Lpc2000

--- In lpc2000@yahoogroups.com, "brendanmurphy37" 
<brendan.murphy@i...> wrote:
>
> I presume you're talking about using some form of external DAC as 
> part of your suggestion? If not, can you explain the statement "Use 
> this to latch the input to the DAC".
> 
> A significant problem with the on-board DAC is that there is no
> latch  signal that can be applied, making some degree of jitter
> inevitable on any software-driven setup.
> 
> We had to abandon the idea of using the on-board DAC for our
> software modem for this very reason: even a few clocks of jitter at 
> 60 Mhz is enough to generate significant noise on the output.
> 
> I suspect anyone else (the original questioner on this topic?)
> trying to drive the DAC at high speeds will hit this problem before 
> they hit the maximum o/p speed problem (unless having a noisy o/p
> isn't a concern). This is unfortunate, as it means that the DAC
> can't really be used for even relatively simple signal generation
> without a noisy o/p.
> 
> A very simple addition to the DAC, namely a latch signal, would be 
> a great help here. 
> 
> Of course, a DMA-fed FIFO arrangement would be even better.....

Yep.  You can add an external "analog latch", i.e. a sample-hold 
circuit with an analog switch and a capacitor, where the analog 
switch is controlled by a PWM output.  The software then updates the 
DAC output (at an imprecise time) while the analog switch is off and 
the capacitor holds the voltage, and the PWM output later, at a 
precise time, turns the analog switch on and lets the new voltage get 
to the capacitor.

Karl Olsen