[sdiy] D/A accuracy - voltage is floating?

[Bret Truchan]

Hello friendly DIY people!

This is probably an easy question.  I've built what's *basically* a small 
sequencer using a microcontroller and a serial D/A converter.  In short, you 
clock the microcontroller which sends values to the serial D/A converter.  I 
take the output of the circuit, plug it into a quantizer, then out to 
control the pitch of my VCO.  (aka - it generates a little tune)

The output of my D/A converter seems to be, oh, not-so-accurate.  The same 
little tune plays over and over again, but sometimes one or two notes in the 
tune get quantized differently.  I believe that the output of the D/A 
converter is floating a little bit.

I have two questions.

#1.  Is it possible that the output of my D/A converter is floating a bit?  
It's a TLC5620IN.  I don't see any noise on my power lines.  I've got 
filtering capacitors all over the place.

#2. If I skip the quantizer and go straight into the VCO, my ear is having a 
tough time deciding wether or not I can _hear_ the fluctuations in pitch.  
Is there some fancy technique that I can use to record the voltages coming 
out of my circuit so I can tell if they're really fluctuating?  I have an 
oscilloscope, but that doesn't help me much.

Thanks a ton in advance!
- Bret

Oh, here's some extra information:

1. The external clock signal is going through a voltage clamp made up of a 
zener diode and two resistors.  If I pay close attention to my power lines 
(magified a LOT), I can see teensie noise introduced by the incoming clock 
signal.  I don't know how to remove the noise.

2. I'm using an Atmel Mega8 microcontroller.

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