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Signal aliasing in ADuCM360's ADC

Question asked by dinrail on Mar 14, 2014
Latest reply on Mar 17, 2014 by dinrail

I am not sure if this is the right forum to post the question. If not, please be so kind to point me to the right one.

 

I clock the ADuCM360 with 16.384Mhz, so the modulator frequency for its ADC is now 128Khz.

 

An RC filter to the input of ADC is present, with 3dB at 5Khz.

 

The above design is powered by your ADuM5401, which has PWM as 625Khz (typical), and I followed the layout instruction around ADuM5401

 

All runs GREAT until it goes to freezer test

 

For the few boards we tested, when temperature gets cold, the PWM frequency drift up, and reaches 640Khz, and its aliasing will begin to show up in the ADC readings at the high gain.

 

We would like to fix the problem

 

Anyway to change the PWM frequency for ADuM5401?

 

Do you have a chart or other info to show the PWM frequency vs ambient temperature for ADuM5401? (For the few boards we tested, the problem is around 25F, if a chart can prove this is always the case, we will just spec it and leave it alone, but some people worry PWM freq may drift to 640Khz at room temperature, and that would post a big problem)

 

Or, do you have other suggestions in regard of this issue?

 

Thanks!

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