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Minimizing noise effects in a Mixed Signal Design

 Post time:  2009-10-13
 

Author:aseem.v

Points:  25

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The following are two good design practices when you have both Digital and Analog/or mixed signal devices such as the ADC [Analog to Digital Convertor] in your system. More recommendations or design practices are welcome from the readers of this post.

1.Using gated clock devices to minimize Digital Switching Noise: Use of gated clock devices is desirable in several system designs. Several mixed signal devices such as the ADCs (Analog to Digital Convertors) support gated clock operation. The serial bit clock which is required to drive the digitized data out of the convertor is active only during the data transfer stage. During the conversion stage (Analog to Digital conversion), the serial bit clock is shut off. This avoids the coupling of digital switching noise in the analog circuitry due to this bit clock thereby achieving higher noise immunity.

2.Isolating Analog circuit ground “AGND” from the Digital ground “GND”: If the Digital and Analog grounds are not isolated (AC isolation), the high frequency switching noise affects the sensitive devices like the ADCs. For example, in case of ADCs, the potential difference between the input signal and its ground is what is quantized. Switching noise on ground will cause the potential difference to vary and thus affect the converted digital output. Use of ferrite beads is a common practice to isolate Analog ground from the Digital ground in a system. This ensures that the switching noise on the Digital ground does not get into the Analog ground.
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Reply: Minimizing noise effects in a Mixed Signal Design

 Post time:  2009-10-17
 

Author:aseem.v

Points:  25

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Can something be done in the software as well??
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Reply: Minimizing noise effects in a Mixed Signal Design Post time:  2009-10-22
 

Author:aseem.v

Points:  25

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There is one approach that is used when you have multichannel ADCs in the system. But, this should not be interpreted as a substitute for the recommendations explained in the original post. When you have multichannel ADCs, one of the unused channels could be connected permanently to analog ground (AGND). Before doing any signal processing of the incoming data on the other channels, this channel who's input is connected to ground should be sampled several times. The averaged signal value should be considered before any valid samples on other channels are processed. This procedure can manage offsets or ambient noise to some extent. But as I pointed out earlier this can't replace the recommended approaches but does act as an additional mechanism to handle certain types of noise.

regards,
-av
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