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DAC161P997: Oscillation at output

Marvin Gasis Arcangel
Expert 8940 points
Part Number: DAC161P997

Hi Team,

Can you help with the issue below?

The customer acquired a DAC for activation with a 4 to 20mA output, as shown in the circuit image below.

We are using the DAC161P997 as a 4 to 20mA loop, the project is working, however, when we have a load cabling close to the printed circuit board (PCB) or close to the current measurement cables, the output current signal starts to oscillate. We suspect that electromagnetic waves are causing interference. We performed a test by placing a relay coil constantly turning on and off next to the circuit. Is there any way to eliminate these oscillations?

Thank you in advance.

Regards,

Marvin

  • 0
    TI__Guru* 92655 points

    Marvin,


    Basically, the DAC161P997 controls the current loop with the bipolar device at the output of BASE. Similar to an op-amp in feedback, there can be stability problems depending on how it is set up.

    Your description says that the output current starts to oscillate when the output cabling is placed close to the PCB. I would guess that the board is close to the instability and that it needs to stabilized a bit more. There is a basic discussion the datasheet describing the capacitances on the board around the device and how they may affect stability. The Stability section is on page 25, and refers to the Figure 24 on page 23.

    Excess capacitance at the positive voltage of the loop, OUT, or BASE can decrease the stability. However, this may include the capacitances from the LDOs that generate the 5V and 3.3V supplies. If this application is loop powered, series resistance (similar to RX1 in figure 24) or reducing the LDO capacitance (similar to CX1) may both help the stability.


    Joseph Wu