MIC557 circuit frequency output issue

Discussion in 'General Electronics Chat' started by Gibson486, May 28, 2015.

  1. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    I made a circuit attached, however, the frequency keeps shifting back and forth by approx +/-10kHz. In other words, it hits my frequency of around 260Khz, but the frequency does not stay constant at around 260kHz. It goes between approx 250kHz and 267kHz. Are there grounding issues I need to be away of?

    Edit...part number should be MIC1557
     
    • pic.png
      pic.png
      File size:
      15.2 KB
      Views:
      33
    Last edited: May 29, 2015
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    Is the temrerature stable?
    What kind of capacitor do you use?
    If you use a ceramic capacitor a np0 would be fine.
    Also a styroflex foil type capacitor could be used.

    Bertus
     
  3. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    It should be stable....If there is any heat, it is is far away from the chip.

    I used an NPO and then an XR7. Both produced the same results.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    What kind of resistors do you use?
    Carbon resistors can change and make noise.
    Metal film resistors will give a much better result.

    Bertus
     
  5. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    Thick Film resistors....
     
  6. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    Ooops...part number should be MIC1557
     
  7. RichardO

    Well-Known Member

    May 4, 2013
    1,235
    384
    100 pF is a fairly small amount of capacitance. because of this you need to keep the wires shot around it. Can you give a picture of your circuit?
     
  8. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    It's on a PCB....I think it may be the output...I have another circuit that I switch it with if I do not want to use the 555, so the output is sort of a long trace. It's really the only thing I can think of... I am thinking about making a circuit board via a panelized pcb shop, that way I can test multiple designs for cheap....
     
  9. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,791
    1. What is the purpose of R7?
    2. What is driving the CS input?
    3. What is the purpose of C12?
    You seem to be trying to do something other than what the chip was designed for. What is that?
     
  10. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    R7 is just a pull down to force it low when it is hanging.

    CS is being driven by a relay contact the goes to 5V.

    C12 is just a AC coupling cap. When I remove the cap, it makes no difference.

    The circuit is going to a integrator to get a triangle wave.
     
  11. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,791
    Are you serious -- a relay contact!! They bounce (hi-low-hi-low..) for 20 to 50 milliseconds. No wonder the frequency output is all over the place. Congratulations, you've uncoverd a new feature of the part.
     
  12. Gibson486

    Thread Starter Member

    Jul 20, 2012
    199
    12
    Umm...it is only used as a turn on, turn off. I do not toggle it it every 2 seconds or something. If it says on, it stays on for 5 hours type of thing.
     
  13. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,791
    Well something we can't see is causing a problem, because your frequency stability should be better than what you have described. I would start with the application circuit in the datasheet with as few modifications as possible.
     
Loading...