Most likely, you could design a small data-acquisition (DAQ) system to embed in a product or a test system. But specifying filters, choosing analog-to-digital converters and determining how to communicate with a microprocessor may divert you from your main task. Recently, I discovered a chip that deserves attention because it provides functions most DAQ applications need. As shown in the diagram below, the Quickfilter QF4A512 IC offers four analog-input channels, anti-alias filters, and an analog-to-digital converter. This device — in a 9-mm-square 32-pin package — takes less space on a circuit board than equivalent circuits and an analog-to-digital converter (ADC).
In addition, the device includes a 512-tap finite-impulse-response (FIR) filter for each channel. This digital signal-processing capability, usually handled by an FIR-filter algorithm within a digital signal processor (DSP) chip, makes the Quickfilter chip worth investigating. FIR filters process sampled data in the digital domain, so engineers specify filter characteristics and calculate filter coefficients instead of choosing passive components for active filters. (I'll cover FIR-filter operation and programming in a future column.)
Alex Kalasinsky, director of corporate development at Oceana Sensor Technologies, explained his engineers uses the Quickfilter chip because it lets them — and customers — quickly configure a DAQ system for specific sensors. The chip can connect directly with sensors and it provides 12 to 16 bits of digital data to a processor through its SPI port. Oceana develops accelerometers engineers use to monitor seismic activity, machinery vibration, and structural characteristics.
Because sophisticated digital filtering can occur within the Quickfilter chip, a DAQ system may not need a DSP chip to perform additional processor-intensive steps. Thus, a system could use a simple microcontroller to manage the Quickfilter chip, to receive and format its data, or to make decisions based on sensor information.
“The direct digital connection of the filter with the ADC front end and the ability to quickly develop a prototype of a digital filter make the Quickfilter chip stand out,” noted Mike Edick, senior electrical design engineer at Wilcoxon Research, a manufacturer and supplier of accelerometers. “But, if you don't need the chip's FIR-filter capability, you can still use the chip as an ADC with anti-alias and programmable-gain filters.”
To help engineers get a fast start, Quickfilter offers a QF4A512-DK Development Kit ($199) that lets engineers exercise the chip's capabilities. Accompanying software includes filter-design and chip-configuration wizards so users can test filters with white noise produced by their host PC's sound-card. Users can supply other test signals, too. The PC displays an FFT plot of the filtered digital data. A programming adapter that mounts on the dev-kit board provides a socket for individual QF4A512 chips, so you can program them and put them into your own circuits.
One designer I spoke with said engineers can use the Quickfilter development kit to verify the chip meets specifications and to try it under lab conditions. But when they start a design, a programmed chip should go into a realistic prototype circuit. That way, the chip runs under actual operating conditions and engineers can adjust parameters as they go along.