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LMP91000: Ferrite bead for EMI

Nicolas Maillard
Prodigy 20 points
Part Number: LMP91000

Hi,

My question is quite simple, do you recommend to add ferrite bead between the electrochemical cell pins and the LMP91000 ?

I've done a research on the net and I didn't find anything about that. There are some applications notes that recommend to do it but it's for other analog front end.

My question is because we have some EMI issue with our design which is based on the LMP91000 + ferrite beads.

I had the opportunity to do some house test with a homemade BCI and I noticed the LMP91000 output is more disturbed with the ferrite beads than without.

I'm wondering if it's better to check another ferrite bead reference or to remove them ?

Thanks in advance for your help

Nicolas

  • 0
    TI__Genius 9310 points

    Nicolas,

      I agree that ferrite beads sounds like the default solution. In this case it may not be a good idea. A sensor is very sensitive typically running on nano-amps of current. Adding a ferrite bead attempts to block skin effects and potentially become high impedance at RF frequencies. This can effect the sensor by changing the impedance and become an antenna to EMI. 

    The proper technique is to layout the PCB using controlled impedances and micro-stripping design rules. You may also want to look at a guardring to protect the sensitive traces and sensor. My favorite technique is to place the sensor traces between two ground planes, while controlling the impedance of the PCB. 

    Long sensor traces can become an antenna. This is the reason why controlled impedance layouts are very important. The layout may not work well if you are trying to use a 2 layer PCB. I always use 4 layer at a minimum in order to create a full ground plane on the second layer, route the sensor traces on the third layer and a partial ground plane on the 4th layer. Again The spacing of the layers and the thickness of the traces is very important to get controlled impedances. 

  • 0 in reply to Gordon Varney
    Prodigy 20 points

    Thanks for your quick feedback Gordon !

    It's really helpful, I think we'll try to do it without these ferrite bead.

    For information, the measurement disturbance I am talking about occurs at the injection transition (no noise -> noise injection) or (noise injection -> no noise). In practice I observe a variation of the DC output signal of the LMP91000 which lasts about 4 seconds before returning to the previous level. This DC variation results in a measurement variation that is outside the authorized limit.

    According to my understanding, the injection of the conducted noise results in a variation which the potentiostat circuit manages to compensate for after a delay of a few seconds. 

    Without the ferrite bead, the phenomenon almost disappeared. 

  • +1 in reply to Nicolas Maillard
    TI__Genius 9310 points

    Nicolas,

       Based on your description, remove the ferrite beads and add a 1000pf capacitor to C1 to ground and another to C2 to ground. Verify measurement accuracy with the capacitors, but this should take care of your EMI problems. If not, then we need to be more specific about the frequencies you are seeing the increase in voltage. but we can look at that later.