> First, as Mike wrote, try it and measure the results. Second, proper
Thank you all for the suggestions. I'll try this with some slowly
varying waveforms as suggested.
> dithering before the converter can help. A requirement for dithering to
> yield the theoretical improvement --sqrt(n), where n is the oversampling
> ratio -- is that the precision and stability of the decision levels of
> your 12-bit converter meet the requirements of a 14-bit converter.
I don't think I'll have the resources available to do dithering before
conversion, unfortunately.
> do, some don't. Some improvement -- in resolution, if not precision --
> should be possible, and more samples help. Can you oversample by 32?
Probably not. I'm sampling over multiple channels and I believe that
an OSR of 32 wouldn't give me enough time to do the processing I need
to do, though I'll take a closer look at it.
> Will you settle for a bit and a half of improvement?
Yes. In fact, just 1 bit would more than suffice, but I was trying to
be conservative.
Once again, to everyone, thanks for all of the input.
Reply by Jerry Avins●October 18, 20032003-10-18
Myron Mychal wrote:
> I'm working with the new 56F630x series of Motorola DSP's (with an
> onboard 12-bit ADC). My application could make good use of 14-bit
> data, so I planned on oversampling by a factor of 16 and filtering
> with decimation to get an extra 2 bits out of the ADC.
>
> I recently spoke with someone from Motorola who suggested that this
> might not work with their ADC because it doesn't ensure equal
> probability within the encoded voltage "bins" of the ADC. Is this
> true? If so, does this mean that I cannot use the
> oversampling/filtering technique to get another 2 bits out?
>
> Myron Mychal
> DSP Consultant
First, as Mike wrote, try it and measure the results. Second, proper
dithering before the converter can help. A requirement for dithering to
yield the theoretical improvement --sqrt(n), where n is the oversampling
ratio -- is that the precision and stability of the decision levels of
your 12-bit converter meet the requirements of a 14-bit converter. Some
do, some don't. Some improvement -- in resolution, if not precision --
should be possible, and more samples help. Can you oversample by 32?
Will you settle for a bit and a half of improvement?
Jerry
--
Engineering is the art of making what you want from things you can get.
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Reply by Fred Marshall●October 18, 20032003-10-18
Mike Rosing <rosing@neurophys.wisc.edu> wrote in message news:<3F8B359B.5060406@neurophys.wisc.edu>...
> Myron Mychal wrote:
> > I'm working with the new 56F630x series of Motorola DSP's (with an
> > onboard 12-bit ADC). My application could make good use of 14-bit
> > data, so I planned on oversampling by a factor of 16 and filtering
> > with decimation to get an extra 2 bits out of the ADC.
> >
> > I recently spoke with someone from Motorola who suggested that this
> > might not work with their ADC because it doesn't ensure equal
> > probability within the encoded voltage "bins" of the ADC. Is this
> > true? If so, does this mean that I cannot use the
> > oversampling/filtering technique to get another 2 bits out?
>
> I don't know, but I'd think an empiracle test would tell you
> more that a data sheet. Put in a really slow ramp or triangle
> and run the over sampling routine. It should be pretty easy
> to compare the stability of the real signal to the measurement.
>
> By adding some noise to the ramp you can see how bad the averaging
> actually is. If nothing else you can characterize the problem
> and then take it into account with software.
>
Myron and Mike,
Mike's idea about an empirical evaluation is a good one. At least it
will tell you what one unit will do and if you test a few of them you
may have a good idea. Don't get caught in the trap that the test
cases don't represent a production run variability. You may never be
able to reproduce or not able to reproduce reliably..... A little
digging with the supplier might go a long way in matching your
application with the underlying characteristics of their product.
Even so, be cautious when using a chip in an application it's not
intended for.
About software:
Once I worked on a system that had been mostly analog with some
digital logic for process management. The system was tightly
integrated with sensors into the real physical world which provided
many of the operational situations / parameters, etc. Microprocessors
had been around for some time by then and the engineers decided it was
time to upgrade to a "much better" product. They assured me it would
be much better because it would "have a computer inside". They were
never able to express just what the computer was going to do!! Can
anyone imagine that I was skeptical about this proposal?
So, gee, I have to be similarly skeptical that software will fix "the
problem" Mike! Probably that's not what you meant, eh?
Fred
Reply by Myron Mychal●October 17, 20032003-10-17
I'm working with the new 56F630x series of Motorola DSP's (with an
onboard 12-bit ADC). My application could make good use of 14-bit
data, so I planned on oversampling by a factor of 16 and filtering
with decimation to get an extra 2 bits out of the ADC.
I recently spoke with someone from Motorola who suggested that this
might not work with their ADC because it doesn't ensure equal
probability within the encoded voltage "bins" of the ADC. Is this
true? If so, does this mean that I cannot use the
oversampling/filtering technique to get another 2 bits out?
Myron Mychal
DSP Consultant
Reply by Myron Mychal●October 17, 20032003-10-17
I'm working with the new 56F630x series of Motorola DSP's (with an
onboard 12-bit ADC). My application could make good use of 14-bit
data, so I planned on oversampling by a factor of 16 and filtering
with decimation to get an extra 2 bits out of the ADC.
I recently spoke with someone from Motorola who suggested that this
might not work with their ADC because it doesn't ensure equal
probability within the encoded voltage "bins" of the ADC. Is this
true? If so, does this mean that I cannot use the
oversampling/filtering technique to get another 2 bits out?
Myron Mychal
DSP Consultant