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Integrated Circuit Interface for SAW Biosensors Applications

by Aggour, Khaled, MS


Page 41

Chapter 3: Literature Review 28

Pierce SAW oscillators have superior stability than their Colpitt’s counterparts
(Timal et al., 2006). Several reports on CMOS Pierce oscillators for SAW utilise
one-port SAW resonators such as in Nordin et al. (2005) and in Zou et al. (2009) use
MEMS resonator. A two-port SAW resonator was used in the CMOS Pierce
oscillator by Nordin et al. (2006). The circuit was implemented with a conventional
CMOS process (0.5µm process).

3.3 Mixer

As the name indicates, the mixer function is to mix two different signals and provide
a desirable mixed output. The mixer block diagram is shown in figure 3-3.

Since mixers are mostly used in Radio Frequency (RF) receivers; the mixer
inputs were denoted as fRF and fLO (RF and Local Oscillator frequencies respectively)
and the output as Intermediate Frequency (fIF). The Intermediate frequency is the
difference between local oscillator and RF frequencies

(fIF= fLO – fRF).

One important specification of the mixer is its conversion gain; the mixer
conversion gain Gc is defined as:


= ��

��

eq.3.1

Figure 3- 3 Mixer block diagram

The conversion gain specifies how the mixer output amplitude is related to the
input signal. As the conversion gain increases; the output signal is more sensitive to
the input signal.


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Chapter 3: Literature Review 29

Another important mixer criterion is its port-to-port isolation. This parameter
defines how the mixer ports amplitude (both inputs and output) are isolated from
each other. Ideal mixers have infinite isolation. This parameter is of a great
importance in RF transmitters and receivers as the mixer output amplitude is the
main concern in RF communications. For SAW sensors applications, the detectable
sensing quantity is contained only in the frequency of the mixer, so only the output
frequency is interesting.

Another interesting specification of the mixer is its output harmonics; it is
desirable to have as low as possible harmonics in order to avoid the requirement of a
sharp filter. In SAW resonators, a low-pass filter with relaxed performance is
sufficient to suppress any unwanted harmonics.

In saturation operation of the MOS transistor, the current passing through the
MOS drain is a function of the square of the gate voltage. It can be approximated as
follows (neglecting channel length modulation):


=

� 휇 퐶
��(
�� − 푉
)eq.3.2

ID the drain current

W and L are the transistor width and length, respectively

µ is the carrier mobility

Cox is the oxide capacitance per unit area

VGS and Vt are gate to source voltage and threshold voltage, respectively.

From the above equation, it can be seen that the gate voltage is squared. If two
signals were applied at the gate, their sum and difference frequencies will occur at
the drain current and consequently the drain voltage.

Mixers can be classified according to their conversion gain into two categories:
passive and active mixers. Passive mixers do not introduce gain while active mixers
produce a more than unity conversion gain.

Active mixers can be categorised into two types: Balanced and Unbalanced
mixers. The schematic diagram of the single balanced mixer is shown in figure 2-5.

Balanced mixers have better port isolation than unbalanced ones (Lann, 2006).
In the double balanced mixer, RF input also is differential.


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Chapter 3: Literature Review 30

Figure 3- 4 Single balanced mixer schematics

The unbalanced mixer does not need a differential input signal. The schematic
diagram of the active unbalanced signal is shown in figure 3-5.

Vdd

VLO

VIF

VRF

Figure3- 5 Unbalanced mixer schematics

The unbalanced mixer implementation is simple and does not need a
differential structure. Its disadvantage lies in its poor port isolation. For SAW signal
conditioning, the frequency of the difference signal is only interesting, so the
unbalanced mixer is convenient for this particular application.


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Chapter 3: Literature Review 31

3.4 Temperature Control

Depending on the SAW device substrate material, the temperature change can affect
the sensor’s output characteristics. The SAW device temperature dependence is
called the Temperature Coefficient of Delay (TCD). While Quartz substrates
produce negligible TCDs, in other substrate materials the temperature variation can
influence the electrical signal more than the measured quantity itself.

