First of all, TANK YOU ALL for all the help you bestowed upon me (so 
generously :)
in the selection of my tolls. For now I will stay with the TI Kick 
start. Should
this not be enough later I will probably switch to the GNU version.
This will give me enough time to familiarize with the processor so I will
have to fight only the new tools learning curve :) You know, TI should 
make the effort
to provide decent free tools if they want to sell their silicon :) But 
this is my opinion only :)
We are all suckers, isn't it :) - as one marketer put it to me once :)

So, THANK you all again.
Now, the ADC question.

It seems that the input impedance/resistance of the ADC is around 2KOHm
which is strange to me to see this low value in such a power aware part.
However, my signal source is 100K minimum output impedance so I need
to adapt somehow the two ends. I do not want to add an other active
device (OpAmp) as this will require some power to run so I think of an
100KOhm resistor on the input pin.
It seems though that the ADC conversion time, and thus the S/H timing
is varying with the source impedance, but this is not quite spelled out
in the documentation, so here is my question:
What is the collective experience with high source impedance sources as 
far the 430
ADC input is concerned? Will the ADC bark at me because of the high 
resistor value?

Thanking you In Advance,
Alex   S.
Sand Labs Research
http://www.sandlabs.com

On page 17-9  User's for x449  they say :
Tsample >(Rs+2k)x9.011x40pF, whre Rs is 100K in your case.
Karl.

>From: alex <alex@alex...>
>Reply-To: msp430@msp4...
>To: msp430@msp4...
>Subject: [msp430] ADC Input impedance/resistance
>Date: Fri, 15 Apr 2005 10:32:33 -0500
>
>
>First of all, TANK YOU ALL for all the help you bestowed upon me (so
>generously :)
>in the selection of my tolls. For now I will stay with the TI Kick
>start. Should
>this not be enough later I will probably switch to the GNU version.
>This will give me enough time to familiarize with the processor so I will
>have to fight only the new tools learning curve :) You know, TI should
>make the effort
>to provide decent free tools if they want to sell their silicon :) But
>this is my opinion only :)
>We are all suckers, isn't it :) - as one marketer put it to me once :)
>
>So, THANK you all again.
>Now, the ADC question.
>
>It seems that the input impedance/resistance of the ADC is around 2KOHm
>which is strange to me to see this low value in such a power aware part.
>However, my signal source is 100K minimum output impedance so I need
>to adapt somehow the two ends. I do not want to add an other active
>device (OpAmp) as this will require some power to run so I think of an
>100KOhm resistor on the input pin.
>It seems though that the ADC conversion time, and thus the S/H timing
>is varying with the source impedance, but this is not quite spelled out
>in the documentation, so here is my question:
>What is the collective experience with high source impedance sources as
>far the 430
>ADC input is concerned? Will the ADC bark at me because of the high
>resistor value?
>
>Thanking you In Advance,
>Alex   S.
>Sand Labs Research
>http://www.sandlabs.com
>
>
>
>
>.
>
>
>Yahoo! Groups Links
>
>
>
>
>
>
>

_________________________________________________________________
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http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/

Hi Alex,

> Thanks Karl, I have seen this. Perhaps I was not
clear enough :)
> I was wandering if any one has _actually_ used the ADC with this high
> input impedance and what it was the outcome.
> Given the 2K internal input resistor, a 100K might work like a
> good attenuator :) Did you personally had the ADC running on a such High
> input impedance ? From paper specs to real life there is a loooong way :)

Your 100 k resistor makes no attenuation.
The 2 k ADC resistance is not the input impedance.
The 2 k resistor is in series to the internal capacitor.
You have to wait that the internal capacitor can charge to the input voltage
(sampling time).
With your 100 k resistor the necessary sampling time must be greater about 50
us.

