audio circuit design help...

Discussion in 'General Electronics Chat' started by newton3375, Jul 2, 2008.

  1. newton3375

    Thread Starter Member

    Apr 29, 2006
    13
    0
    Hi all....

    I had a question about a little audio circuit I am working on....its pretty simple it just consists of an audio amplifier (OPA134PA), an audio balanced line driver (DRV134), a audio differential receiver (INA137), and a few caps and resistors.

    When I hook this circuit up on a bread board everything works fine...the op amp gives a gain of 8 and the line driver outputs two signals 180 degrees out of phase with virtually the same peak to peak voltage.

    I am inputting a .5 volt peak to peak signal into the amplifier and getting 4 volts peak to peak out of the amplifier. Then the line driver provides me with two out of phase signals each 4 volts peak to peak.

    But on my PCB something seems to be wrong....the audio line driver (DRV134) outputs are not equal...the positive output is around .30 volts higher than the negative input which is out of specs for the DRV134. I called Texas instruments and they said I might have a bad chip, but since it works on a breadboard I am convinced it is a issue with the final PCB circuit.

    Any ideas what could cause the difference in the two outputs?

    I have attached the ORCAD files I used for the circuit for reference....any help would be appreciated.....
     
  2. Distort10n

    Active Member

    Dec 25, 2006
    429
    1
    I will take a look later this evening. Who did you speak to at Texas Instruments? I am surprised that you got through.
     
  3. newton3375

    Thread Starter Member

    Apr 29, 2006
    13
    0
    I don't remember who I spoke to....

    I never have a hard time getting through to TI...some companies are a pain but TI is pretty good.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I tried looking at the contents of the zip file, but couldn't find any schematics there, at least no graphics files.
     
  5. newton3375

    Thread Starter Member

    Apr 29, 2006
    13
    0
    You would need Orcad to open it.....
     
  6. newton3375

    Thread Starter Member

    Apr 29, 2006
    13
    0
    There have been some new developments since my last post...

    I think I have discovered what causes the negative output of the audio line driver to have a .3 V peak to peak voltage drop although I do not fully understand it...

    I had taken the output from the audio line driver (DRV134) and used this as the input for the audio line receiver (INA137). When this is connected we observe the voltage drop. I have connected the driver directly to the receiver without any capacitors or resistors, connected it with the resistors only, and connected it with both the capacitors and resistors present and each time the negative output has the .3 volt drop.

    If I take the output of the line driver and I just connect each output to ground through a 2.2 K resistor then the line driver output does not have the .3 volt drop and both outputs look good.

    So the receiver is doing something....but Im not sure what. This happens on both the PCB and the breadboard if the driver outputs are looped back as inputs to the receiver.

    Any thoughts?
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    It is too bad that your ORCAD schematic cannot be seen by most of us.
    If it was saved and posted as a GIF or PNG file type then all of us would see it and comment on it.

    Try PRNT SCRN then save-as.
     
  8. newton3375

    Thread Starter Member

    Apr 29, 2006
    13
    0
    Here is a PDF of the circuit....
     
  9. newton3375

    Thread Starter Member

    Apr 29, 2006
    13
    0
    For some reason it wont let me.....
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, then try it this way:
    1) In ORCAD, open your schematic.
    2) Hold down the CTRL key and tap the PrintScreen (or PrtScrn) button.
    3) Start Paint (Start -> Programs -> Accessories -> Paint)
    4) Hold Ctrl down, and tap the V key (or Edit/Paste)
    5) Save the file as type .png
    6) Add or edit a reply to this thread, and click the Go Advanced button near the bottom. Attach the file you saved from Paint.
     
  11. newton3375

    Thread Starter Member

    Apr 29, 2006
    13
    0

    Ive already tried this, but I couldnt do it because the program "snag it" was interfering.....once I deleted snag it all the issues disappeared.

    Anyway Ive attached the schematic....
     
  12. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Thanks for the PDF.
    I don't see anything that would cause a voltage drop when the signals are direct coupled.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Ahh, ok. It appears that you have quite a bit of "white space" on your schematic. While this may make it somewhat easier to read while you're working on it, you won't be able to fit the whole circuit on one screen and do a screen capture with enough resolution to be able to see what everything is.

    Try moving your components closer together.
     
  14. newton3375

    Thread Starter Member

    Apr 29, 2006
    13
    0
    Will do.......
     
  15. newton3375

    Thread Starter Member

    Apr 29, 2006
    13
    0
    The issue has been resolved. After talking to Texas Instruments a few times they ran a simulation and the cause was simply found to be the receiver inputs.

    The receiver is basically a differential amplifier so the negative and positive inputs have different input impedances. That is what causes the .3volt drop on the negative terminal.

    I incorrectly assumed that TI would have internally compensated for this difference and internally made both inputs equal in impedance. TI has assured me that this does not affect the receivers output, so perhaps there is something internal that eventually takes this into account (although looking at the circuit I don't see it).

    Anyway.....mystery solved and thanks everyone for responding.
     
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