Table 3.1 in Shimizu (1993) shows the minimum Temperature Coefficient of
Delay for different substrate materials. The values of the table are the minimum
possible values at specific cut angles only. In general, leaky SAWs such as SH-SAW
and Bulk Acoustic Wave (BAW) exhibit superior temperature stability than their
Rayleigh counterparts as stated by Kalantar-Zadeh et al. (2003).

Material Mode Minimum TCD (ppm/°C)

Quartz

LiNbO3

Rayleigh 0

leaky 0

Rayleigh 70

leaky 0

LiTaO3 Rayleigh 18

leaky 0

Li2 B4 O7 Rayleigh 108
leaky 112

Bi12 Ge O20 Rayleigh 0

leaky 0

Table 3.1 TCD for SAW devices for different substrates (Shimizu, 1993)

As the SAW measurement is carried mostly by frequency measurement, it is
beneficial to represent the SAW temperature dependency in terms of Temperature
coefficient of Frequency (TCF).

As in Hashimoto (2000) the TCF can be approximated as follows:

푇퐶퐹 = − 푇퐶퐷 eq.3.3


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Chapter 3: Literature Review 32

It can be seen that in substrates such as Lithium Tantalate (LiTaO3), at a
frequency of 200MHz, one degree temperature change will result in at least 3.6 kHz
frequency shift (assuming a linear relationship). So temperature control or
compensation is crucial for SAW devices in sensors applications.

In order to be able to either control the temperature or compensate its effect in
SAW sensors, a temperature measurement scheme should be implemented. Adding a
resistive heater is one approach to calculate the device temperature. Analysing the
temperature effect of ZnO SAW devices via measuring the heater resistance was
done in Tigli et al. (2008). It showed that the SAW temperature can be externally
controlled by changing the heater terminal voltages. A large voltage was applied
because a large N-well heater resistance was used.

Compensating the temperature effect by using two materials with opposite
TCDs was done by Wu et al. (2001) using ZnO and SiO2.

In Nordin et al. (2008) heating the SAW heater to a temperature that exceeds
the ambient temperature was done. The effect of ambient temperature variation on
the SAW resonant frequency was reduced to a third.

In the case of constant ambient temperature, applying a constant power and
consequently heating at a constant temperature could be done. A circuit delivering a
constant power was implemented by Chan et al. (1997) with a 3% temperature
tolerance.

3.5 System Integration

Integrating the biosensors with temperature control in a single chip has been made
by Lauwers et al. (2001) for conductometric sensors. Controlling the temperature
automatically was implemented by Roith et al. (2006) for SAW resonators using an
external micro-controller.

3.6 Conclusion

In this chapter, a brief description of the thesis work was given. The coated SAW
device is the chemical sensing element. Two oscillator circuits are designed one with
a reference SAW and the other with a coated sensing one. Different SAW oscillator
topologies such as Pierce and Colpitt ones were explored and Pierce oscillator circuit
was chosen for this particular application. A comparison between different mixers
architectures was presented. Due to its simplicity, the unbalanced mixer was
designed to provide the frequency difference between the two oscillators. Several
temperature compensation techniques and temperature control mechanisms were
shown.


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Chapter 3: Literature Review 33

3.7 References

1. Chaize, A., 2008. SAW micro-sensor design for biosynthetic infochemical
communication. MSc thesis (Advisor Cole, M.). University of Neuchatel,
Switzerland, University of Warwick, UK.
2. Chan, S.S.W., Chan, P.C.H., 1997. A resistance variation tolerant constant

power heating circuit for integrated sensor applications. 23rd European Solid-
State Circuits Conference, Southampton, UK, pp. 5-8.
3. Hashimoto, K., 2000. Surface Acoustic Wave Devices in

