Development of Low Pass Filtering and Linearization for Piezoresistive Microcantilever Biosensing

Mohd Firdaus Abdullah, Lee Yoot Khuan, Nina Korlina Madzhi

Faculty of Electrical Engineering Universiti Teknologi MARA 40450 Shah Alam, Selangor

badli_84@yahoo.co.uk

Anuar AhmadFaculty of Engineering

Universiti Industri Selangor Berjuntai Bestari, Selangor

Abstract— This paper concerns the development and design of filtering and linearization stage of potentiometric transducer to measure human stress level based on salivary amylase activity. The design of this new way approached is implemented using Cadence OrCAD Capture CIS 15.7 software and hardware construction and troubleshooting. From the previous research, it has been found that for a sensor input range of 1.1 to 1.3 kilo ohms, an output range of -100 to +100 millivolts is obtained. The design for filtering stage used the first order passive low pass filter to accommodate the stress signal frequency for 0.4Hz. Besides the design also focus to boost up the low voltage input of 108.6mV - (-100mV) to 0V-5V output voltage. Its involved low pass filter, voltage follower and linearization stages. The result indicates discrepancy within 4.5% and 3.7% were found between the simulation and experimental results for voltage follower after filtering and linearization circuit respectively, on the average. This development allowed stress measurement with relatively high precision and accuracy.

Keywords- Salivary amylase, Human Stress, Filtering, Linearization, Voltage Follower

I. INTRODUCTION

Human lives are concerned with stress everyday. The amount of stress a person feels depends on both internal factors (the person's personality and mental state) as well as external factors (the amount of stress in their daily home or work environment). Coping with stress can be easy or hard depending on the specific influence inducing stress and anxiety. A person's stress can become so severe that it not only affects their mental state, but it can also cause a physiological response. During stress, the activity of the sympathetic nervous system increases. In response to this, the heart begins to beat more rapidly, muscle tensions, blood pressure rises and pupil dilates [1, 2]. Previous study have reported stress measurement focuses on two major physiologic stress responses exhibit in the autonomic nervous system, i.e. the sympathetic adrenal medullary system (SAM) and hypothalamus pituitary adrenocorticol system (HPA) [3]. Heart rate and blood pressure have been widely used as indices to measure human psychologic stress because they can be easily and promptly measured. However, one major drawback of this approaches is that, heart rate cannot be used to distinguish between eustress and distress, markedly affected by homeostasis and the changes are not large as compared to their normal values [1]

Studies have shown that the amount of alpha amylase, an enzyme secreted by the salivary glands, can be related to the "fight or flight" response in SAM system to stress. It has been proven that the increase in salivary amylase activity is more significant and the reaction is more rapid than that of cortisol when experimented with psychologic stressor, suggesting that it is a better index of stress [4]. The development of the biomedical recording system has been created many years. Due to the weak amplitude characteristic, biomedical signal are easy to be influence by environment and devices [5]. Recording the biosensor in the biomedical electronics is one of the challenges in a biomedical electronics detection system, because the biomedical signal has very weak amplitude and low frequency, usually of few milivolts or less and the frequency below 1 KHz [5, 6]. However, the biomedical electronics is too weak to detect, therefore we need a high gain to amplified biomedical signal [6]. An instrumentation amplifier is such an amplifier of electrical systems for amplifying small differential voltage [6].

In this paper, the development of filtering and linearization stages designed specifically used the first order passive low pass filter to accommodate the stress signal frequency at 0.4Hz. Besides, the designed also focus to boost up the low voltage input of 108.6mV to -100mV to 0V-5V output voltage. Its involved low pass filter, voltage follower and linearization stages.

