Signal evaluation circuits

Detailed Design and Optimization of Sensing Element and Signal-Evaluation Circuit... 

The methods described in this section are used particularly during design of the sensing element (Fig. 4.1.4) and also to some extent in the system-design phase. (Similar approaches are applicable to the design of the signal-evaluation circuit, but we will focus here on the micromechanical aspects.)... 

When designing sensors, an initial decision has to be made between the discrete approach, with separated sensing element and electronic signal evaluation circuit,... 

This can further be enhanced by the so-called lock-in technique, in which modulated signals are used, which after demodulation separate the desired signal from the noise. Both techniques are described here by examples one is a monolithic pressure sensor with moderate piezoresistive bridge signals and the other is a signal evaluation circuit for a high-pressure sensor with very small sensor signal levels. 

The Bosch microsilicon air-flow meter is available with analog (HFM5) or digital (HFM6) signal evaluation circuits. 

Most capacitive evaluation circuits do not achieve the maximum possible resolution but are limited by the electromechanical interface, shortcomings in the electronic circuits, or stray signals coupling into the detector and corrupting the output. Section 6.1.2 below illustrates approaches to maximize the sensitivity of capacitive sensor interfaces, potential error sources, and approaches to minimize them. Electronic circuit options are discussed in Section 6.1.3. 

Before describing the signal evaluation principles, the general observed imperfections of the piezoresistive bridge, which must be taken care of in the evaluation circuit, will be discussed. 

This example shows an analog solution with a V/I converter. Another possibility is an instrumental amplifier instead of the V/I converter. The analog evaluation circuit is economical relating to wafer-area, but there are a lot of design features necessary for a high accurate signal evaluation. All circuit blocks must be designed to be insensitive to temperature and mechanical stress. 

This parameter specifies the potential upon which the AC signal is imposed. Oftentimes, to maintain the steady state of the system, the DC potential is selected as 0 mV vs. open circuit. However, the response of a system can be evaluated over a potential range by running successive experiments with different DC potentials. Thus the impedance response of a system could be mapped to a potentio-dynamic polarization curve by specifying various DC potentials defined by the polarization curve. In all experiments performed in this laboratory, the DC potential was set to 0 mV versus open circuit. 

The flow of electrical current in a circuit with an electrolyte causes ionic transport between the electrodes, which affects the electrode reaction. The application of a small perturbation in a potentiometric sensor has been shown to improve the sensor performance in what is referred to as the current reversal method [24—26]. An analysis of the response to such a perturbation can also be used as a diagnostic tool to evaluate sensor performance during operation [27, 28]. Similarly, the application of a bias voltage to a potentiometric sensor can be used to affect the performance of the sensor, such as the relative responses to NO and NO2 gases [29—31]. The response to voltage or current perturbations can also be used as the sensor signal. 

---