TAP DANCE

Which brings us to the final piece of the QF4A512 puzzle: the integrated digital filters that differentiate it from a regular ADC chip. To make a long story short, these filters rock! And there’s no better way to demonstrate that than to fire up the chip and give it a whirl, taking advantage of Quickfilter’s $199 evaluation board (see Photo 1).

The Quickfilter software shown in Photo 2 makes designing a filter as simple as entering the specifications I discussed earlier (passband, stopband, attenuation, ripple, and so on). Two filter types are supported: Parks-McClellan and Sinc. The former is notable for a sharp transition band at the expense of some passband ripple. The latter is just the opposite (slower transition, but flatter passband). Have it your way.

Filter gurus may debate the relative virtues of particular filter algorithms, but no one can argue that the number of taps a digital filter has makes all the difference. Each tap corresponds to the familiar multiply-and-accumulate (MAC) operation at the core of DSP inner loops. If taps are the currency of filters, the QF4A512 is a rich chip indeed because each filter can have up to 512 taps.

To put that in perspective, let’s do a little back-of-envelope calculation. Start by assuming 2-Msps throughput because it’s an easy number to work with that splits the difference between maximum throughput in Single Channel mode (2.5 Msps) and Multi-Channel mode (1.6 Msps). At 2 Msps, a sample arrives every 500 ns, and every tap has to be run for each sample. That means for a 500-tap filter, you have to do a MAC operation every nanosecond! That’s 1 billion MACs per second, which is high-end DSP/FPGA chip territory indeed. Better yet, having 512 taps at your disposal means you can get filters that come remarkably close to the brick wall ideal and certainly way beyond anything you’d ever hope to get by stringing op-amps together.

You can prove it yourself by plugging the board into your PC headphone jack, feeding it the supplied white noise WAV file, and monitoring the results with the software’s real-time FFT display. Check out the QF4A512 strutting its stuff in Photo 3. It’s all the more impressive because I was using preproduction silicon that purportedly wasn’t quite up to spec.