II. METHODOLGY

This project consists of three major parts, which are theoretical calculation, software design and hardware construction. The project was designed based on theoretical calculation. It is then divided into two parts that is construction of the circuit and software development. In hardware design, the circuit for input and output will be test. If any circuit not working properly, then the troubleshooting action will be implement. The software design and simulation was performed using the OrCAD Capture CIS 15.7. The overall project has been tested. If the output does not show the expected result, the troubleshooting will implement either in the hardware design or software design. The architecture of the potentiometric circuit design has shown in Fig.1. The overall project involved transduction stage, filtering, voltage follower, and linearization stage. This project only focuses on filtering and linearization stage.

14978-1-4244-7406-6/10/$26.00 c©2010 IEEE

The output of the voltage follower after filtering stage entered the linearization stage and lastly the voltage follower circuit.

Figure 1: Potentiometric Circuit Block Diagram

A. Filtering The filtering stage will accommodate the stress signal in a frequency 0.4Hz [7] using low pass filter after the first stage of amplification. A filter is a circuit that passes certain frequency and attenuates or reject all other frequencies. Its constructed using combination of resistors and capacitor [8, 9]. The low pass filter is used to eliminate frequency higher than the stress signal which is 0.4Hz, and to eliminate the change of noise interruption or data error. Low pass filter pass only the frequencies below the designed frequency, blocking all other frequency [8].

R2

0U2

OP-27

+3

-2

V+7

V-4

OUT6

OS11

OS28

R4

R3

C1

C2

V4

12Vdc

V5

12Vdc

0

0

R1

C3

Input v oltage

Output Voltage

Figure 2: Schematic for first order low pass filter

Fig.2 illustrate the structure of low pass filter schematic circuit. The frequency at which the gain starts to decrease is controlled by C3 and R2. This type of circuit is usually configured as an inverting amplifier and is refered to as an active low pass filter. The circuit will pass low frequency signals on to its output gain, Av, but will attenuate signal of higher frequencies. The output gain for this circuit was calculated as:

1

2

R

RAv −= (1)

The cut off frequency defines the end of the passband and its normally specified at the point where the response drops -3dB (70.7%) from the passband response [8]. Cut off frequency is calculated by the familiar formula:

322

1

CRf

Π= (2)

B. Voltage Follower A unique noninverting amplifier is the voltage follower, which has its output connected directly to its inverting input, thus producing an output that is equal to the non inverting input voltage in both amplitude and polarity [11]. Because the output is equal to the input the gain is always 1 and so voltage follower is sometimes referred to as a unity gain amplifier. The output appears to follow or track the input voltage to eliminate loading effects or interface impedances. The interposed buffer amplifier prevents the second circuit from loading the first circuit unacceptably and interfering with its desired operation. Voltage follower also has the ability to buffer a high impedance signal. This is to give more power to a long sensor cable run, lower impedance, and protect circuitry from being overloaded [12]. Figure 3 shows that the output from low pass filtering stage connected as the input to the voltage follower stage.

0

U7

OP-27

+3

-2

V+7

V-4

OUT6

OS11

OS28

V612Vdc

V712Vdc

0

Input v oltage

Output Voltage

Figure 3: Schematic for Voltage Follower

In this project, voltage follower also will be used after linearization stage. As in Figure 4, the output voltage (Vo4) will connect directly to the noninverting input of the op-amp. The inverting input connected to the output of the op-amp. To obtain the unity gain as in:

Ri

Rf

Vin

Vo+= 1 (3)

For buffer 0=Rf and ∞=Ri therefore gain is equal to 1 and VinVo = .

Vin1

Vo1

Vin2

Vo2

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C. Linearization Linearization stage design involved the modification of a basic circuit so that the output was approximately linear functions of its input, in order to facilitate analysis of the system. In this project, the sensor outputs range from filtering stage is 0.1086V to -0.1V. For interface to an analog-to-digital converter, this needs to be 0 to 5V. As a result, it is easiest to develop an equation for the output in terms of output. From this circuit can be envisioned.

VomVinVout += (4)

Using the specified information, we form two equations for the unknown slope (gain), m, and offset (bias), Vo Vom += 1086.00 (5)

Vom +−= 1.05 (6) Clearly, from the equation 5, Vo = 0.1086m, and when this substituted into equation (6), we get mm 1086.01.05 −−= (7) Then solving for m as in:

97.231086.01.0

5−=

−−=m (8)

The transfer function equation is thus: 603.297.234 +−= VinVo (9)

To satisfy the equation (9), schematic circuit for linearization stage was developed as in Fig.4. It consists of two operational amplifier, (OP1 and OP2) and resistor network. OP1 is connected in non-inverting stage, while the OP2 is the simple differential amplifier construction. NonInverting amplifier was chosen to get gain -23.97. Note that a portion of Vo4 is feedback via R6 to the negative input of an op-amp. Resistor value for R5 and R6 was calculated to get the exact gain -23.97 as in:

5

6

R

RAV = (10)

For the offset voltage, a variable resistor (R11) has been used, so both loading of the divider by the op-amp circuit and variation of the supply from exactly 5V can be compensated for by adjusting until the bias exactly 2.603V at the point of Voff as in the Figure 5. To adjust the resistors with the actual value of Voff and Vo4, their value are R7=R8=R9=R10. The output voltage was accomplished by using differential amplifier as in: VVV

−+

−=05

(11)

V+ from offset voltage (Voff), while V

− from

noninverting amplifier output voltage (Vo3).

Vo1

0

0

R11

SET = 0.5Vdc

0

Vo2Input v oltage

OP1

+3

-2

V+

7V

-4

OUT6

OS11

OS28

OP2

+3

-2

V+

7V

-4

OUT6

OS11

OS28

R5

R6

R7

R8

R9R10

Voff

V1312Vdc

0

V1712Vdc

0

V1812Vdc

0

0

V1412Vdc

Figure 4: Schematic for Linearization Stage

D. Hardware Construction The hardware construction was initiated using resistor decades, OP-27 operational amplifiers, resistor, capacitor and a voltage source. The Op-27 operational amplifier was used in this design for its low-noise with high precision performance. The DC power supply voltage for OP-27 at V+ and V- is +12V and -12V respectively. The capacitors C1, C2 and resistors R3, R4 as in Figure 3 are used to keep the circuit stable, especially in cases where the same DC power supplies are used for several stages. The output measurement was taken using digital multimeter. In order to obtain the precise value of the resistor decade equivalent to the fix resistor value, digital meter is used. The construction of the hardware circuit implemented by testing the input and output of all stages on the breadboard. A lot of modification and troubleshooting done to achieve the desired result as in software design.

III. RESULTS AND DISCUSSION

Design for the filtering, voltage follower and linearization circuit has been carried out through theoretical, simulation and experimental study.

A. Simulation Design

The software design and simulation was performed using the OrCAD Capture CIS 15.7. Its offers comprehensive solutions for entering, modifying, and verifying complex system design quickly and cost effectively. PSpice is a derivative of the original Spice program which supports integrated circuit design and general circuit design. The simulation study begins with theoretical analysis of all the components in the design. The result for simulation was produced as in Table I, Table II, Table III and Table IV.

B. Filtering and Voltage Follower The calculation and simulation was done to find the stress signal frequency at 0.4Hz. Table I show the calculation and simulation result for low pass filtering while table II shows the discrepancy difference of simulation and experimental for voltage follower. The gain for the filtering stage was remaining constant to 1. The theoretical calculation for filtering stage is 0.458Hz because the value of capacitor and resistor chosen. The component values of choice must be

Vo3

Vin3 Vo4

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realistic to realize the design on the circuit board. A discrepancy different 0.43% is detected for low pass filtering stage.

TABLE I LOW CUT-OFF FREQUENCY

Low cut-off frequency (Hz)

Theoretical calculation 0.458 Simulation result 0.46 Discrepancy (%) -0.43%

A slight change in discrepancy difference around −

+ 10% is detected comparing both simulation and experimental results for voltage follower stage. It can be said that an acceptable difference is shown thus confirming that the design will meet the requirement. A discrepancy within 4.5% on the average is detected from the results, which could be attributed to tolerance of electronic components and wiring. Figure 5 show a frequency response simulation of frequency versus gain for Low Pass Filter. For frequencies greater than the cutoff frequency, when low cut-off frequency is 0.46Hz, the gain decrease at a rate of 20bd/decade. The cutoff frequency is defined as that frequency of input voltage where the gain is reduced to 0.707 times to its low frequency value [10]. It was calculated from equation (2).

TABLE II VOLTAGE FOLLOWER

Vin2 (mV)

Vo2(mV) from simulation study

Vo2(mV) from experimental study

Discrepancy (%)

108.6 108.9 111.74 -2.61 86.2 86.48 89.45 -3.43 64.1 64.43 67.34 -4.52

42.37 42.76 45.75 -6.99 21 21.46 24.36 -13.51 0 0 3.29 -

-20.66 -20.09 -18.42 8.31 -40.98 -40.35 -38.7 4.09 -60.97 -60.28 -58.89 2.31 -80.65 -79.88 -78.25 2.04 -100 -99.17 -97.49 1.69

Figure 5: Simulation Low Frequency Cut-off

Figure 6 depicts the outcome from a comparative study between Theoretical, Simulation and Experimantal results on identifying the voltage follower. Voltage follower is the part after the low pass filtering stage. As observed in the graph, the voltage follower input is similar each other. The mangnitude and the polarity of the output voltage followed by input voltage.

Figure 6:Comparative study between Theoretical, Simulation and Experimantal results on output volatge of voltage follower

C. Linearization and Voltage Folower Table III and Table IV shows the discrepancy difference of simulation and experimental voltage for linearization and the second voltage follower respectively.

TABLE III LINEARIZATION

Vin3 (mV)

Vo3(V) from simulation study

Vo3(V) from experimental study

Discrepancy (%)

108.6 -0.119 -0.144 21 86.2 0.441 0.404 8.39 64.1 0.992 0.968 2.42

42.37 1.534 1.519 0.98 21 2.066 2.063 0.15 0 2.598 2.601 -0.115

-20.66 3.105 3.161 -1.80 -40.98 3.611 3.67 -1.63 -60.97 4.11 4.175 -1.58 -80.65 4.6 4.66 -1.30 -100 5.082 5.15 -1.34

TABLE IV VOLTAGE FOLLOWER

Vin4 (V) Vo4(V) from simulation study

Vo4(V) from experimental study

Discrepancy (%)

0 -0.119 -0.145 21.85 0.537 0.441 0.404 8.39 1.067 0.992 0.968 2.42 1.497 1.534 1.52 0.91

2.1 2.066 2.06 0.29 2.603 2.598 2.601 -0.12 3.098 3.105 3.161 -1.80 3.585 3.611 3.669 -1.61 4.064 4.11 4.175 -1.58 4.536 4.6 4.66 -1.30

5 5.082 5.15 -1.34

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A slight change in discrepancy difference around −

+ 10% is detected comparing both simulation and experimental results. The output from the first voltage follower will boost up in the range of 0V-5V which required in this project objective. A discrepancy within 3.7% on the average is detected from the results, which could be attributed to tolerance of electronic components and wiring. Figure 7 depicts the outcome from a comparative study between Theoretical, Simulation and Experimantal results on output voltage of linearization stage. The graph proved that the inputs voltage were boost up between 0V-5V in a linear mode. The output of this project was amplified linear to the input. The linearity could be proved by equation (4). The y-axis crossing for the software linear line is -0.119 whereas for hardware linear line is -0.144. Although the y-axis is a little bit different from each other the slope proves that the linearity of the output amplified. Figure 8 depicts the outcome from a comparative study between Theoretical, Simulation and Experimantal results on output voltage of voltage follower. Voltage follower is the last past of linearization stage. The voltage follower input and output were similar to each other. The magnitude and the polarity of the output voltage followed the input voltage.

Figure 7:Comparative study between Theoretical, Simulation and Experimantal results on output voltage of linearization stage

Figure 8:Comparative study between Theoretical, Simulation and Experimantal results on output volatge of voltage follower

IV. CONCLUSION A filtering and linearization circuit for a piezoresistive MEMS sensor to detect human stress is developed. The circuit has been designed and tested through theoretical, simulation and experimental studies. Stress signal was filtered by low pass filter at 0.46Hz. It has found that for a

sensor input range of 1.2 to 1.3 kiloohm, the voltage output range from voltage follower after low pass filter between 111.74mV to -97.47mV and output range of -0.145V to 5.15V is obtained from linearization stage using experimental study. A discrepancy within 4.5% and 3.7% was found between the simulation and experimental results for filtering and linearization circuit respectively. The discrepancy percentage was very low and proved that the software and hardware significant to each other. The software simulation and hardware implementation have been successfully done and this finding is useful for interface to analog-to-digital converter for detecting human stress level.

ACKNOWLEDGMENT

The authors would like to acknowledge the Minister of Science, Technology and Innovation, Malaysia and Universiti Teknologi MARA, Malaysia for their financial and equipment assistant for the project research.

REFERENCES

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[2] M.H Lee, H.K Lee, and S. Bang, "Development Stress monitoring System based on Personal Digital Assistant(PDA)," in 26th Annual International Conference of the IEEE EMBS, San Francisco, CA, USA, 2004.

[3] G. C. H. D. Bond, "Lifestyle-Specific Outcome Measures," The International Electronic Journal of Health Education, pp. 159-168, 2000.

[4] N.Takai, T. Aragaki, K. Eto, K. Uchihashi, and Y. Nishikawa, "Effect of psychological stress on the salivary cortisol and amylase levels in healthy young adults," Achieves of Oral Biology, pp. 963-968, 2004.

[5] H. Chun-Chieh, H. Shao-Hang, C. Jen-Feng, L. D. Van, and L. Chin-Teng, "Front-end amplifier of low-noise and tunable BW/gain for portable biomedical signal acquisition," in Circuits and Systems, 2008. ISCAS 2008. IEEE International Symposium on, 2008, pp. 2717-2720.

[6] C. Hwang-Cherng and W. Jia-Yu, "High CMRR instrumentation amplifier for biomedical applications," in Signal Processing and Its Applications, 2007. ISSPA 2007. 9th International Symposium on, 2007, pp. 1-4.

[7] S. Cerutti, A. M. Bianchi, and H. Reiter, "Analysis of sleep and stress profiles from biomedical signal processing in wearable devices," in Engineering in Medicine and Biology Society, 2006. EMBS '06. 28th Annual International Conference of the IEEE, 2006, pp. 6530-6532.

[8] D. A. Bell, "Active Filter," in Operational Amplifier, Application, Troubleshooting and Design, 1st ed United State of America: Prentice Hall, 1990, pp. 259-288.

[9] J. J. Carr, "Loop Filter Circuits," in Integrated Electronics:Operational Amplifier and Linear ICs with application, 1st ed Florida: Habcourt Brace Jovanovich, 1990, pp. 484-485.

[10] H. S. Kalsi, "Filters," in Electronic Instrumentation New Delhi: Tata McGraw-Hill, 2004, pp. 500-534.

[11] D. A. Bell, "Introduction to operational Amplifier," in Operational Amplifier, Application, Troubleshooting and Design, 1st ed United State of America: Prentice Hall, 1990, pp. 1-20.

[12] Thomas L.Floyd and D. Buchla, "Voltage Follower," in Op-Amp Configurations with Negative Feedback, 2nd ed United State of America: Prentice Hall, 1999, pp. 70-71.

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