Building a DSP board, Part Five: Power supply considerations
This is the fifth in a series on how I went about building a dual Motorola DSP56000 sampling board.
There is a lot of hype these days about using separate power supplies in CD players. I tend to believe this theory. Digital signals are very demanding of power regulators. I have a feeling that most power regulators can't react fast enough to prevent a transient current drain from creating a voltage drop in the regulator. This will appear as a little AC signal floating on top of the power supply voltage. If this gets into your analog power supply, it will modulate itself with whatever analog signal you are processing, causing some distortion.
In my power supply design, I needed a +/- 15V supply (I wanted to use 12V, but my S/H required 15V) and a +/- 5V supply for the analog section (I consider the 5V needed for my A/D digital part as part of the analog section) which consists of the analog filters (+/- 15V), the S/H (+/- 15V), the A/D (+/- 15V and +5V), and the D/A (+/- 5V). I needed a separate +5V for the digital section, consisting of the DSP chip, the SRAM, the EPROM, glue logic, crystal and dividers, and the SM5805. I didn't have to worry about nasty digital hash sneaking into my analog supply since all of the clocking information for the serial A/D is provided by the SM5805, which runs off of a separate +5V supply.
I added up all the power requirements of each chip, and rounded up generously.
The final design consists of 12.6V five amp center tap (of which I use one side of the tap for 6.3V) and a four amp 28V center tap (I use both sides for a bipolar supply). I rectify the voltage off both transformers, smooth it out to 8.9V and +/- 19.8V via 22,000 and 4700 uF caps, and put a 3 amp +5V regulator on the former and the +15V, +5V, -5V, and -15V on appropriate positions on the later. The digital supply got a 3A fuse and the analog supplies got 1A fuses. (Make sure you fuse them... I've blown more than one!) Then, I hooked the center tap of the analog supply to the ground digital supply for a common ground. Oh yeah, don't forget to bypass the regulators with 10 uF tantalum caps to prevent oscillations!
Of course, if you don't like building ugly beasts and want a nice protected power supply in a nice case, it might be worth the money (and it is a lot of money) to go buy a 3A/24V bipolar regulated power supply.
Next: How the SM5805 will drive the board