Telecommunications: Modelling and Simulation, Springer, Berlin, Germany.
4. Kalantar-Zadeh, K., Powell, D. A., Wlodarski, W., Ippolito, S. J., Galatsis,

K., 2003. Comparison of layered based SAW sensors. Sensors and Actuators
B: Chemical, vol. 91, pp. 303-308.
5. Kozaki, M., Hama, N., 2005. A 300-MHz-band, sub-1 V and sub-1 mW

CMOS SAW oscillator suitable for use in RF transmitters. IEICE
Transactions on Electronics, vol. E88–C, no.4, pp. 502-508.
6. Lann, A., 2006. RF CMOS Power Mixer Design for short range wireless

applications with focus on polar modulation. MSc thesis, Linkoping
University, Sweden.
http://webstaff.itn.liu.se/~parha/docs/annamaria_exjobb.pdf.
7. Lauwers, E., Suls, J., Gumbrecht, W., Maes, D., Gielen, G., Sansen, W.,

2001. A CMOS multiparameter biochemical microsensor with temperature
control and signal interfacing. IEEE Journal of Solid-State Circuits, vol. 36,
no. 12, pp. 2030-2038.
8. Lee, T.H., 1998. The Design of CMOS Radio Frequency Integrated Circuits,

Cambridge University Press, NY, USA.
9. Nordin, A.N., Zaghloul, M., Korman, C., Ahmadi, S., 2005. CMOS surface

acoustic wave oscillators. Circuits and Systems, 48th Midwest Symposium
on, OH, USA, vol. 1, pp. 607-610.
10. Nordin, A., Zaghloul, M., 2006. Design and Implementation of 1GHz CMOS

resonator utilizing Surface Acoustic Wave, IEEE International Symposium
on Circuits and Systems (ISCAS06). Island of Kos, Greece, pp. 3514-3517.
11. Nordin, A.N., Voiculescu, I., Zaghloul, M., 2008. On-chip hotplate for

temperature control of CMOS SAW resonators. Symposium on Design, Test,
Integration and Packaging of MEMS/MOEMS, Nice, France, pp.71-76, 9-11.
12. Roith, B., Gollwitzer, A., Lerner, A., Fischerauer, G., 2006. Microcontroller-

Based Temperature Compensation Scheme for Two-Port SAW Oscillators.
IEEE International Frequency Control Symposium and Exposition, FL, USA,
pp. 827-830,
13. Schmitt, R.F., Allen, J.W., Wright, R., 2000a. Rapid design of SAW

oscillator electronics for sensor applications. Sensors and Actuators B, vol.
76, pp. 80-85.


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Chapter 3: Literature Review 34

14. 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, pp. 40,42,44,46,48,50,52,54.
15. Shimizu, Y., 1993. Current status of piezoelectric substrate and propagation

characteristics for SAW devices, Japanese Journal of Applied Physics, vol.
32, pp. 2183–2187.
16. Tanguay, L.F., Sawan, M., 2006. Low Power SAW-Based Oscillator for an

Implantable Multisensor Microsystem. IEEE Asia Pacific Conference on
Circuits and Systems, Singapore, pp. 494 – 497.
17. Tigli, O, Zaghloul, M.E., 2008. Temperature Stability Analysis of CMOS-

SAW Devices by Embedded Heater Design. IEEE Transactions on Device
and Materials Reliability, vol. 8, no. 4, pp. 705-713.
18. Timal, A.T., Singh, M., Mittal, U., Yadava, R.D.S., 2006. A comparative

analysis of one-port Colpitt and two-port Pierce SAW Oscillator for DMMP
vapour sensing, Sensors and Actuators B: Chemical, vol. 114, pp. 316-325.
19. Vittoz, E. A., DeGrauwe, M. G. R., Bitz, S., 1988. High-performance crystal

oscillator circuits: theory and application. IEEE Journal of Solid State
Circuits, vol. 23, no. 3, pp. 774-783.
20. Wu, P., Emanetoglu, N.W., Tong, X., Lu, Y., 2001. Temperature

compensation of SAW in ZnO/SiO2/Si structure, IEEE Ultrasonics
Symposium, GA, USA, vol.1, pp.211-214.
21. Yao, S., Zhu, H., Wu, X., February 2007. Design of Low Power CMOS

