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Jonathan Harris

ADC Noise: Where Does It Come From?

Jonathan Harris
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Victor Lorenzo
Victor Lorenzo
12/1/2013 11:37:55 AM
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Re: noise/jitterin the board
@Scott Elder, "In this case, the switching ground current will generate a switching noise voltage that will conduct directly on to the reference voltage thereby reducing the precision of the reference voltage". You're right, but we should always consider that as part of the design that situation must be avoided. Two points to address here, the high current switching paths should be as short as possible, separated from the analog part of the circuit (they use to have less effect on the digital circuits) and occupy the minimum current path area as possible to minimize emissions (perhaps the most importan rule).

The voltage reference should be placed as close as possible to the reference pin and provided with its own return path (once again, it should occupy the minimun possible area).

Most digitized signal noise I've found has come from other sources, far more difficult to circunvent than the reference voltaje noise.

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Victor Lorenzo
Victor Lorenzo
12/1/2013 11:22:34 AM
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Re: noise/jitterin the board
@Daej, "I wonder where conductive and radiated noise come from in what system" Just a small experiment that could surprise you. Fo this you'll need one high-bandwidth oscilloscope with the highest resolution you can find (10 or more bits per sample).

Radiated Emissions: make two loop antennas, one about 10cm in diameter and another about 3cm, both with about 10 turns. Hold the bigger antenna with your hand at about 50cm from a fluorescent light and look at the 'scope. Take notes. Now take the small antenna and place it over the chips (MCU, FLASH, ADC, Switching regulator, etc) of your board. Once again, take notes. Of course you'll need to adjust the vertical resolution for the 'scope and gain some experience in differentiating the incoming noise from the quantization noise inherent to the 'scope's ADC.

Conducted Emissions: take a piece of wire about 20cm in length and wind it around one of the DC power cables for your board. Take a look at what the 'scope reveals. Now repeat the same experiment with different positions between the power/ground wires (putting them apart, putting them together, twisting them, putting a ferrite clamp, etc.)

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Victor Lorenzo
Victor Lorenzo
12/1/2013 11:11:20 AM
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Re: noise/jitterin the board
@Netcrawl, if you let me, regarding your phrase "We got two type of noise here- conductive and radiated. Its not hard to eliminate these two, its addressable". I don't agree with the bolded part of it. Yes, it is most of the times a hard design task to end up with a correct PCB design showing a high EMI noise inmunity and at the same time featuring low EMI emissions (conducted and radiated). It specially holds true when working with synchronous (clocked) digital signals switching at clock speeds over 50MHz. From my experience on the area, newcomers to high speed PCB design almost always fail on that at least the first time.

"we need to know some electromagnetic mechanisms", I agree on that and I would add, we should also be prepare to think that a piece of wire is not a near zero ohm 'wire' at high frequencies, it is a complex transmission line with many parasitic effects to take into account.

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jonharris0
jonharris0
10/30/2013 4:05:49 PM
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Re: Ground?
Aha, yes, David, you are correct.  What an oversight I have made here!  I shall have to update that in the next blog.  As I once heard Bruce Archambeault say giving a conference paper, "Ground is a place for potatoes and carrots!  We have current return paths!"  It is definitely a potential pathway for noise into the ADC.  As you correctly point out, it is not always a stable reference point.  Thanks for the comment!

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David_O
David_O
10/30/2013 3:56:47 PM
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Newbie
Ground?
Jonathan

Your ADC seems to be missing its ground pin, a very important signal to consider. I suppose you sort of covered it with with the other power lines, but it is often an analgue reference point as well. While it may be less important with differential devices, as shown, the absense of this pin can only help reinforce the assumption that all grounds are the same and provide a safe and stable refernce point. Unfortunately, this is often not the case.

 

 

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yalanand
yalanand
10/27/2013 7:05:26 AM
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Newbie
Re : ADC Noise Ė Where Does It Come From?
All analog-to-digital converters have a certain amount of input-referred noise exhibited as a noise source associated in series with the response of a noise-free ADC. Input-referred noise is not to be chaotic with quantization noise, which is only of notice when an ADC is handling time-varying signals.

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Scott Elder
Scott Elder
10/24/2013 3:02:56 PM
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Re: noise/jitterin the board
@DaeJ, "I wonder where conductive and radiated noise come from in what system."

Consider an ADC system with a switching power supply and voltage reference.

An example of conductive noise is the power supply ground return current flowing through a shared return path also used by the ADC reference.  In this case, the switching ground current will generate a switching noise voltage that will conduct directly on to the reference voltage thereby reducing the precision of the reference voltage.

Using the same system example, radiated noise can likewise couple into the reference ground if the reference ground return signal path is sufficiently parallel to the switching power supply return path such that the two paths are electromagnetically coupled as in an antenna.

A "star" ground system along with large ground planes and shields is usually adequate to solve these noise issues.

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jonharris0
jonharris0
10/24/2013 8:51:52 AM
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Re: what about the Vref circit?
Thanks for the comments Bonnie.  I am assuming the Vref circuitry is internal as is the case on many high speed ADCs today.  And I agree wholeheartedly that the space is limited with the blog, but I hope to split this out and cover many of the topics over several blogs.  I hope all can enjoy the ride as we look over these many doorways! :)

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samicksha
samicksha
10/24/2013 2:25:30 AM
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Student
Re: noise/jitterin the board
I agree you Netcrwal, conducted noise is already in the circuit board by the time the signal arrives at the input of the ADC.

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DaeJ
DaeJ
10/23/2013 8:07:49 PM
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Master
Re: noise/jitterin the board
I wonder where conductive and radiated noise come from in what system.  I heard that there are so many different type noises in the circuit with interface board.

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More Blogs from Jonathan Harris
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.
Keeping the power supply inputs on separate domains can minimize crosstalk and make it much harder for noise to interfere with ADC performance.
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