How to Record a DC voltage in the order of millivolt

Discussion in 'General Electronics Chat' started by ElectAm, Feb 11, 2019.

  1. ElectAm

    Thread Starter New Member

    Feb 11, 2019
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    Hello everyone,
    I searched in the net about a voltage recorder but unfortunately I can not find any with the following performance:
    -Record DC voltage of the order of a few millivolts with a sensitivity of 0.1 mv
    -Sampling frequency: 200 hz (if it is possible an adjustable frequency that can reach 200 hz)
    I want to measure DC Voltage at the terminals of a wire that leads a DC current of 100 mA, the following diagram illustrates the assembly : upload_2019-2-11_14-43-0.png

    Thank you very much for any help.
     
  2. Reloadron

    Distinguished Member

    Jan 15, 2015
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    You are looking at a data logger capable of 200 samples / second. That is doable. You are also looking at an A/D (Analog to Digital) resolution of 100 uV (0.1 mV) which makes things a little more complex. It looks like you want to measure the voltage drop of a section of wire with a 100 mA current through it? You make no mention of the anticipated maximum voltage? How many samples will you take (duration) or where you plan to put the logged data?

    Ron
     
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  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Could you wrap several turns of the wire around a Hall effect sensor (assuming you can source a suitably sensitive one)?
     
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  4. ElectAm

    Thread Starter New Member

    Feb 11, 2019
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    Hi Reloadron,
    Thank you for the reply, the anticipated maximum voltage is 2.1 millivolts.
    Voltage recording must follow the following cycle :
    • Recording at a minimum frequency of 200 Hz (measurement sampling frequency) for one second.
    • Recording at a frequency of 1 Hz every 15 minutes.
    upload_2019-2-11_17-25-8.png
    This cycle will be repeated 500 times, that is to say a recording for almost 130 continuous hours!
     
  5. pmd34

    Member

    Feb 22, 2014
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    You could look at using a "current sense amplifier" such as the INA180 to get your tiny voltage signal up to a more robust voltage. Then use something like an arduino / Atmega microcontroller with ADC and serial communication to do the logging. If the on-board ADCs are not up to scratch, have a look at the Atmel application information on "oversampling" which tells you how to achieve better resolution.
     
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  6. ElectAm

    Thread Starter New Member

    Feb 11, 2019
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    Hi Alec_t,
    Thanks for your remark, but I think this solution is not practical for precision reason.
     
  7. Reloadron

    Distinguished Member

    Jan 15, 2015
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    Here is a little of what you are looking at. The 200 Hz sample rate is 200 samples/Sec. That becomes 200 * 60 = 12,000 samples / min. or 720,000 samples / hour.

    Here is where you will have problems. When doing an A/D (Analog to Digital) conversion we have a bit count. That will be a power of two. You will hear terms like 8 bit (2^8), 10 bit (2^10, 12 bit (2^12) and so forth. That bit count determines the resolution of your measurement plane along with the A/D reference voltage.

    I saw an Arduino mentioned. The Arduino is a 10 bit A/D device and can use a 3.3 or 5.0 volt reference so using a 5.0 volt reference with a 10 bit ADC we get 10 bits = 1024 quantization levels (2^10). 5 volts / 1024 = 0.00488 or 4.8 mV resolution. Not all that great for what you want. How about a 12 bit A/D? Now we get 5 volts / 4096 = 0.00122 or about 1.22 mV resolution. See what is happening here? Also, as we increase the bit count we also increase the cost of the A/D converter. Also, with very low level signals the noise becomes a factor along with any A/D errors. So while a 16 bit is nice and would yield 5 volts / 65536 = 0.0000763 or 763 uV resolution there will be noise. Another "trick" is to take samples using software and then average the samples.

    Next problem is when we data log or collect data it's like collecting anything else, we need someplace to put it and a format to store the data in. Using a personal computer based data acquisition system we can store the data on the hard drive. I could suggest a DI-710 Data Acquisition and Data Logger Products or similar but you can see the cost. You can likely find less expensive systems.

