Integrated Circuit Interface for SAW Biosensors Applications
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
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
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.
fref - fsens
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
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
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
The motional resistance affects the SAW quality factor. It can be calculated as
� = ���� ��
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.
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):
� = 휀
� × 휀
� × 푊 × 푁
εp is the relative permittivity of the piezoelectric substrate
εo is the free space permittivity
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
퐺� = 퐺�
���� �������� ����
Ga is the radiation conductance
� = 8푘�푓
k2 is piezoelectric coupling coefficient
fr is the resonator frequency
δ is half of the effective cavity length calculated as :
훿 = 퐿
� + 퐿� + ���
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:
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.
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Chapter 3: Literature Review 26
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.
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
(a) MOS Colpitt
(b) Bipolar Colpitt
Figure 3- 2 Colpitt Oscillator connections
a. MOSFET b. Bipolar