tindle@ms.uky.edu (Ken Tindle) (01/11/90)
I have a problem that has me thrown for a loop- I've got a project running digital and audio side-by-side, which I've always known was trouble; it's fun keeping the audio clean. So I have three power supplies- +12 for audio; +12 for 60 multiplexed LED's; and +5 for microcontroller power. My problem is a whine in the audio, caused by the LED's turning on and off. All these supplies are separate from each other; I did this to (ha) keep the audio quiet. The digital and LED power is coming from a new, surplus TI 99-4A board. Is this thing known to be unstable, when drawing current pulses out of it's +12 output? It should be providing me with some isolation, but using the audio +12 for everything actually works slightly better! The audio power supply (a Lambda LSS series 25W) and the TI supply have a common ground- am I gonna have to separate them? --------------------------\ /----------------------------------------------- INTERNET:tindle@ms.uky.edu | "Could you please continue the petty bickering? BITNET: tindle@ukma.bitnet | I find it most intriguing." --- Data, Ken Tindle - Lexington, KY | Star Trek, The Next Generation, "Haven" --------------------------/ \-----------------------------------------------
torkil@psivax.UUCP (Torkil Hammer) (01/12/90)
In article <13647@s.ms.uky.edu> tindle@ms.uky.edu (Ken Tindle) writes:
#I have a problem that has me thrown for a loop-
#
#I've got a project running digital and audio side-by-side, which I've always
#known was trouble; it's fun keeping the audio clean.
Make sure that,
1. Each audio low signal amplifier references ground as just one _point_
in the circuit and feed that point from the output amplifier ground.
This feed wire will carry essentially no current.
2. Digital currents are not flowing in any audio ground wire.
Which means connect the digital and audio ground at just one point,
which is also the point where you join all the power supply grounds.
3. Audio amplifier loops have physically zero area.
You do so by keeping the input feedback loop small and in a figure of eight,
since areas can be positive and negative. This cancels magnetic induction.
4. Audio amplifiers are shielded.
This cancels electric induction.
-------------------------------------------------
Wires (such as ground) have ohmic resistance, and current in them cause
voltage differentials to build up. They also have inductance, which
makes microsecond spikes worse than slow variations.
You can share a power supply (e.g. +12 volt) between digital and analog
sections if you insulate the analog side by a series resistor followed by
a parallel capacitor, and do so at the power supply, not the audio board.
This removes spikes on the supply. Analog IC's usually are insensitive
to slow supply variations; microsecond spikes are another matter.
Torkil Hammer