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OPA2333: Questions related to SBOA379 High-Performance 16-bit PWM to 4- to 20-mA DAC for Field Transmitters paper

Viktorija Cecil
Mastermind 18231 points
Part Number: OPA2333
Other Parts Discussed in Thread: REF3330, SN74LVC2G240, TPS7A16, TIDA-00167

Team, 

Follow up questions on High-Performance 16-bit PWM to 4- to 20-mA DAC for Field Transmitters article http://www.ti.com/lit/an/sboa379/sboa379.pdf:

  1. What were the PWM frequencies?
  2. What were the PWM clock source/specs?
  3. PWM scaling & Ripple suppression circuit (math and implementation)?
  4. Please give the 2nd Order LPF circuit (Vp output) information.
  5. Current consumption (< 100 µA) measurement, to which circuits does this apply?
    1. TPS7A16 + REF3330 + SN74LVC2G240 + OPA2333 + mysterious PWM scaling & Ripple suppression circuit ?  Anything else?
    2. Is the relevance of this spec the fraction it takes from the power budget 4mA maximum ?

  1. Please explain the current limiter stage: I-limit, V-drop, implementation.
  2. Please give the complete schematic and parts list.

Thank you!

Viktorija

  • 0
    TI__Genius 10850 points

    Hi Viktorija,

    I've sent you an email about this and looped in the author of the paper for further assistance.

  • 0
    TI__Expert 3510 points

    Hello Viktorija,

    1. PWM frequency is 16 kHz, but lower frequencies as low as 4 kHz has been tested as well.

    2. Most measurements were done with AWG as source, when MCU was used as a source, 16 MHz clock frequency is used.

    3. Details about the circuit and some mathematical analysis will appear in a publication on TI.com in ~July 2020

    4. Simple Butterworth, Sallen Key filter with about 1 kHz BW.

    5. This applies to all parts except pass transistor of the transmitter, so including all mentioned parts. and yes, the benefit of having low power consumption is more available current for the rest of sensor circuit.

    6. TIDA-00167 design guide gives more information about the current limiting circuit, its operation and simulation.

    7. files provided in a separate communication.

    Best Regards,

    Ahmed

  • 0 in reply to Ahmed Noeman
    Guru 140200 points

    Hi Ahmed,

    the resolution is 16bit and the DNL is +/-4LSB according to the appnote. What about the precision? And, important for control loops, does the circuit guarantee a monotonic transfer function?

    Kai 

  • 0 in reply to kai klaas69
    TI__Expert 3510 points

    Hi Kai,

    I believe the DNL is better than +/-4 LSB. I think this number is related to our test setup using AWG which has a step of 1/1000 rather than 1/256 which is expected from the PWM generator. approximating the 1/256 step resulted in systematic error. I will try to fix that by running the sweep with actual PWM generator.

    Precision need to be calculated given the tolerance of different components, I think It is in the range of 0.2% over temperature range as indicated in figure 5, and 0.1% in room temp.

    The code sweep shows that circuit is systematically monotonic. we just need to note that rms noise can reach ~2uA, while LSB = 24mA/2^16= 0.3uA. so noise fluctuations can result in non monotonic behavior (1. on a small scale of few bits, 2. if no averaging is used). but I believe this is common in any high resolution system.

    Best Regards,

    Ahmed

  • 0 in reply to Ahmed Noeman
    Guru 140200 points

    Hi Ahmed,

    thanks for the response! I highly appreciate it :-)

    Kai