The chips from the PICOCAP® series are well suited for capacitive-based MEMS devices. This is mainly because of the outstanding versatility the chips provide and the fact that they are programmable. The first refers to a great flexibility in regards of the capacitance range (a few pF up to hundreds of nF), the wide range of possible sensor connections and wide parameter set which provides a high level of adjustability (e.g. optimization of configuration for low-current, high update rate or high resolution). The programmability refers to the powerful internal 48-bit DSP with integrated memory space to accommodate user programs. This allows for easy and convenient product adaption to your needs, examples of how to use the DSP are given below.
The PICOCAP® chips are “intelligent IC solutions” which basically means that the ASICs or ASSPs provide high-end measurement quality combined with on-board processing capability. We distinguish the ASICs (Application Specific Integrated Circuit) from the ASSPs (Application Specific Standard Product) on the degree of customization. An ASSP is a proven component open to a wide audience, but still programmable by the user. Meanwhile the ASIC is usually tailored to one specific application and one (or several) customers exclusively. The internal standard DSP, is a programmable 48-bit DSP, which allows for convenient realization of complex tasks such as doing sensor linearization (for e.g. pressure, humidity or fill-level sensors) or mathematical calculations like for example the calculation of the dewpoint with humidity sensors. Acam provides for some applications a base firmware which is ready-to-use and can be easily adapted for the own application. For a convenient development process there is a development platform available which allows for fast evaluation and prototyping with the own sensor.
The development platform is a complete hardware/software kit for fast and convenient evaluation of the PICOCAP chips in combination with your MEMS sensor. The kit’s hardware consists of a base board (power supply and interfacing) and a plug-in module which hosts the PCap-chip. It can be connected by an included programmer device called PICOPROG to a computer via USB. On the software side there are two main tools available; the front-panel software for quick and easy direct evaluation and the assembler software which allows making modifications to the firmware. In the following some screenshots of the kit’s hard- and software (click to enlarge):
Capacitive pressure sensors in the range from 0.05 to 25 bar are widespread in industrial, automotive and consumer applications. The advantages lie in the high overload capability and the lower temperature sensitivity compared with piezoresistive pressure sensors. To increase the overall accuracy, linearization of the sensor is in the focus of many sensor manufacturers. Traditionally the linearization is done by means of an external circuitry, e.g. an operation amplifier combined with a resistor network and by using look-up tables. Today, calibration algorithms for linearization and also temperature compensation is becoming more and more common in ASICs; however often limited to given algorithms and a fixed set of calibration/temperature points. This is different with the PCap series.
By using the fully-programmable DSP application specific firmware can be developed to meet the requirements of the particular sensor. More than that, the process can be fully automated. A non-linear function, such as the output of a pressure sensor can be described by mathematical equations, which approximate the characteristic of the curve. Thereby, the description of the non-linearity under consideration gets better the higher the order of the polynomial. The plot in the next figure illustrates the “best-fit” – curves ranging from a straight line to a 3rd order polynomial.
- Pressure sensor non-linearity
- Polynomial curve fit
The bold red-curve is the assumed non-linearity of a sensor which is then first approximated by a straight-line (orange), 2nd order polynomial (green) and 3rd order polynomial (blue/gray). Obviously the approximation is the better the higher the order of the polynomial – especially if the non-linear curve bends several times and/or has a turning point. For the linearization with PCap01 a 3rd order polynomial was laid at the bottom of the DSP’s firmware. This means to have an optimal approach to do the sensor linearization. To get rid of the overlaid dependency on temperature, a 2nd order polynomial is furthermore implemented for each coefficient of the pressure linearization polynomial. The result of the pressure linearization (3rd order) with a sample sensor is displayed in the next figure along with the calculated error deviation.
- Linearized pressure sensor output
- Measurement error
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