Does ground plane affect Hall effect sensor?

Thread Starter

ebeowulf17

Joined Aug 12, 2014
3,274
I'm redesigning a board (details here) which uses analog Hall Effect sensors to measure magnet proximity, relying on output swings in the tens of millivolts, and uses a comparator to set a distance threshold.

In my redesign, I added a ground plane which didn't exist in the first version. The magnet is in front of the sensor, and the ground plane would be behind it, so the ground plane won't be in between the two.

The question is whether the proximity of a ground plane could effect the Hall Effect sensor sensitivity.

I could pull the ground plane away from the sensors a bit and still leave it behind other components, or I could eliminate it entirely. The current generation of boards has worked fine without one, but it seemed like a generally good idea to add one in the redesign until I got worried about altering the magnetic field.

Any thoughts?

Google has mostly failed me. One stack exchange discussion and one TI support forum discussion both make me think that the ground plane shouldn't be an issue since the plane is behind the sensor and the magnet isn't going to move terribly quickly. Two internet opinions don't amount to much though - I'd love to have a few more from people I trust.

Thanks!
 

Thread Starter

ebeowulf17

Joined Aug 12, 2014
3,274
It could if you were monitoring a high-frequency AC magnetic field; but I can't see how it would have any influence in your application.
I'm glad to hear that. Thanks for your help!

It does raise other questions for me (not regarding this circuit, but just understanding electromagnetic interactions in general.) I never really thought about how shielding does (or doesn't) work before. I'm guessing that common shielding methods are frequency dependent? Lower frequencies are relatively unaffected, but higher frequencies are blocked? If so, there must be variables that alter those characteristics - does thicker shielding equal greater effectiveness at lower frequencies? Again, I realize none of this matters much in the circuit I originally asked about; I'm just wondering in general terms now. Thanks!
 

OBW0549

Joined Mar 2, 2015
3,544
I'm guessing that common shielding methods are frequency dependent?
I think that's generally true.

Lower frequencies are relatively unaffected, but higher frequencies are blocked?
It's a matter of the degree to which different frequencies are blocked, but yes.

If so, there must be variables that alter those characteristics - does thicker shielding equal greater effectiveness at lower frequencies?
I think that's usually true. Also, at low frequencies ferrous materials will do a better job of shielding against the magnetic component of the field, with high-permeability materials like mu-metal performing better than low-permeability like ordinary iron or steel.

DISCLAIMER: I'm no expert on shielding; perhaps someone with more knowledge/experience than I will chime in with better info.
 

Alec_t

Joined Sep 17, 2013
11,627
Shielding is frequency dependent. A conductive mesh, for example, can form a Faraday cage providing the mesh apertures are a lot smaller than the incident wavelength.
 

nsaspook

Joined Aug 27, 2009
7,585
I'm glad to hear that. Thanks for your help!

It does raise other questions for me (not regarding this circuit, but just understanding electromagnetic interactions in general.) I never really thought about how shielding does (or doesn't) work before. I'm guessing that common shielding methods are frequency dependent? Lower frequencies are relatively unaffected, but higher frequencies are blocked? If so, there must be variables that alter those characteristics - does thicker shielding equal greater effectiveness at lower frequencies? Again, I realize none of this matters much in the circuit I originally asked about; I'm just wondering in general terms now. Thanks!
Most electronic applications of AC signal shielding depend on cancellation or redirection (reflection) of energy rather than pure absorption until you get to lower frequencies. Once you get beyond a few skin-effect thicknesses of material the extra shielding from more material is minimal.

You can directly shield the 60 Hz magnetic component with non-magnetic good conductor like copper but it will be thick ( ~ 1cm) so it's usually more effective to redirect the field with ferromagnetic materials if energy cancellation can't be used with things like twisted pairs and differential signals..
https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/depthcurrentdensity.htm

http://www.analog.com/media/en/training-seminars/tutorials/MT-095.pdf
Conductive enclosures can be used to shield sensitive circuits from the effects of these external fields. These materials present an impedance mismatch to the incident interference, because the impedance of the shield is lower than the wave impedance of the incident field. The effectiveness of the conductive shield depends on two things: First is the loss due to the reflection of the incident wave off the shielding material. Second is the loss due to the absorption of the transmitted wave within the shielding material. The amount of reflection loss depends upon the type of interference and its wave impedance. The amount of absorption loss, however, is independent of the type of interference. It is the same for near- and far-field radiation, as well as for electric or magnetic fields.
 

Thread Starter

ebeowulf17

Joined Aug 12, 2014
3,274
Most electronic applications of AC signal shielding depend on cancellation or redirection (reflection) of energy rather than pure absorption until you get to lower frequencies. Once you get beyond a few skin-effect thicknesses of material the extra shielding from more material is minimal.

You can directly shield the 60 Hz magnetic component with non-magnetic good conductor like copper but it will be thick ( ~ 1cm) so it's usually more effective to redirect the field with ferromagnetic materials if energy cancellation can't be used with things like twisted pairs and differential signals..
https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/depthcurrentdensity.htm

http://www.analog.com/media/en/training-seminars/tutorials/MT-095.pdf
Lots of good stuff there. Thanks!
 
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