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DAC8775: Open Wire Detection and Other Questions

Part Number: DAC8775

Hi team,

  1. Can you please help to elaborate on how DAC8775 can be used for Open Wire detection ?
  2. Open wire detection is used channel wise or all channel at once ?
  3. Is the accuracy for this DAC measured ? Can you provide any calculations for same ? (if any)
  4. Can you also please how the operation of internal watchdog for DAC ?
  5. Can we control the outputs of all channel during booting of this DAC ? how can this be done ?

Thank you! 

  • PY,

    1. For the DAC8775, the DAC can detect an open circuit for any output used as a current output. Much of this is described in the /ALARM pin section 8.3.8 on page 40. The /ALARM will be pulled low when the output is open for greater than 500us. There are internal voltage levels sensed in the amplifier driving the output current. When the amplifier is overdriven (as if trying to drive into an open circuit), these voltage levels set off the /ALARM. Note that with the buck-boost DC-DC converter enabled with full tracking, the /ALARM may be momentarily triggered as the converter voltage adjusts to drive the current load's voltage output. Also, the open circuit check is only for current output. It does not check the voltage output. However, this check is similar to the voltage output check for a short circuit current error.

    2. The open circuit detection is used for all channels. The /ALARM checks all outputs at the same time. It cannot be used to look at only one channel.

    3. The accuracy for the current and voltage outputs are checked based on the slope of the output at zero (or near zero for any voltage output zero-code errors) and at full scale. The gain error is then calculated with any errors in the reference removed. For example if the reference is 5.005V instead of 5.000V, the gain error is calculated as if the reference were ideal, with the 0.1% error of the reference removed.

    4. The watchdog timer is an alarm that ensures that communication is constantly maintained with the device. This is described in section 8.3.13 on pages 41 and 42. The idea is that there must be an SPI communication frame sent to the device within the programmed timeout period. If the device does not receive the SPI communication in that amount of time, then it triggers the /ALARM.

    5. When powering up the device, the output DAC (current or voltage) starts up disabled. For the most part, this means that the output is disconnected from either the voltage output or current output. After that, the DAC outputs are configured and enabled individually. The only thing that may be connected at startup/powerup of the device is a VOUT_x to 30kΩ resistor that may be enabled in the Power-Off-Condition. This is dependent on the Reset Config Register (address=02, bit 1). If this bit is 0, then when the device comes out of power-on reset, the VOUT_x is connected to an internal 30kΩ resistor. In the EVM, the VOUT_x and IOUT_x are basically connected together, and this resistor is another path for current when the current output is enabled.

    Joseph Wu