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High Pass Filter-XTR105

Saif Malik
Prodigy 100 points
Other Parts Discussed in Thread: XTR105

Hi there,

Is there any way to put a high pass filter on XTR105 and how to connect it please?

Thank you

Regards,

SM

  • 0
    TI__Guru 62085 points

    Hello SM,

    The XTR105 is designed to create 2-wire 4-20mA RTD sensor transmitter designs.  RTD sensors operate at low frequencies near dc and the 4-20mA output is dc as well, therefore we're not sure how you're trying to use the high-pass filter.  Are you trying to apply a high-pass filter to the input signal or on the output?

    Please provide a little more information and we'll work towards a solution.

  • 0 in reply to Collin Wells
    Prodigy 100 points

    Hi there,

    I have highlighted in color in pins 13 and 2 where i put high pass filter. I actually want the filter to block any frequencies less than 225 MHz. I also added a ferrite bead in input signal for same purpose.

    Thanks for your help

    Regards,

    SMCircuit Diagram.pdf

  • 0 in reply to Saif Malik
    TI__Guru 62085 points

    Hello SM,

    The XTR105 has a bandwidth between 5kHz and 50kHz depending on the system gain as shown in the Figure below.  Therefore, operation of the XTR105 above these frequencies is not recommended.  What type of 0-100C temperature sensor system are you designing that operates at frequencies above 225MHz?  Perhaps you're interested in low-pass filtering the inputs of the XTR105 to prevent high-frequency noise from being transmitted to the outputs?  If so, an example of low-pass filters for the inputs is shown in the second figure below.

    Input Filtering XTR105:

  • 0 in reply to Collin Wells
    Prodigy 100 points

    Hi Collin,

    Thank you for your useful reply. This filtering is required for EMC purposes (Radiated Immunity Test from 80MHz-1GHz) as the design in datasheet without low-pass filters is not good and this causes the temperature reading to drop massively and the whole system TRIPS because no input temperature sensor is detected due to reading below 0C. I hope that by putting low pass filters as you suggested and adding two ferrite beads on supply lines should resolve the issue otherwise there are serious issues with XTR105 in the field.

    P.S. Is it possible to simulate these circuits in TINA software having XTR105 and other components in it?

    Thank you

    Regards,
    Saif Malik

  • 0 in reply to Saif Malik
    TI__Guru 62085 points

    Hello Saif,

    2-wire transmitter designs that must remain robust against radiated emissions must be properly filtered and protected or they will commonly experience issues such as the ones you're experiencing.  You can set the cutoff filter of the low-pass filters based on the standard RC low-pass filter equation:  fc = 1/(2*pi*R*C).  Depending on the desired bandwidth of your transmitter you could set the filter to a pretty low value. 

    Be sure to use surface mount or non-wire wound through-hole resistors in this design to minimize the chances the resistors will act like antennas for the frequencies you're trying to reject.

    In addition to the input filters and output ferrites we also recommend that you consider placing a transient voltage suppression element and the recommended 10nF decoupling capacitor.  The reference design linked below shows a low cost 2-wire 4-20mA sensor transmitter design that has been tested against the IEC61000-4 suite of transient and radiated immunity tests.  Consider using the diodes, capacitors, and ferrites shown in that design to protect your design as well. 

  • 0 in reply to Collin Wells
    Prodigy 100 points
    Hi Colin,

    Thank you for your reply. I just got my PCB fabricated and tested it. It does not work as it should. I always get the temperature creeping up. My room temperature reads 25C and my PCB goes from 25.4C to 29C in just less than 5 minutes and continues creeping up.

    Could you please provide any solution? It's a 2-wire PT100 temp sensor for 0-100C using XTR105U.
    My values for resistors are:
    Rz=100 Ohm
    Rg=82 Ohm
    Rcm=1K Ohm
    RLin1=33K Ohm
    RL=249 Ohm

    Looking forward for your reply. Thank you

    Regards,
    Saif Malik
  • 0 in reply to Saif Malik
    TI__Guru 62085 points

    Hello Saif,

    Can you provide more details about what you mean by "I always get the temperature creeping up" ?

    Is the 4-20mA output continuously increasing, or are you saying that the temperature of the PCB with the XTR105 is increasing?  Please attached your schematic and PCB layout so we can take a look at the design.