• AVR Freaks

How to reduce I2C bus noise

Author
yerunzhao
New Member
  • Total Posts : 28
  • Reward points : 0
  • Joined: 2006/11/03 10:25:26
  • Location: 0
  • Status: offline
2009/11/05 10:15:52 (permalink)
0

How to reduce I2C bus noise

ALL,
 
The SDA/SCL bus is pulled up to 3.3V via 1.5K resistor. And 200 pF cap to ground.
 
When the Bus is free, the noise on the bus is 600mV now. I want to reduce the noise to 300mV.
 
The frequency of the major noise is about 50MHz. I have tried to add(in series) ferrite Bead(with impedance is 200 ohm at 50MHz).
But it made no sense.
 
In addtion, if in series 100 ohm resistor on the bus, the noise can be reduced to 400mV or so.
 
Why the 100 ohm resistor could reduce the noise and the ferrit bead can't? 
Will this 100 ohm resistor effect the communication?
How to get 300mV leavel?
 
 
Thanks.
RZ.
 
#1

2 Replies Related Threads

    markp
    Super Member
    • Total Posts : 397
    • Reward points : 0
    • Joined: 2006/01/26 13:54:56
    • Location: 0
    • Status: offline
    RE: How to reduce I2C bus noise 2009/11/05 11:34:04 (permalink)
    0
    Not sure what you mean about noise, relative to what?
     
    You've got 200pF of capacitance on each signal, this is not really necessary. I suspect you've actually got local ground noise which is being coupled into your signals through these capacitors. What happens if you remove the caps?
     
    First thing, verify how much noise there is on the power supply relative to various ground points around the board (e.g. on each IC) by getting a close ground with your 'scope. Do you have a solid ground plane? It sounds a bit to me like that the grounds might be just tracks causing local ringing of the ground signals which in turn couple into the signals via the caps - make sure each IC has 100nF decoupler close to the power pins, and that there's some bulk capacitance (say 10uF caps) spread around the board on your 3.3V power.
     
    Mark.
     
     
    #2
    Stefan Uhlemayr
    Super Member
    • Total Posts : 4292
    • Reward points : 0
    • Joined: 2005/05/12 12:25:46
    • Location: Germany
    • Status: offline
    RE: How to reduce I2C bus noise 2009/11/05 12:51:02 (permalink)
    0
    ORIGINAL: yerunzhao

    ALL,

    The SDA/SCL bus is pulled up to 3.3V via 1.5K resistor. And 200 pF cap to ground.

    When the Bus is free, the noise on the bus is 600mV now.  ...
    Some questions:

    - How long is your bus?
    - How clean is your 3.3V (supply-filtering / decoupling etc. properly done)?
    - How many parts are connected to the bus?

    Greetings,
    Stefan

    ps: And remove the 200 pF, they make no sense and might only cause problems (max. allowed capacitance on the i2c-bus is 400 pF including wires, pin-capacitance etc...).
    #3
    Jump to:
    © 2020 APG vNext Commercial Version 4.5