Jens Schröder

INTERFACE CIRCUITS FOR QUARTZ-CRYSTAL-MICROBALANCE SENSORS

The utilization of quartz-crystal-microbalance sensors in liquids yields new requirements to the applied interface circuits. In the present article, the fundamentals of the measuring principle and advantages and drawbacks of suitable interface circuits are discussed. Special requirements of oscillators as interface circuits are outlined. Possible solutions to those requirements are investigated and two recently developed oscillator circuits are presented.

Is an oscillator-based measurement adequate in a liquid environment?

Oscillator-based measurements with quartz crystal resonators are analyzed. The investigations have shown that classical thickness monitors as well as many chemical vapor sensors based on a quartz crystal microbalance (QCM) work properly, even with simple oscillators. It was demonstrated that, for applications in a liquid environment, more sophisticated electronics are necessary. Also a comparison between the experimental results in liquids and the theoretical predictions is hardly possible without the knowledge of the oscillator behavior. As our solution, we present an automatic gain-controlled oscillator with two output signals, the oscillator frequency, and a signal that represents the damping of the quartz resonator. A calibration method is introduced, which allows one to calculate the series resonance frequency f/sub s/ and the series resistance R/sub s/ from these oscillator signals.

Network analysis based interface electronics for quartz crystal microbalance

The application of resonant sensors such as quartz crystal microbalance (QCM) resonators requires interface electronics to measure parameters that characterize sufficiently the resonant behavior of the sensor due to effects under investigation. Common oscillators as sensor electronics have two major disadvantages. Since they measure the resonant frequency shift as the only parameter their employment is restricted to mass sensing of thin and rigid films in terms of QCM. Because of spurious phase shifts the measured resonant frequency may be erratic. In order to characterize the sensor itself or a material on the sensor surface more parameters such as damping are to be measured. Therefore, a sensor electronics was developed that precisely acquires the impedance spectrum of the resonant sensor in a small frequency range which reflects properties like thickness of a sensor coating, its density, and shear moduli or the density-viscosity product when measuring in liquids. Originally developed as a sensor interf...

Analogue and digital sensor interfaces for impedance spectroscopy

Impedance spectroscopy is a non-invasive method of sensing electrical material properties. This approach is also suited for sensors transducing non-electrical properties into an electrical impedance. As yet, impedance spectroscopy has only been applicable with laboratory instrumentation, covering a broad frequency and impedance measurement range. By restricting to measurement ranges required by specific applications it is possible to implement the method in sensor interfaces. An interface with substantially analogue circuitry was developed and applied to quartz crystal resonators for sensing density and viscosity of liquids. Additionally, a fully digital interface was realized which is mainly intended for capacitive measurements in liquids. As it samples occurring signals directly and performs a real time sine regression of the acquired probes, it is faster and more precise than the analogue sensor electronics. However, both interfaces have the same precision like laboratory devices for their specified application which is demonstrated with reference measurements.

Advanced interface electronics and methods for QCM

In order to characterise the acoustic properties of a liquid by quartz crystal microbalance (QCM) the measurement of a frequency shift and of a resistance change is suitable. Two interface electronics for QCM measuring these parameters are presented. The first one follows the oscillator concept and measures two complex quartz impedances from which the parameters can be calculated. The second interface is network analysis based and acquires an impedance spectrum in a small frequency range. The parameters can be directly found in the spectrum. In contrast with commercial devices this interface is realised on single board and thus suited for industrial application.

Universal impedance spectrum analyzer for sensor applications

A compact impedance spectrum analyzer has been developed that is well suited for the use in process control. Various sensor elements connected to the sensor electronics allow an adaptation to different measuring applications. Digital signal processing minimizes the insertion of noise and distorting effects to the system. Facilities are provided to calibrate the sensor with a reference.

Recent trends in bulk acoustic wave resonator sensors

Devices based on piezoelectric materials, which allow transduction between electrical and acoustic energies, have been developed in a number of configurations for sensor applications and materials characterization. Bulk acoustic wave resonators like QCM and ultrasonic sensors can be classified in the category of acoustic wave resonators sensors. The latter will not be considered here. In the following the state-of-the-art of bulk acoustic wave resonator sensors will be discussed. Models for the description of the transduction mechanisms of acoustic-wave based chemical or biochemical sensors are introduced and compared to derive the application chance and limitations of these sensors. The importance of suitable sensor interface electronics for getting exact sensor data in damping fluid systems will be shown. In this context the signal-to-noise-ratio (SNR) plays a dominant role. Selected application examples are given. Aspects of future trends are discussed.

Sophisticated Interface Electronics for QCM

In order to characterise the acoustic properties of a liquid by QCM (quartz crystal microbalance) the measurement of a frequency shift and of a resistance change is suitable. Two interface electronics for QCM measuring these parameters are presented. The first one follows the oscillator concept and measures two complex quartz impedances from which the parameters can be calculated. The second interface is network analysis based and acquires an impedance spectrum in a small frequency range. The parameters can be directly found in the spectrum. In contrast with commercial devices this interface is realised on single board and thus suited for industrial application.