Lpc2000

Hello,

I'm trying to read an analogue signal using the internal ADC, but it seems to have problems readding small signals. My analogue signal goes from 0 to 3V, in increments of about 1mV, but when reading the signal I get a range of readings, and it doesnt seem to settle. I'm reading every 100msecs this signal.

p.e. when the signal is locked at, lets say, 10mV I get an oscillatory reading that ranges from 6 to 10mv.

Has anyone dealt with this problem? any clever solution?

thanks in advance!

MAURO

At 01:47 PM 9/16/04 +0100, you wrote:
>I'm trying to read an analogue signal using the internal ADC, but it seems 
>to have problems readding small signals. My analogue signal goes from 0 to 
>3V, in increments of about 1mV, but when reading the signal I get a range 
>of readings, and it doesnt seem to settle. I'm reading every 100msecs this 
>signal.
>
>p.e. when the signal is locked at, lets say, 10mV I get an oscillatory 
>reading that ranges from 6 to 10mv.
>
>Has anyone dealt with this problem? any clever solution?

Umm, isn't that on the order of +/1 count?  That basic nature of discrete 
conversion will give you that much uncertainty.

If you need to resolve to better than 1 part in 1000 you will need a higher 
resolution (and accuracy) convertor.  You could get a greater dynamic range 
with a higher resolution convertor, a PGA (programmable gain amplifier) or 
a non-linear amplifier.

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,
be they legal, genetic, or physical.  If you don't believe me, try to
chew a radio signal. "

                         Kelvin Throop, III

ok, that makes sense, but using this ADC how can I get the most of it?

-----Original Message-----
From: Robert Adsett [mailto:subscriptions@aeolusdevelopment.com]
Sent: 16 September 2004 13:57
To: lpc2000@yahoogroups.com
Subject: Re: [lpc2000] internal AD converter


At 01:47 PM 9/16/04 +0100, you wrote:
>I'm trying to read an analogue signal using the internal ADC, but it seems 
>to have problems readding small signals. My analogue signal goes from 0 to 
>3V, in increments of about 1mV, but when reading the signal I get a range 
>of readings, and it doesnt seem to settle. I'm reading every 100msecs this 
>signal.
>
>p.e. when the signal is locked at, lets say, 10mV I get an oscillatory 
>reading that ranges from 6 to 10mv.
>
>Has anyone dealt with this problem? any clever solution?

Umm, isn't that on the order of +/1 count?  That basic nature of discrete 
conversion will give you that much uncertainty.

If you need to resolve to better than 1 part in 1000 you will need a higher 
resolution (and accuracy) convertor.  You could get a greater dynamic range 
with a higher resolution convertor, a PGA (programmable gain amplifier) or 
a non-linear amplifier.

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,
be they legal, genetic, or physical.  If you don't believe me, try to
chew a radio signal. "

                         Kelvin Throop, III




 
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At 01:59 PM 9/16/04 +0100, you wrote:
>ok, that makes sense, but using this ADC how can I get the most of it?

That depends.  What is your input (frequency, bandwidth, noise level, 
impedance...)?  What are you trying to do with it?

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,
be they legal, genetic, or physical.  If you don't believe me, try to
chew a radio signal. "

                         Kelvin Throop, III

Hi,

assuming that the signal you are scanning is relatively slow, you can
do something like adding up 4, 8 or 16 ADC-results and then shift the
sum by 3, 4 or five to get an average number. 
If you need the speed of the ADC, the programmable gain amplifier
might be the next best solution. 
There is always an option that the least significant bit toggles, as
Robert already pointet out.
An external ADC with 12-bit and connected through SPI is another
expensive option.

Cheers, Bob


--- In lpc2000@yahoogroups.com, Robert Adsett <subscriptions@a...> wrote:

> At 01:59 PM 9/16/04 +0100, you wrote:
> >ok, that makes sense, but using this ADC how can I get the most of it?
> 
> That depends.  What is your input (frequency, bandwidth, noise level, 
> impedance...)?  What are you trying to do with it?
> 
> Robert
> 
> " 'Freedom' has no meaning of itself.  There are always restrictions,
> be they legal, genetic, or physical.  If you don't believe me, try to
> chew a radio signal. "
> 
>                          Kelvin Throop, III