Regards, Florian

> 
> Sicerely,
> Alex
> 
> 
> 
> 
> Karl Adler wrote:
> 
> 
>>On page 17-9  User's for x449  they say :
>>Tsample >(Rs+2k)x9.011x40pF, whre Rs is 100K in your case.
>>Karl.
>>
>> 
>>
> 
> 
> 
> 
> .
> 
>  
> Yahoo! Groups Links
> 
> 
> 
>  
> 
> 
> 
> 

Thanks Karl, I have seen this. Perhaps I was not clear enough :)
I was wandering if any one has _actually_ used the ADC with this high
input impedance and what it was the outcome.
Given the 2K internal input resistor, a 100K might work like a
good attenuator :) Did you personally had the ADC running on a such High
input impedance ? From paper specs to real life there is a loooong way :)

Sicerely,
Alex




Karl Adler wrote:

>On page 17-9  User's for x449  they say :
>Tsample >(Rs+2k)x9.011x40pF, whre Rs is 100K in your case.
>Karl.
>
>  
>

Yes, I am using LTC1787  high side current sense ,and with no buffer looks 
like a current generator, going straight in channel 5.

>From: Florian Finsterbusch <news.flo@news...>
>Reply-To: msp430@msp4...
>To: msp430@msp4...
>Subject: [msp430] Re: ADC Input impedance/resistance
>Date: Fri, 15 Apr 2005 18:56:59 +0200
>
>
>Hi Alex,
>
> > Thanks Karl, I have seen this. Perhaps I was not clear enough :)
> > I was wandering if any one has _actually_ used the ADC with this high
> > input impedance and what it was the outcome.
> > Given the 2K internal input resistor, a 100K might work like a
> > good attenuator :) Did you personally had the ADC running on a such
High
> > input impedance ? From paper specs to real life there is a loooong way

>:)
>
>Your 100 k resistor makes no attenuation.
>The 2 k ADC resistance is not the input impedance.
>The 2 k resistor is in series to the internal capacitor.
>You have to wait that the internal capacitor can charge to the input 
>voltage (sampling time).
>With your 100 k resistor the necessary sampling time must be greater about 
>50 us.
>
>Regards, Florian
>
> >
> > Sicerely,
> > Alex
> >
> >
> >
> >
> > Karl Adler wrote:
> >
> >
> >>On page 17-9  User's for x449  they say :
> >>Tsample >(Rs+2k)x9.011x40pF, whre Rs is 100K in your case.
> >>Karl.
> >>
> >>
> >>
> >
> >
> >
> >
> > .
> >
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
> >
> >
> >
> >
>
>
>
>.
>
>
>Yahoo! Groups Links
>
>
>
>
>
>
>

_________________________________________________________________
Dont just search. Find. Check out the new MSN Search! 
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OK Florian :)
I see the combination of S/H and ADC as a single unit namely the ADC :) 
Because the actual ADC is hidden behind the S/H
the governing input impedace becomes the S/H's impedance :)
My mistake was perhaps that I was not considering the C in series with 
the resistances at the end of the chain
but at the junction of the 100K and the 2K :)
It pays to read attentively the papers :)

Thank you Florian,
Sincerely,
Alex

PS... I run the example given in the user's guide with the 10K resistor.
I guess had to much coffee to drink as I could not come up with the same 
answer of 4.33uS/Sample.
A




Florian Finsterbusch wrote:

>Hi Alex,
>
>  
>
>>Thanks Karl, I have seen this. Perhaps I was not clear enough :)
>>I was wandering if any one has _actually_ used the ADC with this high
>>input impedance and what it was the outcome.
>>Given the 2K internal input resistor, a 100K might work like a
>>good attenuator :) Did you personally had the ADC running on a such High
>>input impedance ? From paper specs to real life there is a loooong way
:)
>>    
>>
>
>Your 100 k resistor makes no attenuation.
>The 2 k ADC resistance is not the input impedance.
>The 2 k resistor is in series to the internal capacitor.
>You have to wait that the internal capacitor can charge to the input voltage
(sampling time).
>With your 100 k resistor the necessary sampling time must be greater about
50 us.
>
>Regards, Florian
>
>  
>
>>Sicerely,
>>Alex
>>
>>
>>
>>
>>Karl Adler wrote:
>>
>>
>>    
>>
>>>On page 17-9  User's for x449  they say :
>>>Tsample >(Rs+2k)x9.011x40pF, whre Rs is 100K in your case.
>>>Karl.
>>>
>>>
>>>
>>>      
>>>
>>
>>
>>.
>>
>> 
>>Yahoo! Groups Links
>>
>>
>>
>> 
>>
>>
>>
>>
>>    
>>
>
>
>
>.
>
> 
>Yahoo! Groups Links
>
>
>
> 
>
>
>
>
>
>
>  
>