Grounding problem//ADC (ADS1115)

Discussion in 'Analog & Mixed-Signal Design' started by sohamkul, Aug 10, 2016.

  1. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    Hello Everyone,
    I am making a data acquisition system using a raspberry pi .
    Ive posted many questions on the forum and got great answers ,my project is in its final stage and i am having some problems,
    Let me tell you What ive done so far,
    Stage one :
    POWER and SENSOR circuit:
    so I have made an Instrumentation amplifier circuit to get differential data from the sensor which is some thing like this
    (sensor has 5 wires +- 5V output and shielding wire is grounded to smps ground)
    sensor.PNG (Pressure sensor 2mV/V)

    now the instrumentation amplifier circuit is something like this
    IMG-20160628-WA0006.jpg
    (it also ha a low pass filter)
    Sensor circuit is powered by +-5 v supply

    The output of this circuit is given to adc
    (ads1115 which is )
    index.jpg


    PROBLEM:Now the output of this adc is connected to raspberry pi as i have uploaded the pdf of full circuit
    there seems to be some grounding problem
    as The circuit is powered by smps(supply no1)
    and raspberry pi by (supply no 2)
    how do i connect grounds of adc and raspberry pi or smps ground

    heres the pdf of my circuit . I have used arrangement for 2 sensors :

    (arrangement near pin 1 and 3 is of rotary encoder +5v which works fine )
    http://forum.allaboutcircuits.com/a...8/?temp_hash=51722f05aa3ae2f4f48f1becf092af95

    Please review and help
    Thankyou in advance !!!
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,129
    266
    What is the "grounding problem"?
     
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  3. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    There are two supplies and i think that there is some problem in grounding of both circuit and raspberry pi\
    the sensor doesnot give expected output....output either goes to saturation or stars dropping down ...(voltage)
     
  4. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,129
    266
    Both circuits need a common reference point- GROUND - connect them all together.

    But understand that ADC noise performance is greatly influenced by HOW and WHERE you connect them together.
    Cross that bridge when you come to it, it may not be a problem in your application.
     
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  5. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    Yes !! Thank you for suggestion but i have already tried it... i dont know whats going wrong ...
    Should i connect raspberry pi and Smps grounds together ?? Can you please review my circuit
    ...
    Thank you !
     
  6. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,129
    266
    As usual... Break the problem down into blocks- test the blocks...

    1) do you see the expected voltage DIRECTLY ON THE SENSOR?

    2) do you see the expected analog signal at the input of the computer ADC?

    3) to get better advice, layer your documentation, start with a high level block diagram, then add detailed schematics.
    It's fun helping people succeed, but it's boring wading through schematics when you cannot see a clear picture of the whole system. When you speak of the power supply, I have no idea where or what that is, or how it's connected, and little patience to bother figuring it out.
     
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  7. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    ill do that asap and add details photos and block diagrams !!!
    Thank you !!:):):)
     
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  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,907
    2,168
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  9. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    Yup i have read it !! i am just confused as like :
    when i dont connect raspberry pi across the circuit the output on pin 0 and 1 of adc is exactly what i want :
    its output of differential amplifier somewhere between 0-3.3 volt i.e 0-32767 adc counts but as soon as i connect raspberry pi the output starts to go bizzare like fluctuale and go to saturation ...
     
  10. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,144
    203
    I'm having a hard time understanding the stuff presented, so I'll pick something:

    The shielding wire should be connected to earth at one end only. "Ideally" at the sourced or sensor end.
    You want to enclose the output with Earth's potential without any loops.
     
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  11. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,907
    2,168
    Have you disconnected the sensor and used a jumper to set both differential inputs on your amp to the amp circuit ground for a null output signal. Then connect the ADC/RPi to the circuit to see if the problem happens.
     
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  12. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    I ll check right away !!!
     
  13. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    i just checked the circuit by shorting ground and two inputs (sensor) output still jumps to 3.3v i.e saturation !!
    so raspberry pi is out of the picture // what might be the reason ?
    also what is the procedure to connect two circuits which have different power sources and different grounds //

    Thank you !!
     
  14. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,144
    203
    Shorts are bad things. There''s no place for Ib to go, the OP amp saturates.

    I'm not sure what the out put of your sensor interface looks like to the A/D, but if not present add some series resistance. Even 50 to 100 ohms will do.
     
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