AD converter

Hi,

I work with an 24bit evaluation-adc-board from AKM. The board sends the
data in 32bits to the DSP. I can choose whether I would justify the
24bits at the MSB or the LSB of the 32bit word which is send to the
DSP. Now my question: what`s the difference and what makes more sense.
The DSP can both formats.

Markus

Markus wrote:

> Hi,
> 
> I work with an 24bit evaluation-adc-board from AKM. The board sends the
> data in 32bits to the DSP. I can choose whether I would justify the
> 24bits at the MSB or the LSB of the 32bit word which is send to the
> DSP. Now my question: what`s the difference and what makes more sense.
> The DSP can both formats.
> 
> Markus
> 
The difference should be obvious and what makes sense depends on you.

Early in a design project I like having things left justified because I 
can change the ADC resolution without changing the meaning of the word. 
  There are other reasons that left-justifying everything makes sense, 
but it really boils down to personal preference.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Is your DSP a 32-bit fixed point processor?  If so, the way you justify controls 
whether you have 8 bits of "headroom" (also called guard bits) for the signal to 
grow during processing or 8 bits extra at the end to deal with round-off during 
processing.  Based on what type of processing you are doing, it might make sense 
to choose one or the other.  If you are planning on mixing a lot of signals 
together so that result may grow, the former may be a better choice.

Some type of compromise might be the best bet.  I haven't done much 32-bit 
fixed-point processing, but I would imagine you would leave a few guard bits at 
the top to avoid overflow and use the rest for additional precision at the low 
end.

If you are using a floating-point DSP or aren't doing any significant 
processing, then it's a moot point and you can choose whichever is easiest or 
you like best.  Some people light LSB-justifying so that the digital word put 
out by the ADC is exactly what ends up in the DSP (although I guess you still 
have to deal with sign-extending for negative values).

-- 
Jon Harris
SPAM blocker in place:
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"Markus" <Markus.Schweikhardt@web.de> wrote in message 
news:1125948240.082138.139480@z14g2000cwz.googlegroups.com...
> Hi,
>
> I work with an 24bit evaluation-adc-board from AKM. The board sends the
> data in 32bits to the DSP. I can choose whether I would justify the
> 24bits at the MSB or the LSB of the 32bit word which is send to the
> DSP. Now my question: what`s the difference and what makes more sense.
> The DSP can both formats.
>
> Markus
> 

"Jon Harris" <jon99_harris7@hotmail.com> wrote in
news:K83Te.20215$cy4.1003@trnddc05: 

> Is your DSP a 32-bit fixed point processor?  If so, the way you
> justify controls whether you have 8 bits of "headroom" (also called
> guard bits) for the signal to grow during processing or 8 bits extra
> at the end to deal with round-off during processing.  Based on what
> type of processing you are doing, it might make sense to choose one or
> the other.  If you are planning on mixing a lot of signals together so
> that result may grow, the former may be a better choice. 
> 
> Some type of compromise might be the best bet.  I haven't done much
> 32-bit fixed-point processing, but I would imagine you would leave a
> few guard bits at the top to avoid overflow and use the rest for
> additional precision at the low end.
> 
> If you are using a floating-point DSP or aren't doing any significant 
> processing, then it's a moot point and you can choose whichever is
> easiest or you like best.  Some people light LSB-justifying so that
> the digital word put out by the ADC is exactly what ends up in the DSP
> (although I guess you still have to deal with sign-extending for
> negative values). 
> 

Most of the time, MSB first is used. 

-- 
Al Clark
Danville Signal Processing, Inc.
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