Re: Analog Input Board Recommendations

On Wed, 2 Oct 2002, David Schleef wrote:

> On Wed, Oct 02, 2002 at 02:12:00PM -0400, Calin A. Culianu wrote:
> >
> > Hi Tim,
> >
> > I was looking at the Measurement Computing specs for the PCI-DAS 6402
> > board.  They say their board has an input settling time (to 1 LSB) of 6
> > us, and an A/D conversion time of 5us.  Now let's say I'm switching
> > channels, do I have to add those two numbers up, or do I take the worst
> > case of 1 of those numbers to arrive at my settling time?  What I need to
> > figure out is if it takes 11us or 6us to switch channels on that board?
>
> The actual time is probably somewhere between.
>
> Even though the A/D converter is "busy" for 5 us converting analog
> to digital, it is only sensitive to the analog signal for a short
> period.  The A/D converter on that board (Analog Devices AD976A),
> for example, is done with the analog signal about 40 ns after the
> convert pulse.  So assuming that the board is designed to switch
> the mux to the next channel immediately after this time, you can
> acquire at a 6.040 us rate and be within the 1 LSB mux settling time.
> I would expect this from a well-designed board.
>


Dave,

I understand what you are saying... but I am still a bit confused.  Then
what do they mean by 5us A/D conversion time?  Is that time used by other
parts of the board to populate voltage values into registers and whatnot?
So the real bottleneck is not the A/D converter itself, but talking to the
board's hardware, etc?

BTW, thanks for taking the time to explain this to me... :)


Also, I remember a while back we were talking about some sort of mux
settling time infrastructure within comedi (so that comedilib and
kcomedilib application programmers have a way of determining how long to
wait for their muxes -- without having to maintain board-specific
information in their application).  I forget where we left off on that...
but the little bit of research I have been doing in comparing boards has
me motivated now to compile a mini-database of 1LSB MUX settling time
numbers...  Is that still a good idea for comedi, or are we going to leave
that up to the application programmers to sort out for themselves? :)

Cheers,

-Calin

Received on 2002-10-02Z20:02:13