Help with scope monitoring? (Differential inputs etc)

Discussion in 'The Projects Forum' started by peskywinnets, Feb 23, 2009.

  1. peskywinnets

    Thread Starter Active Member

    Jan 19, 2009
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    Hiya,

    (not exactly a total noob, have had some scope useage previously, but on simple circuits!)

    I may have a faulty board, but I'd be interested to hear comments as to whether I should have a viewable 'signal' as seen on a scope between pins 4 & 5 on the main chip on the attached diagram.

    I'm not seeing anything at the moment, but that may well becaUse of the way I'm scoping the signal (or becuase of a fault!). My scope groud is on the mainboard ground...I don't see anything on pins 3 or 4. Becuase these are differential inputs, do I need to do something fancy?!!!

    I have a Hantek DSO2150 USB scope if it helps! ( http://www.hantek.com.cn/english/produce_list.asp?unid=63 )


    As an aside, can anyone explain hat going on there with those inputs on pins 4 & 5.?!

    Pin 4 I can grasp grasp (Left positive...L+), but what's happening with the (seemingly to me) weird place they've tapped pin 5 from? (Left negative....L-) I'd have expected the L- to be to ground or the inverse of the L+ signal?
     
    Last edited: Feb 23, 2009
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Yeah, they're creating a single ended input out of what would normally be differential. Pin 5 has a low-pass filter on it that will keep any DC level on it constant, whle the signal voltage will appear on pin 4. You won't see anything on pin 5, except for maybe a DC voltage, while your variable will be on pin 4.

    Eric
     
  3. peskywinnets

    Thread Starter Active Member

    Jan 19, 2009
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    0
    Thanks Eric...so with my scope probe grounded (short croc clip to the circuit ground)...I should normally expect to see a signal on pin 4?

    (All this voltage/signal referencing confuses me sometimes!)
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Yes. Though I'm not sure what sort of signal you're looking for. I see some sort of Piezo transducer. Is this a sonar or something?

    By the way....whenever in doubt, use your scope probes in the differential mode. That way, you keep all the grounding issues irrelevant. It makes for a little more cluttered prototype at times, but it saves a LOT of grief!


    Eric
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    If you have a 2+ channel scope, you can probe both pins with channels 1 and 2, one on each channel, turn on the "Add" and "Ch2 Invert" functions to get the signal. (Differential mode on old school scopes)

    A Single Channel Differential probe that doesn't need to be referenced from ground is another option, but an expensive one.

    From the diagram, There shouldn't be a signal on Pin 5, as it is fed only by C40 (1nF), and is then HF bypassed to ground with C99(10uF) and C38 (47nF). Depending on the IC internals, a DC bias level may exist, but not a lot/any of signal, compared to pin 4.
     
    Last edited: Feb 23, 2009
  6. peskywinnets

    Thread Starter Active Member

    Jan 19, 2009
    47
    0
    Many thanks for your answers...I'm going to give this (guitar) cct another go over the next night or two!
     
    Last edited: Feb 25, 2009
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