Measuring Voltage up to 4 decimal places using TDS3032 Oscilloscope

Discussion in 'General Electronics Chat' started by Tahsina Hossain, May 11, 2015.

  1. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
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    Hi Everybody,

    I am trying to capture waveform data from TDS 3032 oscilloscope through RS232 port. All samples have 1 decimal place data only (i.e. 20.3V) whereas I need something like 20.2985V. Is there any way to get 4 decimal places from oscilloscope?

    I really appreciate any help.

    Tahsina
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    No way. Most digital scopes are 8 or 9-bit resolution.
    The TDS3032 is 9-bit vertical resolution, that is 1 part in 500 resolution.

    Do the math. Four decimal places such as 20.2985 is a resolution of 1 part in 200,000.
    You would need an 18-bit ADC and less than 100μV noise.
     
    Last edited: May 11, 2015
    Tahsina Hossain likes this.
  3. crutschow

    Expert

    Mar 14, 2008
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    To get higher resolution you would need a fast, high resolution digitizer (A/D converter). But you likely won't be able to reach 18 bits.
    What is the highest frequency of interest in your signal?
     
  4. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
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    Thanks a lot for your reply. Can you suggest any oscilloscope that can do this?
     
  5. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
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    Well, the voltage signal is 50Hz. Is this what you are asking?
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Yes. That eases the problem somewhat.
    So why do you want such high resolution for a 50Hz signal (I presume this is the main's voltage)?
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    I am not aware of any oscilloscope with that kind of resolution.
    You need to find a DAQ system. Search google for high resolution DAQ.
     
  8. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
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    Basically I have a sense resistor (0.156ohm) at an inverter output. I am using two probes to acquire voltage level before and after the sense resistor so that I can deduct one from another to get the voltage drop across the sense resistor. The objective is to find inverter output current. However, since I am getting only one decimal place, the two probes are giving something like 19.5V and 19.4V but this is not giving me proper voltage drop. According to my theoretical calculation, the inverter output current is 62mA, which means there should be 0.009672V drop across the sense resistor. I hope I could explain my problem to you.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    Leave the two probes where they are and select the MATH SUB feature of the oscilloscope.
     
  10. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
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    That is the obvious thing to do, right? So I tried that a lot of times. The probe voltages are almost similar in amplitude (around 20V) which means the result of MATH function is a very low voltage (milli-volts). So when I change the vertical volts/division to milli-volts level, say 100 mV/division what I see in the scope is almost like a noise rather than expected outcome. I captured the data (which seemed like noise) and observed manually that the data values are not consistent, means its noise plus data as the data is at milli-volts level. What can I do?
     
  11. MrChips

    Moderator

    Oct 2, 2009
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    Have you tried using a DMM to measure the voltage across the resistor?
     
  12. crutschow

    Expert

    Mar 14, 2008
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    You can't change the scale to mV levels when using the math function since that causes the the scope input amplifiers to saturate.
    You need a differential amplifier with good common mode rejection at a high voltage to measure that voltage. TI, among others, sells such amps.
     
  13. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
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    Hmm, seems like I am out of options at hand at the moment :-(
     
  14. crutschow

    Expert

    Mar 14, 2008
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    Why not use a DMM?
     
  15. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
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    I have fluke 39 but it cant measure current below 1A, my output current should be around 62 mA
     
  16. DickCappels

    Moderator

    Aug 21, 2008
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    You might want to use a high accuracy DMM to measure the voltage across the resistor.
     
  17. tcmtech

    Well-Known Member

    Nov 4, 2013
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    You could just change the resistor out to a larger value one and increase the voltage drop across it or you could do a direct millivolt reading across the resistor and calculate your current from that.
     
  18. Tahsina Hossain

    Thread Starter Member

    Jan 16, 2015
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    Hi, exactly what I did last night and things seem to work now. Thanks a lot for your great suggestion.
     
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