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4-20mA input with hight precision (0.1%)

Arthur Deniau
Prodigy 50 points
Other Parts Discussed in Thread: TIDA-00119, ADS8638

Hi,

We need to measure 8 channels 4-20mA, up to now it is easy. But we need an accuracy of 0.1%.

To convert the analog input we will use an 12bit or better ADC.


I found schematic of your TIDA-00119 board, but in the design feature it is write that the accuracy of the input current is 0.35%.

Can you have an exemple of analog input with overcurrent protection and 0.1% of accuracy?

Arthur

  • 0
    TI__Guru** 113765 points

    Hi Arthur,

    Your accuracy will depend on the total error of your system as RSS (root sum squares) of each of the error sources.  You must not only include the initial front end portion of the input, but also the ADC errors as well.  Some of these errors can be calibrated out of the system.  So there is calibrated error and uncalibrated error.

    Error sources can be from leakage paths (TVS diode leakage for example), drift, component tolerance, offset, gain and linearity to name a few.  The circuit you have shown in the post may work just fine to meet your goal with the right ADC/temperature range.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 50 points

    Hi Bob,


    Thanks for your answer,

    I think we will use this circuit, with a software two point calibration.
    The unknow error source is non linearity of component with temperature (+60,-40). We will use a climatic chamber for that.


    About the ADC, I think use the ADS8638. A 12 bit ADC is enough or do you think is better to use a 16bits?

    We want add a fuse, do you think that is better to integrate it after R89 or between C33 and R31?


    Arthur

  • 0 in reply to Arthur Deniau
    TI__Guru** 113765 points

    Hi Arthur,

    The 12-bit performance of the ADS8638 may be good enough, but I'm really not that familiar with the SAR product line or this particular device.  If the max spec of the INL performance is used, then there may be some problem hitting the accuracy target.

    As far as the fuse and placement is concerned, the fuse is a protection mechanism and really depends on what you are trying to protect in the circuit.  D16 will clamp at a voltage high enough so that the current could be significant through the R31 resistor.  The same is true for R89, and in fact will most likely act as a fuse for any prolonged overvoltage condition.  You need to determine what condition(s) will most likely occur that can damage your circuit, then provide the proper protection.  The TI Design circuit is primarily to protect from ESD and EFT events.

    Best regards,

    Bob B