    I would think about some of what has been mentioned not just by me but in the thread. You may really want to consider an IA (Instrumentation Amplifier) to get your low level DC up to a more practical range. ADC devices like my link are available from dozens of manufacturers you need to look at the features you need/want against cost of what is available out there. The link is a turn key plug and play solution and if you can write your own code there are certainly less expensive alternatives.

    Ron
     
    Last edited: Feb 11, 2019
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  8. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
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    Keysight 34970A Data Acquisition / Data Logger Switch Unit might work.
     
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  9. ElectAm

    Thread Starter New Member

    Feb 11, 2019
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    Thank you for the idea KeepItSimpleStupid
     
  10. ElectAm

    Thread Starter New Member

    Feb 11, 2019
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    Reloadron thank you so much for all the information you mentionned, this is really helpful and made lot of things clear. But I think I'm not going to make the A/D conversion with arduino because the device I look for is needed for an industrial application and I'm looking to buy a sophisticated device with precision, that directly records with integrated functions of sampling and amplifying, cost is not a matter.
     
  11. pmd34

    Member

    Feb 22, 2014
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    Hi ElectAm, if cost is not an issue, then you could purchase a "keithley" meter, they are top of the range for sensitivity and can be interfaced with via the RS-232 or USB so can be polled to take reading. Depending on which meter you chose, some can also be programmed to record a set number of readings.
     
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  12. danadak

    Well-Known Member

    Mar 10, 2018
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    Are you looking for a turn key solution, or a development where you do the coding ?
    Reason I am asking is there are micros with DelSig that can resolve to uV.

    Is this a one off design ?


    Regards, Dana.
     
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  13. ElectAm

    Thread Starter New Member

    Feb 11, 2019
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    Hi Dana,
    Thanks for your comment, well, because of time constraint, I prefer to look for a turn key solution, meaning if there is a sophisticated product in the market that meets the requirements to avoid time delay and further development problems.
    Kind regards
     
  14. ElectAm

    Thread Starter New Member

    Feb 11, 2019
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    Hi pmd34,
    Very good device the keithley product, I'm enjoying choosing a multichannel one.
    Thank you so much and thanks for all of you.:)
     
  15. Reloadron

    Distinguished Member

    Jan 15, 2015
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    I would not use an Arduino for the reasons I mentioned. It's only a 10 bit A/D. Fot a commercial application I would look to DATAQ Instruments or Measurment Computing or any of the other commercial lines of data acquisition units available. Those and others offer good commercial application equipment and include adequate software packages to get started. That includes data logging software.

    Ron
     
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  16. ElectAm

    Thread Starter New Member

    Feb 11, 2019
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    Ron,
    Once again, thank you for this information,I'm checking the site now I hope I found one:)
     
  17. BobTPH

    Senior Member

    Jun 5, 2013
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    Lots of bad advice in this thread. He needs 2.1mV range with 0.1mV resolution, and people keep telling him that an 8 bit ADC, or even 12 bits is not enough. Bullstuff. He only needs accuracy of one part in 21. 5 bits of ADC is sufficient. 8 bits is perfectly reasonable.

    The problem is not the resolution of the ADC it is the range. You need to amplify the signal to get it up to the range of the ADC. Someone suggested an instrumentation amplifier. This is good advice. Once you have turned the 0 - 2.1mV range into 0 to 5V or 0 to 3.3V depending on what micro you are using, a micro with an 8 bit ADC will do what you want.

    Bob
     
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  18. ElectAm

    Thread Starter New Member

    Feb 11, 2019
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    Thanks Bob for clarifying that:)
     
  19. danadak

    Well-Known Member

    Mar 10, 2018
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    The observation about the limited range implies low res ADC a real good one.
    I missed that :(.

    But do an end to end error analysis to see if you are getting the accuracy desired.
    IA costs money, and there is a temptation to roll your own IA and that leads to
    some serious inaccuracies due to Aol differences between OpAmps and R ratio
    accuracy and CM issues. And Vref onchip errors.

    Regards, Dana.
     
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  20. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
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    Keithley and Keysight (the newest HP) are the places to go. I basically remembered that HP and Fluke of yesteryear had a lot of stand-alone data loggers. For DC measurements, I used a lot of Keithley stuff - mainly electrometers.
     
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