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XTR117 and Arduino PWM analog Interfacing

user4599672
Prodigy 190 points
Other Parts Discussed in Thread: XTR117, LM35, OPA347

Dear Experts,

I am trying to interface Arduino PWM out in which 1-5 volts as required by XTR117 input is mapped into PWM and filtered out. Then it needs to be connected to XTR117 input. Can you please suggest a suitable circuit diagram and input filter component values and XTR117 input resistor. I am very new to current loops and need to design a demo tryout version. Sincere thanks.

Ronie.

  • 0
    TI__Guru 62035 points

    Hi Ronie,

    Here's a schematic for a design that performs with good results.  As always be sure to build a prototype and test thoroughly before going to production.

      PWM+XTR117.pdf

  • 0 in reply to Collin Wells
    Prodigy 190 points

    Why is the rectifier required for in the output current loop ? I am using 24 VDC Industrial DC supply. Is it still required ?

  • 0 in reply to user4599672
    TI__Guru 62035 points
    We recommend including the protection components including the TVS diode, ferrite beads, input capacitor, and rectifier if you're going to release this as a production system. TVS diode, input capacitor and ferrite beads help with ESD, EFT, surge, and other transients that may otherwise damage the transmitter. These transient disturbances are very common in industrial environments and every production transmitter should feature this type of protection circuitry.

    The diode bridge provides a simple method for reverse polarity that allows the transmitter to function with the two wires connected in either polarity.
  • 0 in reply to Collin Wells
    Prodigy 190 points

    In the PDF I think the resistor R6 shall be 24.75K for 40uA at 1V and 200mA at 5V inputs. Can you please clarify.?

  • 0 in reply to user4599672
    TI__Guru 62035 points

    Hi User,

    We recommend using the values shown in the schematic we attached.  The reason the gain (R6) and offsetting (R1) resistors were set the way they were is that the output of the PWM signal never reaches true 0V or 5V, and neither will the op amp output.  Therefore the values shown worked well for the digital output levels coming out of the ISO7310 product.  As you can see in the TINA simulation file I attached, a ~120mV input results in a 4mA output and a 4.5V input results in a 20mA output.  I've attached the simulation file for you to test with

    XTR117_PWM_DC.TSC

  • 0 in reply to Collin Wells
    Prodigy 190 points

    I am using OPA347 rail to rail opamp with single supply and arduino pwm that can reach 0 and 5 volts. I think I have to experiment and see what happens in either configurations. I rememver before there was a post for interfacing LM35 with xtr117 with same configurations.

  • 0 in reply to user4599672
    TI__Guru 62035 points
    Hi User,

    No op amp can provide an output voltage equal to the supply voltages. It can get close, but not all the way there.

    The Arduino PWM outputs will have similar limitations. Go look for the "Voh" and "Vol" logic levels for the Arduino. They will be close to VCC and GND, but depending on the output current they will not reach all the way to either supply voltage.
  • 0 in reply to Collin Wells
    Prodigy 190 points

    I got it....The offset is for compensating the loss for imperfect rail voltages. Thanks, will test and get back to you.

  • 0 in reply to Collin Wells
    Prodigy 190 points

    Hello sir,

                    Can you please give me exact 40V TVS diode model used for the schematic you forwarded me ? I would like the part number for getting an equivalent model from my local TI reseller.

  • 0 in reply to user4599672
    TI__Guru 62035 points

    It was a 36V TVS diode from Bourns, part # below:    

    CDSOD323-T36SC

    The SMAJ series devices would work as well.

  • 0 in reply to Collin Wells
    Prodigy 190 points

    Dear sir,

                If I don't want ISO7310CDR digital isolator then shall I connect AVR microcontroller PWM directly to RC filter network ? Are there any changes in component values if I do so ? Anything to be taken care of for this ? I am trying to make a tabletop demo system for now.

  • 0 in reply to Collin Wells
    Prodigy 190 points
    If I remove ISO7310 then shall there be any changes in component values ? I am only making a tabletop demo now.
  • 0 in reply to user4599672
    TI__Guru 62035 points
    You may remove the ISO7310 from the system if the AVR is powered solely between the VREG and IRET pins of the XTR117. If the AVR is powered from another source or is receiving information over a non-isolated interface (JTAG, USB, etc.) then you may have isolation issues.

    You may need to adjust some of the gain and offsetting resistor values if the output levels of the AVR are different from the ISO7310.