solderless breadboard noise

Discussion in 'General Electronics Chat' started by aeroguy, Jul 21, 2011.

  1. aeroguy

    Thread Starter Member

    Sep 16, 2009
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    2
    Hello,

    I'm working to prototype a 2-stage thermocouple amplifier, using a 2-channel general purpose op amp IC.

    I reviewed design notes from datasheets, Microchip, and my college EE textbooks.

    I then assembled my simple design (which is a clone of other common designs) onto a small solderless breadboard.

    I find that there is significant noise and oscillation in the output from the amplifier. The output fluctiates wildly if a handle the breadboard - particularly if I push on part of it or tap it.

    I used bypass caps for my power supply, and also tried adding caps in parallel with the feedback resistor(s).

    My questions are:

    1) Has anyone else experienced this kind of noise using solderless breadboards? Is it just a crappy board?

    2) Is there a better way to prototype? I read in a Microchip AN that using perf board for prototyping can introduce noise and oscillations.

    Any comments or suggestions would be appreciated.

    thanks
    Eric
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    1,605
    Solderless breadboards are notoriously a poor choice for high gain amplifiers as they not only introduce stray capacitance everywhere, coupling thing you do not want coupled, but they tend to isolate things that should be close.

    It sounds like you have an oscillation. Put the op amp bypass caps directly on the ICs, I mean directly over the top of the chip with the shortest leads to span the holes.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    I'll suggest that you really need to look at using an instrumentation amplifier rather than trying to "roll your own" from individual opamps and resistors. At the level of gain you'll need, you will have big problems with trying to balance the thing.

    Besides, general-purpose opamps have a good bit of offset and lots of drift. You'll wind up having to re-tweak them pretty frequently.

    Have a read through Intersil's AN1298: http://www.intersil.com/data/an/an1298.pdf
    It's a good review of various types of IA's, and why it's so frustrating trying to make them out of opamps.

    Then have a look at this app note: http://www.analog.com/static/imported-files/application_notes/34661261AN369.pdf
    just to get acquainted with the AD594/595 and what they will do for you.
    That product page is here: http://www.analog.com/en/special-linear-functions/thermocouple-interface/ad594/products/product.html
     
  4. aeroguy

    Thread Starter Member

    Sep 16, 2009
    40
    2
    Thank you for the info and links.

    I am motivated to design my own amplifier with cost in mind. I'm prototyping an idea that I might be able to sell, and I don't want $10 ICs showing up in the BOM.

    However, it does seem to be quite troublesome to make my own..

    Much appreciated.

    Eric
     
  5. aeroguy

    Thread Starter Member

    Sep 16, 2009
    40
    2
    Perhaps I spoke too soon. The ISL28271 is quite cheap!

    thanks again,
    Eric
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    What you might save in parts by going with discrete amps, you'll pay 10x as much for the additional board real estate, technician time in tuning the things, and parts failures.

    If you can get a single IC and a resistor to get the gain you need, you'll be way ahead of the game.

    Don't forget to use the recommended bypass capacitors, and then some.
     
  7. strantor

    AAC Fanatic!

    Oct 3, 2010
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  8. Audioguru

    New Member

    Dec 20, 2007
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    Breadboards are used for slow DC and slow AC circuits that have low voltage gain.
    Many requests for help on these websites are caused by the problems (also intermittent contacts) caused by breadboards.
     
  9. aeroguy

    Thread Starter Member

    Sep 16, 2009
    40
    2
    Thanks for the comments.

    Is there a preferred method of prototyping, or should one just etch a "final" PCB and try it?

    Eric
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Making a PCB is preferable.

    However, you can use the "dead bug" technique, which is gluing the parts upside-down on a piece of copper-clad circuit board, and doing point-to-point wiring between the various terminals/components. It can look pretty messy, but it has the advantage of a large ground plane. This technique was used quite frequently at a former employer for prototyping of RF circuitry.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    I made thousands of projects with stripboard. The copper strips form half of a pcb and the parts and a few jumper wires form the remainder. Some of my projects were extremely large and complicated. They all worked perfectly.
     
  12. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    A while ago I posted a link to the "manhatten style prototyping"
    http://forum.allaboutcircuits.com/sh...9&postcount=30

    This hold a kind of inbetween the breadboard and the dead bug.
    Becouse the wiring can be shorter as in the dead bug style, it is fine for RF.

    Greetings,
    Bertus
     
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