• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 46 subscribers
  • Views 1154 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DAC161P997: DAC161P997 output current oscillation (ringing) problem

user4457438
Prodigy 30 points
Part Number: DAC161P997

We need help regarding DAC161P997, an oscillation waveform (ringing) is observed in the loop output current.

An oscilloscope snapshot is attached. The DAC’s output is connected to GND via a 100Ohm resistor when the DAC is commanded to output 20mA, and the oscillator probe is attached to the resistor, the voltage across the resistor is expected to be 2.0V DC (20mA * 100Ohm), but  big saw tooth oscillation can be seen.

PCB Drawing.pdf

We initially suspected the PCB layout, yet we made several attempts to improve it but failed.

We are confident that the timing of digital signal (DIN pin) is correct, the output current’s DC content is correct.  

All related schematics and PCB layout (4-layer) is attached in the PDF file, please help us. Our design progress are blocked at this stage.

  • 0
    TI__Guru 53563 points
    Hello,

    Firstly, some clarification on how you testing your device would be helpful. You say that you are using a 100Ω to GND. On this device you might consider GND the COM connections, but if you want to measure the current output of the from the OUT pin, it does not really make sense to measure it against the COM connections. The device is trying to regulate the current flowing through the supply loop. I think it would make more sense to put your 100Ω in series with the ISO_+24V or 4-20MA connections and measure voltage across it.

    In addition, I do not believe the capacitor C23 is necessary, and could possibly be causing your issue as well. In the DAC161p997 datasheet we show a suggested application in section 8.2. You will notice that we have a capacitor connected across the loop inputs, but not to the COM net. In addition, the input capacitor for the DC/DC is isolated with a series resistor. You should try removing C23 and see if the oscillation reduces.

    Paul
  • 0 in reply to Paul_Frost
    Prodigy 30 points

    Wow. you're a genius, the oscillation stopped as soon as the capacitors you mentioned are removed. And regarding the first 'clarification', I know how to connect the resistor very well, thank you.

    The problem is therefore solved. If I need further help I will re-open this thread.

    Again, thank you very much, Paul