Hello:

I'm using the MSP430F149 ADC unit to sample a signal generated by a 
waveform generator (a sine wave). Whenever I run the program a high 
frequency sinusoid is added to the input sinusoid resulting in noisy 
signal. I noticed that the frequency of the noise sinusoid is 4.8 MHz 
which is the exact frequency of the external crystal oscilator I am 
using as system clock. If I use the MSP internal clock and turn the 
external oscilator off, the 4.8MHz sinusoid disappears. Still, I get 
some noise (with a lower frequency) in the input signal 
caused ,aparently ,by the internal system clock. 

How should I connect the crystal oscillator, could this be the cause?

Does anyone know the cause of this and how can I solve it? 

Any ideas will be really appreciated.

Joel

On Wed, May 18, 2005 at 11:30:11PM -0000, Joel Acevedo wrote:

> How should I connect the crystal oscillator, could
this be the cause?
> 
> Does anyone know the cause of this and how can I solve it? 

1. Switch the XTAL off during ADC sampling.
or
2. Do not connect the XTAL capacitors to the GND-plane 
   use a single wire to the GND pin of the MSP430 instead
   (should not be in series with any AGND current path)
 
        Matthias

----- Original Message ----- 
From: "Matthias Weingart" <msp430@msp4...>
To: <msp430@msp4...>
Sent: Tuesday, May 24, 2005 7:45 PM
Subject: Re: [msp430] ADC noise created by external crystal oscillator


> On Wed, May 18, 2005 at 11:30:11PM -0000, Joel
Acevedo wrote:
>
>> How should I connect the crystal oscillator, could this be the cause?
>>
>> Does anyone know the cause of this and how can I solve it?
>
> 1. Switch the XTAL off during ADC sampling.
> or
> 2. Do not connect the XTAL capacitors to the GND-plane
>   use a single wire to the GND pin of the MSP430 instead
>   (should not be in series with any AGND current path)

I always connect the crystal that way on any board I design. I never have 
any ADC problems, even on single-sided PCBs.

Leon
--
Leon Heller, G1HSM
http://webspace.webring.com/people/jl/leon_heller/