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Industrial signal interface to 3.3V microcontroller ADC

Other Parts Discussed in Thread: PGA308, PROFIBUS, PGA281

As an assignment I have to design an industrial data acquisition system based on a microcontroller, using it's integrated ADC. Thus signal input range is limited to 3.3V. I am looking for a solution or reference design,  to measure the common industrial transmitter outputs with my ADC. 4-20mA signals are easy to deal with.  My main concern is voltage  measurement of signals higher than 3.3V. I have been adviced to use a programmable gain amplifier which is capable of attenuation. Although I have doubts using it, one reason being is that I hardly find any out there. Any suggestion or reference design for such a system?

Thank you in advance!

  • Hi Tanyi,

    I am going to move this post over to an amplifier forum since I feel that this question would be better answered there. We deal with interlace technologies in RS-485, RS-422, RS-232,  etc. in this forum.

  • Hi Tanyi, 

    Sounds like an interesting project you have been assigned.  You stated the VIN must be <3.3V, but did not indicate the type of signal and the parameters (min/max voltage, frequency, etc) for the signal you are to measure.  I would need to know these parameters to further direct your inquiry and provide a solution.

    Also, you can explore some suggested solutions from other E2E posts.  For example the PGA308 is a versatile device specifically for conditioning sensor signals, as described here: https://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/t/378431

    Please provide additional information and we will find a solution.

    At your service,

    ~Leonard

  • My assignment is to design a microcontroller based data acquisition system, with Profibus DP connectivity, and practically that's the only restriction. I am free to design the analog input myself. And I thought, that since this is going to be a Profibus DP device, my DAQ should be capable of measuring common industrial analog transmitter output signals, so 4-20mA and a 0-10V analog input is a reasonable choice in my opinion. The problem is with 0-10V measurent. Should I use a simple voltage divider for such measurement? My teacher's advice was to use programmable gain amplifier which is capable of attenuation. But this doesn't seem to be a common solution to me.

  • Hi Tanyi,

    Thanks for the additional info.  Yes, for a basic measurement, you could use a simple voltage divider for the input to the ADC.  Much will depend on how much resolution is required.  If you have to discern millivolts on a 10V signal, then the basic divider will not provide that level of measurement resolution.  But if you just need to see, for example, 0.5 differences of VIN, the divider will provide a quick and easy solution.

    For more detailed resolution, then yes, a PGA could offer a suitable solution.  Since it is programmable, the means to accomplish this will also have to be included in the design.  For example, the PGA281 has a programmable gain of 128V/V all the way down to 1/8VIN: http://www.ti.com/product/PGA281  .  This would allow you to set the PGA gain to 0.3, read the 10V signal as 3.3V into the ADC.  However, unless you adjust the gain depending on the voltage you are measuring, it really does not afford you any increased resolution over the simple divider.  You will still be reading a 0-3.3V signal with the ADC, and the voltage level discernment is still dependent upon the ADC resolution.

    Let me know if you have any other questions for this function.

    ~Leonard

     

  • Thank you very much! You helped me a lot deciding how to go on with my design.