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Jonathan Harris

ADC Noise: A Second Look, Part 1

Jonathan Harris
Victor Lorenzo
Victor Lorenzo
12/1/2013 2:42:18 PM
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Blogger
Re: Most important factor
"Another important point while bypassing is to place several capacitors in parallel instead of lumping it into a single capacitor".

We could add also that not only to place several capacitors in parallel, it should be taken into account the capacitor's behaviour at high frequencies when selecting the capacitors that will 'filter' the high frequency noises (usually ceramic, commonly 0.01uF-to-0.1uF).

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amrutah
amrutah
12/1/2013 9:49:42 AM
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Master
Re: Most important factor
@Yalanand:  Another important point while bypassing is to place several capacitors in parallel instead of lumping it into a single capacitor.

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amrutah
amrutah
12/1/2013 9:45:17 AM
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Master
Re: Most important factor
@Yalanand: There are many techniques to reduce the noise from power supplies and depends on the scope of application, few of them that I know

1> Reduce the effect of noise by using cascode devices, avoid any high switching digital and sensitive analog sharing the same supply (this can be done to some extent by proper laying out the powerlines)

2> bypassing the power supplies at the load so that the peak current (di/dt), which usually leads to noise, can be taken care to some extent

3> Low pass filters can be used to reduce the effect of noise at the load (but this may cause functional problems).

4> Snubber circuits to isolate the noisy elements.

   As far as ground is concerned, which is a reference node for the entire application, I think making it strong (wide routings) is the trick that suffices.

 

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yalanand
yalanand
11/30/2013 9:15:45 AM
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Newbie
Re: Most important factor
The main culprits I consider are the noise from Supply and Ground.  

@amrutah, thanks for sharing your analysis. I am curious to know how can we reduce noise from ground and supply ? What techniques can be used to reduce noise from ground and supply.

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amrutah
amrutah
11/15/2013 5:26:49 PM
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Master
Most important factor
@Jonathan:

  Thanks for the Blog and also updating th e sources of Noise.  I think the noise from reference or common mode is very less due the huge capacitors on that node.  The main culprits I consider are the noise from Supply and Ground.  We may have star connections to these nodes but they are still significant.  Also the inherent devices noise will add a lot to the output noise.  The network used for switching, sampling will introduce new set of noise.

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More Blogs from Jonathan Harris
In this blog we’ll take this one step further and look at driving the ADC power supplies directly from a DC/DC converter.
Previously in this blog series we looked at using a DC/DC converter (switching regulator) in combination with an LDO to drive the power supply inputs to an ADC. What we found was that using the DC/DC converter to step down the input voltage for the LDO was a much more efficient way to drive the power supply inputs to an ADC.
We look at using a DC/DC converter along with an LDO to drive the ADC power supply inputs.
I thought it would be good to continue looking at the example I gave in my last blog where we looked using fewer LDOs and combining power supply rails on an ADC while maintaining isolation with ferrite beads.
There are some disadvantages when driving low input supply voltages, where multiple LDOs may be required.
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