A Current-mode Interface Circuit for a Piezoresistive Pressure Sensor

— An interfacing circuit for piezoresistive pressure sensors based on CMOS current conveyors is presented. The main advantages of the proposed interfacing circuit include the use of a single piezoresistor, the capability of offset compensation, and a versatile current-mode configuration, with current output and current or voltage input. Experimental tests confirm linear relation of output voltage versus piezoresistance variation.

Abstract-An interfacing circuit for piezoresistive pressure sensors based on CMOS current conveyors is presented. The main advantages of the proposed interfacing circuit include the use of a single piezoresistor, the capability of offset compensation, and a versatile current-mode configuration, with current output and current or voltage input. Experimental tests confirm linear relation of output voltage versus piezoresistance variation.
Index Terms-CMOS analog integrated circuit, current conveyor, current mode, interface circuit, pressure sensor.

I. INTRODUCTION
D URING the last few years, integrated pressure sensors on silicon substrate are of growing interest because of their compatibility with CMOS technology. The main advantages of these systems are their improved sensitivity, small size, and cost effectiveness. Silicon pressure sensors that utilize the piezoresistive effect are usually based in a fourelement Wheatstone bridge configuration, formed by two elements (their resistance increases with pressure) and two elements (their resistance decreases with pressure). The output voltage of such a transducer is sensed by an interface circuit usually based on operational amplifiers [1]. In these four-element pressure sensors, accurate resistance matching between and and equal variation of absolute resistance with pressure are two essential characteristics for proper operation to reduce the offset and increase the sensitivity of the sensor. These constraints complicate the technological process to implement and elements. The use of a single piezoresistance element eliminates the need to match closely the four resistors that form the Wheatstone bridge design [2]. A novel cost-effective currentmode interfacing circuit designed to operate with input from a single piezoresistance element is presented in this paper. The proposed circuit, based on second-generation current conveyors (CCII ) [3], also offers current output which can be very useful in a number of applications in which relatively long wires are used to connect the sensor to the monitoring system. Such is the case in biomedical systems where the proposed configuration is to be used. The pressure sensor integrated on the chip with the proposed interfacing circuit forms a sensing element. Such an element may be directly connected to the external patient-monitoring system with increased signal-noise immunity and transmission efficiency.

II. CIRCUIT DESCRIPTION
The second-generation current conveyor CCII is a known current-mode three-port analog building block with the following characteristics: and [3]. The proposed interface involves three such CCII cells, three conventional resistors (the last one at the monitoring instrument side), and a single piezoresistor, denoted as where is the resistance at reference pressure and is the relative change proportional to pressure variation. The resulting circuit schematic diagram is depicted in Fig. 1.
A current source configuration is obtained via the first . This current value is equal to and therefore determined by reference voltage and resistance . The applied pressure produces an absolute change in the piezoresistor . The small change in piezoresistor value affects the voltage at node (1) According to the operation, the output current of the second also follows the input current since is a high impedance node and does not affect the value of the current through the piezoresistor. On the other hand, the output current of the third will be controlled by and . The two currents and are then added to obtain the output current and, consequently, the output voltage will be (2) Therefore, a linear dependence between the output current and the relative piezoresistor variation is obtained and can be expressed as follows: In order to eliminate the constant output offset value formed by the last two terms, either has to be matched with the initial piezoresistor value and or it could be also compensated by an appropriate external voltage applied to the input . Then, after the output offset value correction, the final output value is (4) A current output proportional to the pressure variation is obtained while the resistors' ratio may be used to control the output voltage gain of the compensated circuit. It should also be noted that both reference and offset inputs are high impedance nodes and therefore are practically not affected by the length of connection wires, although may be also integrated with this circuit. Moreover, the proposed configuration can be used in a true current-mode operation with current reference input at node and current output, and this is difficult to obtain with operational amplifiers.

III. EXPERIMENTAL RESULTS
The proposed circuit of Fig. 1 has been implemented using CMOS current conveyors cells previously fabricated in an 1.2m CMOS process [4]. Experimental results were obtained using a typical piezoresistance (with k ) with a 10% variation due to pressure changes [5]. The experimental fixed resistance values were set to k k k (in order to set the gain factor to 10). Reference voltage is fixed at V and an offset compensation is applied at mV. A typical experimental measurement set for the proposed circuit and its linear regression are shown in Fig. 2. A linear dependence of output versus resistance variation is obtained. The nonlinearity measured using a best fit straight-line for absolute minimum squared error is less than 1% (full scale output). Furthermore, the whole interfacing circuit provides a relatively good frequency response up to 250 kHz.

IV. CONCLUSION
A novel configuration for a pressure sensor interfacing circuit based on CMOS current conveyors has been designed and tested. This circuit uses a single piezoresistive element, thus avoiding the problems of the traditional four-element pressure sensors related to the matching between and piezoresistances. Monolithic implementation of the pressure sensor plus the interface circuit in a single chip is currently in progress. This interface circuit is well suited for surgical cardiovascular applications for accurate intravascular pressure measurements. The pressure sensor plus this electronic interface circuit fulfill specific characteristics: minimum number of wires, minimum power dissipation, high noise immunity of the signal transmission, and accurate pressure-voltage conversion. He is currently Assistant Professor at the Electronics Laboratory, Physics Department, of the same university. His research interests are in the fields of circuits and systems for low-frequency instrumentation, measurement systems and techniques, sensor interfacing and control electronics, and currentmode analog integrated circuit design and applications. He is actively involved in research projects (National and European), collaboration projects with other European laboratories, has contributed more than 30 papers to scientific journals and conference proceedings, and has authored lecture notes in electronic circuits and applications.
Dr. Lapoulos is a member of the IEEE Imstrumentation and Measurement Society. He is currently serving as a Member of the Editorial Board of the new Instrumention and Measurement Magazine and as Associate Editor of the IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT.