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Integrated Circuit Interface for SAW Biosensors Applications

by Aggour, Khaled, MS

Page 31

Chapter 2: Theoretical Background 18

The delay-line biosensor consists of two IDTs; input and output. The input IDT
generates a mechanical acoustic wave in the piezoelectric material. The wave
reaches the output IDT after a delay time (Rocha-Gaso et al., 2009). A sensitive
layer coated with biological reagent is inserted between the IDTs. When the layer is
exposed to the specific excitation; a change occurs at the amplitude and frequency of
the acoustic wave according to the mass change (figure 2-8). An example of delayline
biosensors was implemented by Berkenpas et al. (2003) for pathogens detection
in liquids.

2.6.3 SAW Resonator Biosensor

Like the delay-line SAW, it consists of two IDTs. Its working principle is to convert
an oscillating wave at the input IDT into a mechanical wave travelling across the
piezoelectric surface. The output IDT reconverts the mechanical energy back into an
electrical wave (Manooningh, 2004). The sensing mechanism is similar to the delayline.

2.6.4 Smart SAW Resonator Biosensor

As described before; the resonator sensors are considered quasi-digital sensors
according to their sensing topology. Measuring the frequency of SAW resonator is
the simplest and most cost effective method for signal conditioning for the following
reasons (Gardner et al., 2001):

- It provides the highest dynamic range.
- It is less complex than phase or amplitude measurement.
- There are less component parts which reduces noise sources.

The most efficient way to extract the frequency is to place the SAW resonator as a
feedback element in an oscillator topology (Figure 2-9).

Figure 2- 9 SAW resonator oscillator block diagram

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Chapter 2: Theoretical Background 19

In order to convert the frequency into an intelligent (digital) output, a
frequency to digital conversion is necessary. For high frequencies in the Very High
Frequency (VHF) and Ultra High Frequency (UHF) bands, the translated frequency
will require expensive instrumentation (Gardner et al., 2001). To overcome this
problem a mixing technique is employed.

Figure 2-10 shows the block diagram of the system utilising a mixer and a low
pass filter. Two SAW resonators are used; one as a reference and the other as a
sensing one. The mixer produces both the sum and the difference frequencies. A low
pass filter is inserted to have only the difference frequency. The mixer sinusoidal
output is then converted to a square wave which is applied to a counter to be
converted to digital.

Reference Frequency


SAW resonator
SAW resonator


Low pass

fref - fsens

SAW resonator
SAW resonator

Amplifier Sensing Frequency


Figure 2- 10 Sensing system block diagram

2.7 SAW Resonator Modelling

Simulating the SAW resonator and its interface circuit together allows the
verification of the interface circuit functionality. This can be done by implementing a
circuit model for the resonator. There are two main categories of SAW resonators:
one port and two-port. One-port resonator is obtained by multiple reflections

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Chapter 2: Theoretical Background 20

between the fingers of a long IDT or reflections by the reflectors at a short IDT both
ends (Krishnan et al., 2006). The two port resonator utilises the reflection in a pair of
IDTs (transmitting and receiving). This is illustrated in figure 2-11.

Figure 2- 11 SAW resonators

a) 1- port long IDT b) 1-port two short IDTs c) 2-port

(Source: Krishnan R., Nemade, H.B., Paily, R., 2006. Simulation of one-port SAW
resonator using COMSOL multiphysic, COMSOL Users Conference, India, pp. 2.)

The electrical circuit model of the one-port SAW resonator can be modelled as
a series RLC resonant circuit and a parallel IDT capacitance (Hashimoto, 2000).
Figure 2-12 shows the circuit model for the 1-port SAW resonator



Lm Cm Rm


Figure 2- 12 One-Port SAW resonator electrical circuit model

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Chapter 2: Theoretical Background 21

The resonator frequency is represented by motional capacitance and motional
inductance Cm and Lm, respectively. So the SAW resonator frequency (fr) can be
calculated as:



eq. 2.1

The motional resistance affects the SAW quality factor. It can be calculated as

= ���

eq. 2.2

Q is the resonator quality factor. The quality factor defines the losses of the

The two-port SAW resonator can be modelled as a series RLC circuit and two
IDT capacitances at both input and output IDTs. If the IDTs encounter a 180° phase
shift; an ideal 1:1 transformer will be added to the model (Schmitt et al., 2000b). The
two port resonator electrical circuit model is shown in figure 2-13.




1:1 Transformer


Figure 2- 13 Two-port SAW resonator electrical circuit model

As in single port resonators, the resonant frequency determines the motional
capacitance and inductance while the motional resistance depends on the SAW
quality factor. Since there are two IDTs; there are two parallel capacitances
determined by the aperture width which is defined as the length of effective fingers,
and the number of finger pairs for IDTs.

The IDT capacitance Co can be calculated as in Nordin et al. (2007):

× ×

εp is the relative permittivity of the piezoelectric substrate

εo is the free space permittivity

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Chapter 2: Theoretical Background 22

W is the SAW aperture width

Nt is the number of finger pairs

The motional resistance Rm is calculated as shown in equation 2.4



Gs is the effective radiation conductance calculated in equation 2.5

Г is the total array reflectivity
���� �������� ���


Ga is the radiation conductance

= 8


k2 is piezoelectric coupling coefficient

fr is the resonator frequency

δ is half of the effective cavity length calculated as :
+ +


Lg is distance between reflector and transducer

Lp is the effective penetration length

λ is the periodic distance

The motional inductance is calculated as


��� �

���� �������� ���


v is the acoustic wave velocity

L is the distance between reflectors

Consequently, the motional capacitance can be calculated as:




2.8 Conclusion

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Chapter 2: Theoretical Background 23

In this chapter, a brief description of transducers, sensors and actuators was
presented. The smart sensor concept and specifications were described. The role of
the interfacing circuit is vital for smart sensors with examples of interfacing circuits
for several sensor types such as capacitive, resistive or resonator to implement a
smart sensing system. The Quasi-Digital sensors such as the resonator have excellent
advantages due to their semi-digital output.

An introduction to biosensors was also given and the importance of SAW
sensors in the biological sensing world demonstrated. The SAW sensor concept and
the main types of SAW according to the wave propagation were introduced. SAW
delay lines and SAW resonators were discussed. The electrical circuit models of both
1-port and 2-port were described in details. The 2-port SAW resonator was modelled
as a series RLC and a parallel IDT capacitance with an ideal transformer to add the
180° phase shift.

2.9 References

1. Berkenpas, E., Bitla, S., Millard, P., Pereira da Cunha, M., 2003. LGS shear
horizontal SAW devices for biosensor, Ultrasonics, IEEE Symposium on,
vol. 2, pp. 1404- 1407.
2. Berkenpas, E., Millard, P., Pereira da Cunha, M., 2006. Detection of

Escherichia coli O157:H7 with langasite pure shear horizontal surface
acoustic wave sensors, Biosensors and Bioelectronics, vol. 21, pp. 2255–
3. Deobagkar, D.D., Limaye, V., Shinha, S., Yadava, R.D.S., 2005. Acoustic

wave immuno sensing of Excherichia coli in water. Sensors and. Actuator B,
vol. 104, pp. 85–89.
4. Frank, R., 2000. Understanding smart sensors, Artech House, Norwood, MA,

5. Gardner, J.W., Varadan, V.K., Awadelkarim, O.O., 2001. Microsensors,

MEMS and Smart Devices. John Wiley and Sons, Sussex, UK, ch. 9-11, pp.
6. Hall, E.A.H., 1990. Biosensors. Open University Press, Milton Keynes, UK,

Biotechnology Series.
7. Harsanyi, G., 2000. Sensors in biomedical applications: Fundamentals,

technology and applications.CRC Press, Boca Raton, FL, USA.
8. Hashimoto, K., 2000. Surface Acoustic Wave Devices in

Telecommunications: Modelling and Simulation, Springer, Berlin, Germany.
9. Hur, Y., Han, J., Seon, J., Pak, Y.E., Roh, Y., 2005. Development of an SH-

SAW sensor for the detection of DNA hybridization, Sensors and Actuators
A: Physical, vol. 120, pp. 462–467.
10. Jaervinen A. M., Saukoski, M., Halonen, K.A.I., 2008. A 12-bit ratioindependent
algorithmic A/D converter for a capacitive sensor interface.
IEEE Transactions on Circuits and Systems, I-Regular Papers, vol. 55 (3),
pp. 730-740.

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Chapter 2: Theoretical Background 24

11. Jordana, J., Pallàs-Areny, R., 2006. A simple, efficient interface circuit for
piezoresistive pressure sensors. Sensors and Actuators A: Physical, Vol. 127,
Issue 1, pp. 69-73.
12. Kogai, T., Yatsuda, H., Shiokawa, S., 2008. Improvement of Liquid-Phase

SH-SAW sensor device on 36 degrees Y-X LiTaO3 substrate. IEEE
Ultrasonics Synopsium, Beijing, China, vol 1-4 and Appendix, pp. 98-101.
13. Kwon, Y., Roh, Y., 2004. Development of SH-SAW sensors for underwater

measurement, Ultrasonics, vol. 42, pp. 409–411.
14. Kress-Rogers, E., 1997. Handbook of biosensors and electronic noses:

Medicine, food, and the environment, CRC Press, Boca Raton, FL, USA,
15. Krishnan R., Nemade, H.B., Paily, R., 2006. Simulation of One-Port SAW

Resonator using COMSOL Multiphysic. COMSOL Users Conference,
Bangalore, India.
16. Länge, K., Rapp, B.E., Rapp, M., 2008. Surface acoustic wave biosensors: a

review, Analytical and Bioanalytical Chemistry, vol. 391, pp.1509-1519.
17. Manooningh, L.L., 2004. Design of a Chemical Agent Detector Based on