Crystal Oscillator with Tuning Capacitors. Engineering Letters, vol. 14:1,
EL_14_1_8 (Advance online publication).
22. Zou, X., Xu, X., Yao, L., Lian, Y., 2009. A 1-V 450-nW Fully Integrated

Programmable Biomedical Sensor Interface Chip. IEEE Journal of Solid
State Circuits, vol. 44, no. 4, pp. 1067-1077.


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Chapter 4: SAW Sensor Interface Circuit Design 35

Chapter 4

SAW Sensor Interface Circuit Design

4.1 System Design

The smart biological sensing system function is to convert the biological (chemical)
sensed quantity into a digital output suitable for digital micro-controllers. The
Integrated Circuit (IC) interface target is to translate the frequency shift caused to the
SAW resonator by chemical reaction to a digital word.

The SAW resonator is the sensing element and its resonant frequency is
sensitive to the measured chemical liquid or gas. The output frequency is extracted
using an oscillator circuit utilising the resonator as a frequency determining element.
The oscillator output is then converted to digital output. The system block diagram is
shown in figure 4-1.

The sensing quantity affects the sensing SAW series resonant frequency so the
oscillator output frequency changes. The oscillator output frequency is in the range
of hundreds of mega-Hertz which requires expensive digital equipment to extract.
Moreover, a poor accuracy will result from the measurement of the absolute SAW
resonator frequency. So a mixer is used to produce mixing between the reference
(unperturbed) oscillator frequency and sensing oscillator frequency. The difference
in the oscillators’ frequencies determines the effect of the biological stimulus.
Filtering the other mixer output signals is necessary.

The mixer filtered sinusoidal output is then converted to a square wave with
the same frequency; the square wave is converted to a digital word representing the


Page 49

Chapter 4: SAW Sensor Interface Circuit Design 36

output frequency of the mixer. The digital signal is ready for digital signal
processing.

Figure 4- 1 Sensing system block diagram

4.2 Oscillator

The oscillator is the first interface with the SAW resonator. The SAW device is
connected as a feedback element in the oscillator loop. The oscillator output
frequency depends on the SAW device resonant frequency which is stimulus
sensitive.

Figure 4- 2 Oscillator open loop block diagram

In order to achieve oscillation, two conditions must be met simultaneously in
the open loop circuit shown in figure 4-2:

- Open loop gain must exceed unity (above 0 dB).


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Chapter 4: SAW Sensor Interface Circuit Design 37

- Total phase shift must be 360° or multiple integers of 360°. This condition is
stated in equation 4.1


���� ���� =
��� 180° eq. 4.1

In order to effectively design the circuit, the SAW resonator electrical circuit
model should be implemented.

As discussed in chapter 3, the 2-port SAW resonator is modelled as a series
RLC circuit and two parallel IDT capacitances and an ideal transformer to
implement the 180° phase shift between the two IDTs as shown in figure 4-3.

Rm

Lm
Cm

1:1
Transformer

Co Co

Figure 4- 3 SAW circuit model

The SAW resonator used in the project is implemented in Chaize (2008). Table
4.1 indicates the parameters given for the resonator at frequency 228.79MHz.

Frequency 228.79 MHz

Wavelength 18 µm

Type of Device

IDT Type

Distance between Reflectors

Delay Path Length

Acoustic Aperture

Resonator

Split finger

720 µm / 80 λ

405 µm / 22.5 λ

1440 µ / 40λ

Number of finger pairs per IDT 2.5

Heater resistance 150 Ω

Table 4.1 SAW resonator parameters (Chaize, 2008)

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