MIDEM 2013 CONFERENCE
ABSTRACT SUBMISSION for Invited paper
SIGNAL PROCESSING IMPLEMENTATION FOR
INTEGRATED, HIGH PERFORMANCE, LOW NOISE,
CHEMICAL/BIOLOGICAL SENSOR INTERFACE
Drago Strle
University of Ljubljana, EE Department, Tržaška 25, Ljubljna
e-mail: drago.strle@fe.uni-lj.si
ABSTRACT:
Detecting vapour traces of different materials with low vapour pressure is a
potentially powerful method to reveal the presence of target molecules in the
air with applications in security, medicine, environmental control. The
detection method is based on the fact that most materials constantly emit
rather small, but detectable number of different molecules that constitute the
material. In this work, we describe a design procedure of the sensor
measurement system and signal processing implementation for integrated,
high performance, low-noise sensor interface capable of measuring extremely
small capacitive changes (smaller than 1aF) caused by adsorption of target
molecules to the functionalized surfaces of the sensor. To provide appropriate
sensitivity, the extremely low noise integrated analogue and mixed-signal
processing electronics is needed together with area and power efficient digital
signal processing hardware.
Common limitations of existing detection systems are rather large size, high
power consumption and unreliable detection with false alarms, insufficient
sensitivity, poor chemical selectivity, and hyper-sensitivity to environmental
influences like temperature, acceleration, etc. In addition, such systems are
extremely expensive. We wanted to build a sensor measurement system,
which can be very cheap for high volume production, has a very small
physical dimension, is powered by a small battery and in addition, is
extremely sensitive and selective.
The central part of the detection measurement system is an array of surfacemodified differential capacitive sensors together with integrated electronics.
Each differential sensor is composed of two unreleased MEMS COMB
capacitors covered with a thin layer of SiO2; one is chemically modified [1].
Because both capacitors in a pair are very close to each other, an equal
number of target molecules are on the average present in the space between
the plates. The target molecules adsorb preferentially for a short time to the
surface of the modified capacitor changing its relative dielectric constant and
the distance between the plates [2]. Using appropriate excitation signals,
extremely low noise input transducer electronics [3] and appropriate lownoise digital signal processing blocks, one can efficiently measure the
difference of the two capacitances in each pair; therefore, a detection of a
small number of adsorbed target molecules in the air is possible. All elements
described above are integrated and packed together with differently modified
sensors as a SiP (System in Package), which is capable to detect less than
3ppt of TNT in the atmosphere (3 TNT molecules in 10 +12 molecules of air) at
25oC in 1Hz bandwidth using only a few mm3 and approximately 10mA
current at 3.3V supply voltage [4].
The focus of the contribution is the description of the design steps for the
whole measurement system including: system definition and high level
simulations, circuit design issues of low noise analogue input amplifiers and
other low-noise components like: analogue filters, mixers, mixed-signal
modules for A/D and D/A conversion, and design steps to achieve low-noise
digital signal processing electronics using 0.25um BCD technology. A
demanding characteristic of the system is possible to achieve in a short
design time frame using appropriate design flow, which is also shortly
described. At the end, an example sensor measurement system for vapour
trace detection of explosives is presented together with some exciting
measured results [4].
REFERENCES
[1] J.Chen, P. Xu and X.Li, “Self-assembling siloxane bilayer directly on
SiO2 surface of micro-cantilevers for long-term highly repeatable sensing to
trace explosives,” IOP Science Nanotechnology, vol. 21, Jun. 2010.
[2] Z. Peiji, W. Dwight, “Electrostatic characteristics of tether atoms in
connecting organic molecules to the surface of silicon,” Applied Physical
Letters, vol. 91, No. 6, Aug. 2007.
[3] D. Šiprak, M. Tiebout, N. Yanolla, P. Baumgartner, C. Fienga, “Noise
reduction in CMOS circuits through switched gate and forward substrate
bias,” IEEE JSSC, vol. 44, No. 7, July 2009.
[2] STRLE, Drago, ŠTEFANE, Bogdan, NAHTIGAL, Uroš, ZUPANIČ, Erik,
POŽGAN, Franc, KVASIĆ, Ivan, MAČEK, Marijan, TRONTELJ, Janez,
MUŠEVIČ, Igor. Surface-functionalized COMB capacitive sensors and
CMOS electronics for vapor trace detection of explosives. IEEE sens. j..
[Print ed.], May 2012, vol. 12, no. 5, str. 1048-1057.