Connect with us

2khz noise

Discussion in 'General Electronics Discussion' started by luke67, Sep 18, 2012.

Scroll to continue with content
  1. luke67

    luke67

    46
    0
    Mar 18, 2012
    Hi people !
    I am doing some input-referred noise tests on a Pcb. This Pcb is composed by 8 quad-op amp that filter and give gain to signals. Actually I made this test also on his twin boards and I didn't notice frequency noise. Instead in this Pcb there's a big noise at ouptut at 2Khz frequency even when the signal input is linked to the ground (see the attachment). I give supply to the board with a laboratory dc power supply (7 Volt) and obviously there are 2 electrolytic capacitors on the board. Anyone has some idea to how reduce this noise?
    Thank you very much!
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,651
    2,692
    Nov 17, 2011
    Try a 9V battery to eliminate any chance of noise from the power supply.
    Show us the schematic and the PCB layout. It is possible the noise is coupled from the outside. Also: what kind of OpAmp do you use? Not one with a built-in autozero feature, do you?
     
  3. luke67

    luke67

    46
    0
    Mar 18, 2012

    Attached Files:

  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,651
    2,692
    Nov 17, 2011
    The inputs of your OpAmps are all open circuit. This can cause pickup of noise. Add a large resistor (1MOhm) from each "+" input to GND.

    Do both boards have an identical layout? Or are the breadboarded?
     
  5. luke67

    luke67

    46
    0
    Mar 18, 2012
    breadboarded... do you think that covering the circuit that 2khz noise will disappear ?
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    11,651
    2,692
    Nov 17, 2011
    Try it. Har d to say from a distance.
    Also: are the OpAmps' supplies well buffered by capacitors as near as possible to the supply pins?
     
  7. luke67

    luke67

    46
    0
    Mar 18, 2012
    yes. Here is the exemplum of one quad-op amp. What do you think ? Also you think there may be the presence of parassitic capacitor ?
    THX
     

    Attached Files:

  8. luke67

    luke67

    46
    0
    Mar 18, 2012
    and what is the auto-zero feature ?
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    11,651
    2,692
    Nov 17, 2011
    One end of the bufffer capacitors is well near the OpAmp. The other end, however, seems to be connected to VCC or GND, respectively, by a comparatively long trace. Or do you have Power and GND planes underneath?

    There sure will be parasitic capacitors, but rather small ones and the presence of parasitic capacitors alone does not explain a 2 kHz noise.

    Did you try the battery instead of the bench supply?

    What is the output load of the OpAmps? Are there any capacitors (that are not shown in the exceprt from your schematic)? The ad8644 should not be operated with more than 200pF load capacitance. Otherwise it can become instable.

    Have you looked at the waveform of the 2 kHz signal with an oscilloscope? Is is a sine or some distorted waveform?

    This is a feature where the OpAmp automatically nulls the offset. This is done by measuring and calibrating the offset internally or by chopping Dc and low-frequency signals. This feature can inroduce switching noise. The ad8644 you are using is not of this type, so this can not be the source of the 2 kHz signal.



    btw: that's not what I consider a breadboard.
     
  10. luke67

    luke67

    46
    0
    Mar 18, 2012
  11. Harald Kapp

    Harald Kapp Moderator Moderator

    11,651
    2,692
    Nov 17, 2011
    That OpAmp should also not be critical.
    What happens if you short circuit the capacitors C9 ...C12? Still 2kHz noise? If yes, the phase shift of the lowpass filter in the feedback loop is responsible for oscillation. Again: what is the waveform of the 2kHz signal? Is it a reasonable periodic waveform (sinusoidal or square), or is it very noisy?
     
  12. luke67

    luke67

    46
    0
    Mar 18, 2012
    short-circuit replacing them with a wire, that is putting in parallel to a wire as if there weren't? Can you make a picture of what do you mean ?
    THX!
    The waveform: I'am gonna to analyze it this afternoon! I'll post it !
     
  13. Harald Kapp

    Harald Kapp Moderator Moderator

    11,651
    2,692
    Nov 17, 2011
    No picture: yes, replace these capacitors by a piece of wire.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Similar Threads
There are no similar threads yet.
Loading...
Electronics Point Logo
Continue to site
Quote of the day

-