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XTR115: Oscillation Issue

gaston vallasciani
Prodigy 95 points

Part Number: XTR115
Other Parts Discussed in Thread: ISO7310C,

Hello,

I need to control a 4to20mA loop with a microcontroller. I send you the schematic. 

For the load i am using a RD-125D source (so i can simulate the 24volt source) a 250 ohm resistor connected in series between the bornier and the V- connection of the source.

i am using the potenciometer to adjust the zero current at 4mA.

The problem is that there are oscillations at the output. Here i send you an image of that.

 

Maybe the problem is the filtering of the pwm i think. I am using a TIP31C for the output TBJ.

Can you check the circuit and tell me if it is ok ? or if you have an improvment or modification for the circuit i will be very grateful.

From now thank you.

Greetings,

Gastón

  • 0
    TI__Mastermind 23050 points

    Hi Gastón,

    The output of the ISO7310 is likely not stable with a load capacitance of 100nF and only 10 Ohms of isolation resistance. I would recommend reducing C4 and balancing your input resistance between R8 and R9 to achieve the desired input level at 100% duty cycle.

  • 0 in reply to Zak Kaye
    Prodigy 95 points

    Hi Zak,

    well i have seen on the datasheet that i have to put a 100nf capacitor between 1 and 3 pin and other 100nF capacitor between 7 and 8 pin of the ISO7310c for improving the performance.

    So i do that with the next schematic,

    And the amplitude of the oscillations decreased but i still have the oscillations. I think i have a stability problem with the op amp of the xtr115.

    Also, i have seen on the xtr115 datasheet that there is a compensation circuit for Vref if you have a capacitive load attached to it ,but this is not my case because i have R7 and R9 before the capacitor.

    Don't you have a compensation circuit that i can try ?

    Regards,

    Gastón

  • 0 in reply to gaston vallasciani
    TI__Mastermind 23050 points

    Gaston,

    I do not believe the XTR is the source of the instability because you have 20kOhms between C4 and the input of the internal XTR op amp. If this is a stability issue, it is likely stemming from the ISO7310 trying to drive 100nF capacitance through 10 Ohms. I would suggest changing R8 and R9 to 10kOhms and changing C4 to 100pF to see if this improves performance.


    The cutoff frequency of your low pass filter seems rather high though if you are trying to convert the PWM signal to a dc input voltage. What is your modulation frequency?

  • 0 in reply to Zak Kaye
    Prodigy 95 points

    Hello Zak,

    You were right. i have devided R9 in two resistances of 10kohm and between them i used a capacitor of 1uF instead of 100pF because i need more filtering on the pwm (it has a frequency of 15kHz). And the ouput now is stable.

    i don't know why i put thar resistor of 10 ohm the cut off frequency was too big like 159Khz. 

    An other doubt i have is about R7. As i am going to control the pwm with a microcontroller with a voltage between 1 and 4 volt, in that case i don't need R7. R7 only set an offset to the current loop. what i say is fine ?

    Regards,

    Gastón

  • 0 in reply to gaston vallasciani
    TI__Mastermind 23050 points
    Hi Gastón,

    You are correct, R7 is used to add an offset to the current loop. So for example, a 0V input could correspond to a 4mA output by adding a 40uA offset. In your case, a 1V input is going to correspond to a 5mA output with no offset, so you may actually need to increase your input resistance if you want your minimum output to be 4mA.
  • 0 in reply to Zak Kaye
    Prodigy 95 points

    Hi Zak,

    Thank you, you were very helpfull and accurate with your answers.

    Regards,

    Gastón