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XTR116: Overshoot pulses in PWM isolated 4-20mA

Pablo Liberman
Prodigy 130 points
Part Number: XTR116
Other Parts Discussed in Thread: ISO7310C

Hi,

I made the following circuit from Collin Wells' blog:

Everything works well except for aprox. 100uA overshoot pulses on output current. Frequency was the same of PWM (10Khz):

I couldn't find out where the pulses are generated.

Any idea?

Thanks

  • 0
    Guru 140200 points

    Hi Pablo,

    two issues come into my mind. There could sit too much undesired stray capacitance across the barrier between GND1 and GND2. Or the low pass filter labelled with "Rflt Cflt" is not properly working.

    Can you measure the voltage across Cflt with a scope? (But take care, most scopes have signal ground connected to protection earth!)

    A cap across Rload will usually decrease the unwanted overshot.

    By the way, are you sure that the current flowing out of the Vreg pin and the Vref pin are under 3.7mA (see the datasheet)?

    Kai

  • 0 in reply to kai klaas69
    Prodigy 130 points

    Hello Kai,

    Thank you for your comments.
    I checked everything you said.
    ISO7310C current is 1.8mA:

    Finally I replaced 10nF output capacitor by 100nF and the overshoot disappeared.

    Best regards,
    Pablo

  • 0 in reply to Pablo Liberman
    TI__Mastermind 49405 points

    Pablo,

    Thank you for letting us know, let us know if you have other questions.

    Kai,

    Many Thanks for your inputs,

    Best Regards,

    Luis