Lpc2000

I need a little advice with using external ADC's with lpc 
processors. 

Apparently, the reference, analog ground, and input signals looks 
somewhat distorted (spiky) on the oscilloscope whenever the 16-bit 
(unipolar) Converter is busy converting. As a result, the readings 
don't come out as consistent as it should (even when both inputs are 
grounded). Is there any way to avoid this problem?

This seems to be a power supply decoupling problem because the ADC seems to
draw more current during the conversion process. Did you follow the basic
rules for high resolution ADCs regarding board layout (ground planes,
massive power traces, sufficient high frequency decoupling capacitors 100n
in closest proximity to the ADC, electrolytics somewhat further from the
device)? You could also try LC filtering the supply lines for the ADC and
reference IC.

Regards,

Jan

----- Original Message ----- 

From: "Leighton Rowe" <leightonsrowe@...>
To: <lpc2000@yahoogroups.com>
Sent: Wednesday, February 09, 2005 3:41 PM
Subject: [lpc2000] External ADC Problem


>
>
> I need a little advice with using external ADC's with lpc
> processors.
>
> Apparently, the reference, analog ground, and input signals looks
> somewhat distorted (spiky) on the oscilloscope whenever the 16-bit
> (unipolar) Converter is busy converting. As a result, the readings
> don't come out as consistent as it should (even when both inputs are
> grounded). Is there any way to avoid this problem?
>
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
>
>

Many reasons could be for that, first let me know witch type is your A/D converter? SAR Sigma Delta? and who is manufucter of that ADC.
 
do you have buffered imputs with amplifires?
how many digits of readings are unstable?

Leighton Rowe <leightonsrowe@...> wrote:

I need a little advice with using external ADC's with lpc 
processors. 

Apparently, the reference, analog ground, and input signals looks 
somewhat distorted (spiky) on the oscilloscope whenever the 16-bit 
(unipolar) Converter is busy converting. As a result, the readings 
don't come out as consistent as it should (even when both inputs are 
grounded). Is there any way to avoid this problem?




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[Non-text portions of this message have been removed]

> Many reasons could be for that, first let me know witch type is 
your A/D converter? SAR Sigma Delta? and who is manufucter of that 
ADC.
>  
> do you have buffered imputs with amplifires?
> how many digits of readings are unstable?

- Made by "Analog" (AD7666)
- 16-bit Unipolar
- buffer on the input, 
- External Reference

The instability can be from 2 to 5 bits, or even 20 with switching 
supplies.