Need ADC expertise in solving this problem

Discussion in 'Analog & Mixed-Signal Design' started by alwc, Oct 31, 2018.

  1. alwc

    Thread Starter New Member

    Sep 11, 2018
    10
    0
    Hi guys,

    I have connected the circuit in the following configuration as shown below. It seems ok when I used the stimulation. However, when I connect it in a breadboard at first it gives me a full wave rectifier waveform but I realised that it has negative components in the waveform. The negative part is somehow more than as shown in the second picture the input is from a function generator at 5vpp. I have tried many ways to solve the problem including connecting the circuit in PC board but still futile. I hope I am able to solve this with the help of you guys.

    Thanks
    upload_2018-10-31_17-9-23.png
    upload_2018-10-31_17-13-44.png
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
    5,954
    1,121
    hi alwc,
    Welcome to AAC.
    You say the negative component doesn't show up in the LTS sim, could you post your LTS asc file.
    Is the neg offset coming from the FuncGen, have you checked that output on a scope.
    E
     
  3. alwc

    Thread Starter New Member

    Sep 11, 2018
    10
    0
    Hello Eric,

    upload_2018-10-31_20-2-5.png

    This is the output as shown in LTspice. There are parts of negative but it seems alright as the magnitude of it is small.
    I don't think is the FuncGen, the scope is the normal scope with 50 ohms resistance like this.

    [​IMG]
     
  4. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    Please post your LTS asc file.
    E
     
  5. alwc

    Thread Starter New Member

    Sep 11, 2018
    10
    0
    Hi,

    What is asc file and how do I get it? Sorry but I am new to LTSpice

    Thanks.
     
  6. ericgibbs

    Moderator

    Jan 29, 2010
    5,954
    1,121
    hi,
    OK,
    When you create a LTS circuit layout, you save it as an asc file.
    eg: say you named and saved your circuit as ADC, it would appear in the saved folder as ADC.asc.
    You can directly upload the ADC.asc file to your AAC post, using 'Upload a File' menu button on the post window.

    We can download that file and run it in our PC's with LTSpice.

    E
     
  7. alwc

    Thread Starter New Member

    Sep 11, 2018
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    Hi,

    There you go but take note that this asc is with a smoothing capacitor so just remove it will do.

    Thanks.
     
  8. ericgibbs

    Moderator

    Jan 29, 2010
    5,954
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    hi,
    Temporarily removed the 22uF smoothing cap.
    There is a negative swing of ~0.45V below zero on the output, is this causing the problem you are querying.?
    E
    AA1 31-Oct-18 13.17.gif
     
  9. alwc

    Thread Starter New Member

    Sep 11, 2018
    10
    0
    Hi,

    I am not sure if that's is the reason. I thought that -0.45V will not cause any harm to my circuit as it is insignificant. What are your suggestions and solutions to this? And as I increase the frequency in the practical connections the magnitude of the negative will increase as well.
     
  10. ericgibbs

    Moderator

    Jan 29, 2010
    5,954
    1,121
    hi,
    If you use LTS to see the va and vb inputs on the final OPA inputs , they are not asymmetrical about zero, so there will be an offset.
    E
     
  11. alwc

    Thread Starter New Member

    Sep 11, 2018
    10
    0
    Hi,
    Oh, I didn't notice about the offset. What can I do to solve that issue?
    Thanks.
     
  12. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
    2,870
    505
    MOST of us don't consider a 50 ohm Z scope normal. 1 M || 22 pf is considered NORMAL. When you add a 10:1 properly compensated probe that gives you 10 M resistive input Z. This is about the input Z of a common DMM.

    "compensation" is only available in xA probes where A>1 The adjustment squares up the corners of a square wave. Te Fourier series of a square wave is made up of the sum of the sum of infinate odd harmonics of a sine wave. Usually x10, x100, x500 for probes.

    Mostly RF and transmission line measurements need 50 ohms.
     
  13. ericgibbs

    Moderator

    Jan 29, 2010
    5,954
    1,121
    hi,
    Could you give a little background to the project.?
    ie: why 1MHz at 6Vppk, what is the real source signal.
    E
     
  14. alwc

    Thread Starter New Member

    Sep 11, 2018
    10
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    The vpp should be around 1 to 5 as the output of this is connected to an Arduino. The input signal is taken from a voltage controlled current source.
    thanks.
     
  15. ericgibbs

    Moderator

    Jan 29, 2010
    5,954
    1,121
    hi alwc,
    You need to modify the design of the U1 and U3 circuitry in order to get 'ideally' a symmetrical input about zero into U2.
    This image showing the va and vb superimposed highlights the problem.
    E

    AA1 31-Oct-18 13.49.gif
     
  16. alwc

    Thread Starter New Member

    Sep 11, 2018
    10
    0
    Hi,

    When I place the smoothing cap inside the stimulation and change the 15K resis to 10K resis I get 3vpp as shown here. Does this helps? I still get the smoothing waveform.
    Thanks

    upload_2018-10-31_22-7-58.png
     
  17. ericgibbs

    Moderator

    Jan 29, 2010
    5,954
    1,121
    hi,
    You have moved the location of va.!
    What does the Vout waveform look like.?
    As an example I have added a voltage follower on each of the U1 and U3 outputs, so the U2 gain is not effected by the diodes on U1 and U3.
    E
     
  18. alwc

    Thread Starter New Member

    Sep 11, 2018
    10
    0
    Hi,
    Yes, I have moved it because it will eventually pass through the resistor or does it have a different meaning to that? This is the output as shown.
    Okay, I will try with the voltage follower also.
    Thanks.
    upload_2018-10-31_22-23-28.png
     
  19. ericgibbs

    Moderator

    Jan 29, 2010
    5,954
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    hi,
    The problem is that the Gain of U2 is not just simply set by R5/R6 , because R6 is connected to oV via R4 which is in parallel with the diode resistance in series with the output impedance of U1.
    So as the voltage on U1 changes, so does the diode resistance, [ which is not a linear change] and therefore the Gain of U2 changes.
    E

    AA1 31-Oct-18 14.27.gif

    EDIT:
    Try this asc file.
     
    Last edited: Oct 31, 2018
    Bordodynov likes this.
  20. ebp

    Well-Known Member

    Feb 8, 2018
    2,042
    720
    If the input is to come form a current source, then I suggest your simulations should use such. Right now they are being done with a zero impedance source. A current source (ideally) has infinite impedance.
     
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