawbacks in terms of larger area and cost. The previously mentioned difficulty can be alleviated by using Fig. 2 Current-mode Wheatstone bridge configurations a single resistor as shown in Fig. 1d. One drawback of voltage-driven Wheatstone bridges is that the bridge TABLE I COMPARISON OF WHEATSTONE BRIDGE sensitivity (S= Vref/(ΔR/R0) is proportional to Vref and CONFIGURATIONS inversely proportional to the baseline resistance of the Linearity resistors. Therefore, to obtain high sensitivity, large Vref and Input Output voltage or current error small resistance are preferred, which prevent low voltage configuration (%) operation and lead to high power consumption of the bridge. ΔR Fig. 1a Vout = ⋅Vref 0 R V V ref ref ΔR Fig. 1b Vout = ⋅Vref 0 2R R ± ΔR + V R m ΔR R V R m ΔR ΔR ΔR out - + out - Fig. 1c Vout = ⋅Vref ≅ ⋅Vref 0.5 2R + ΔR 2R 1 ΔR ΔR R ΔR R ± ΔR Fig. 1d Vout = ⋅ ⋅Vref ≅ ⋅Vref 0.5 m R R ± ΔR 2 2R + ΔR 4R ΔR Fig.2 ΔI = I2 − I1 = ⋅ Iref 0 R (a) (b) Vref V ref III. CIRCUIT DESIGN OF THE PROPOSED CMWB R This paper proposes a new simplified circuit design for R ± ΔR V R V R + out - + out - CMWB, employing configuration of Fig. 2. Fig. 3 shows MOS implementation of the proposed CMFB. It uses two R ± ΔR R R ± ΔR R sensitive resistors (R1, R2), a constant excitation current ( I ref ), an operational amplifier (A1) and three current mirrors (formed by transistors M1-M6). One end of both resistors is (c) (d) tied together, while the other end is forced to be equi- Fig. 1 Conventional voltage-mode Wheatstone bridge configurations potential, that is, VX = VY . This can be done by a number of circuit arrangements with second generation current conveyors [5] or operational floating current conveyors [8]. In B. Current-mode Wheatstone bridge Fig. 3, the voltage change of node X will mirror a similar One approach to design Wheatstone bridge is based on the change to node Y owing to the virtual short-circuit provided use of current-mode as an alternative to conventional voltage- by the operational amplifier A1. It can be shown that the mode Wheatstone bridge [6], which employs the circuit circuit in this Figure exhibits the same properties and behavior duality concept [5]. A current-mode dual network for the all- as those reported before [5, 8] while severe reduction of the element varying Wheatstone bridge is shown in Fig. 2. It is elements number. International Science Index, Electronics and Communication Engineering Vol:5, No:10, 2011 waset.org/Publication/4743 straightforward to show that the current difference, ΔI = I1–I2, From Fig. 3 the currents of sensitive resistors R1 and R2, as is linearly proportional to the change in resistance, ΔR, as the arms of CMWB, are calculated as follows: shown in Table 1. Due to the circuit duality, the current-mode Wheatstone bridge inherits all characteristics and behavior of V −V I = Z X (1) its voltage-mode counterpart in the current domain, such as 1 R1 sensitivity, linearity, stability, and so on. The input sensitivity VZ −VY is proportional to the constant excitation current value, Iref. I = (2) 2 R 2 in which I1 and I 2 are the currents of resistors R1 and R2, respectively. International Scholarly and Scientific Research & Innovation 5(10) 2011 1369 scholar.waset.org/1307-6892/4743 World Academy of Science, Engineering and Technology International Journal of Electronics and Communication Engineering Vol:5, No:10, 2011 Dividing (1) by (2) and using equation VX = VY , it is obtained ΔR as follows: x = (10) R I1 R2 = (3) From (9) and (10), the output current in Fig. 3 is linearly I 2 R1 related to the relative resistance variation. As Fig. 3 shows, the number of components in the proposed Incorporation KCL for nodes (Z) and (OUT) gives: circuit is much less than those reported before [5, 8]. Also, in this circuit, without degradation in the performance, two I = I + I (4) ref 1 2 resistors are used, so it occupies smaller area.
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