Polymer Coated Surface Acoustic Wave (SAW) Resonator Technology. PhD
thesis, University of South Florida, USA.
18. Nordin, A. N., Zaghloul, M. E., 2007. Modeling and Fabrication of CMOS

Surface Acoustic Wave Resonators. IEEE Transactions on Microwave
Theory and Techniques, vol. 55, no. 5, pp. 992-1001.
19. O'Dowd, J. Callanan, A. Banarie, G., Company-Bosch, E., 2005. Capacitive

sensor interfacing using sigma-delta techniques. IEEE Sensors, Orange
County, CA, USA, pp. 951-954.
20. Pallas-Areny, R., Webster, J.G., 2001. Sensors and Signal conditioning. John

Wiley and Sons.
21. Papadakisa, G., Tsortosa, A., Gizeli, E., 2009. Triple-helix DNA structural

studies using a Love wave acoustic biosensor, Biosensors and Bioelectronics,
vol. 25, pp. 702-707.
22. Ripka, P., Tipek, A., 2007. Modern Sensors Handbook, ISTE, Wiltshire, UK.
23. Rocha-Gaso, M.I., March-Iborra, C., Montoya-Baides, A., Arnau-Vives,

A., 2009. Surface Generated Acoustic Wave Biosensors for the Detection of
Pathogens: A Review. Sensors, Vol. 9, issue. 7, pp. 5740-769, URL:
24. Sakong, J., Roh, H., Roh, Y., 2007. Surface Acoustic Wave DNA Sensor

with Micro-Fluidic Channels, Japanese Journal of Applied Physics, vol. 46,
pp. 4729–4733.
25. Schmitt, R.F., Allen, J.W., October 2000b. Designing an EMC-compliant

UHF oscillator, RF Design Cardiff Publishing Co., Englewood, CO. U.S.,
vol. 23, no. 10.

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Chapter 2: Theoretical Background 25

26. Varadan, V.K., Vinoy, K.J., Gopalakrishnan, S. 2006. Smart Material
Systems and MEMS: Design and Development Methodologies. John Wiley
and Sons, pp. 97-102.
27. Wessa, T., Rappa, M., Sigristb, H., 1999. Immunosensing of

photoimmobilized proteins on surface acoustic wave sensors, Colloids and
Surfaces B: Biointerfaces Vol. 15, Issue 2, pp. 139-146.
28. Zhang, C., Caron, J.J., Vetelino, J.F. 2001. The Bleustein–Gulyaev wave for

liquid sensing applications. Sensors and Actuators B: Chemical, vol.76, issue
1-3, pp. 64-68.

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Chapter 3: Literature Review 26

Chapter 3

Literature Review

3.1 Introduction

Sensor interfacing has always been a major research area. Several publications report
circuits for sensors signal conditioning (interfacing). SAW sensor interfacing has
become as challenging as SAW design itself. This chapter presents some of recent
work in SAW interfacing. The first block for the interface circuit is the oscillator.

3.2 SAW Oscillator

Oscillators are classified with respect to oscillation analysis into two types; feedback
oscillators and negative-resistance oscillators. The later were implemented in Vittoz
et al. (1988) and Yao et al. (2007) with low frequency (few MHz) using crystal
SAWs. A high frequency negative-resistance SAW oscillator was reported in Kozaki
et al. (2005) and Tanguay et al. (2006). Traditional CMOS fabrication processes are
not capable of providing the required transistor trans-conductance at very high

For feedback loop oscillators, a gain required with more than the SAW losses.
Typical commercial SAW resonators have losses between 10-15 dB (Schmitt et al.,
2000a). Several literatures reported the SAW resonator oscillator in discrete circuits
such as in Schmitt et al. (2000a, 2000b) using separate amplifiers ICs.

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Chapter 3: Literature Review 27

The most common topologies for SAW oscillators are Pierce and Colpitt
topologies. In Pierce oscillator topology, the SAW is connected between the gate and
the drain of the MOS transistor or the base and collector of a Bipolar Junction
Transistor (BJT). The connection is shown in figure 3-1. The oscillator feedback is
created by the SAW resonator and the gain is provided by a common source
amplifier (common base in BJT). The gain should exceed the resonator losses to start
oscillation. The Pierce oscillator works in series resonance. The parasitic capacitance
appears in series with the resonator motional capacitance. Accordingly, the resonant
frequency is relatively insensitive to slight changes in the parasitic capacitance.

Figure 3- 1 Pierce Oscillator Connections

a. MOSFET b. Bipolar

In Colpitt oscillators, the SAW resonator is connected between the source and
the gate of the MOS transistor or emitter and base of BJT. This is shown in figure 3-
2. The amplifier is in source follower mode (emitter follower in BJT). The capacitors
provide the feedback. At resonance, the resonator shorts the gate or the base
allowing the loop to have a sufficient gain to start the oscillation (Lee, 1998). The
oscillator works in parallel resonance and its resonance frequency depends on the
components capacitances.

(a) MOS Colpitt

(b) Bipolar Colpitt

Figure 3- 2 Colpitt Oscillator connections

a. MOSFET b. Bipolar

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