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DAC8562: DAC8562 has some noise on VOUTA

Chase Jeong
Mastermind 7365 points
Part Number: DAC8562

Hello TI experts,

My customer uses DAC8562 for their new product, and they have some problems.

VOUTB is okay, but VOUTA has noise about 2mVpp sometimes.

customer said that it does not occur always, and when they operate DAC8562 over 5 minutes it occurs randomly.

i attach a schematic for your convenience. (they measured the noise after removing R22, R4, R28, R11.)

and here are more information;

1. they use 5V SMPS power supply -> isolated DCDC converter 5V -> LDO(3.3V) and voltage reference (3.0V) for DAC8562.

2. they changed the value of output capacitor. 10uF and 100nF had no effects, and only they removed all capacitors, less noise observed.

3. they have one XTAL for MCU, but it is far away from DAC8562.

4. there are any specific activities from device or PCB when the noise occurs.

please check this issue, and please let me know if there are anything to test more, or need more information. Thanks.

Best regards,

Chase

  • 0
    TI__Mastermind 41573 points

    Hi Chase, 

    The capacitive load stability on the outputs is 3nF, so the two capacitors are too large. What was the noise observed when the capacitors were removed?

    Best,

    Katlynne Jones 

  • 0 in reply to Katlynne Jones
    Mastermind 7365 points

    Dear Katlynne,

    Thank you for your reply.

    Actually they did not found any noise so far, without capacitor, they said that 

    and they are using trans-impedance circuit from the output of DAC8562, they need output capacitor.

    1. is the maximum value of capacitor 1nF based on the datasheet?

    2. is there any possibility to make a noise from power circuit into DAC8562?

    3. Does the LSB affect to this noise?

    (DAC8562 is 4 LSB, and the part(other supplier) which used before is 1 LSB -> they did not found any noise with it.)

    Best regards,

    Chase

  • 0 in reply to Chase Jeong
    TI__Mastermind 41573 points

    Hi Case,

    1. Yes, the max value of the capacitor is taken from the electrical characteristics table of the datasheet. The snip I shared is from page 6 in the output characteristics section. 

    2. I do not understand what you are asking, can you elaborate on this question please?

    3. You are referring to the 4LSB INL. INL is the integral non linearity and represents the deviation of the values on the actual transfer function from a straight line. The actual value will be within 4LSBs of the ideal value. This wouldn't translate to any output noise. The noise was likely due to the output buffer oscillating from the large output capacitance. 

    Best,

    Katlynne Jones

  • 0 in reply to Katlynne Jones
    Mastermind 7365 points

    Dear Katlynne,

    Thank you for your reply.

    for question #2, i meant that is there any reason of making noise because of this power rail? (SMPS->isolated DCDC->2 LDOs->DAC8562

    and I understand that it is okay up to 1nF for output capacitor. please confirm it.

    Best regards,

    Chase

  • 0 in reply to Chase Jeong
    TI__Mastermind 41573 points

    Hi Chase, 

    Thank you for clarifying. The noise observed should not be due to any noise on the power rail or voltage reference. If so, a similar noise would be seen on channel B. The customer can check the supplies to verify, or disconnect the LDOs and check the DAC outputs using a clean supply. Yes the total output capacitance is ok up to 1 nF with infinite load impedance. 

    Best,

    Katlynne Jones

  • 0 in reply to Katlynne Jones
    Mastermind 7365 points

    Dear Katlynne,

    Thank you for your support.

    I want to ask one more question

    - Does it have possibility to have output noise if I use over 1nF capacitor on the output?

    because the part of another supplier have the same characteristics (Capacitive Load Stability=1nF) but it does not have noise at all with 1uF capacitor.

    Best regards,

    Chase

  • 0 in reply to Chase Jeong
    TI__Mastermind 41573 points

    Hi Chase,

    Can you ask the customer to send us an oscilloscope shot of the noise they are seeing on the DAC output?

    Best,

    Katlynne Jones

  • 0 in reply to Katlynne Jones
    Mastermind 7365 points

    Dear Katlynne,

    They do not have an oscilloscope shot, Just have a capture by Labview. Can you refer this waveform?

    the red dotted rectangle is the noise that they said. it is about 2mVpp.

    Best regards,

    Chase

  • 0 in reply to Chase Jeong
    TI__Mastermind 41573 points

    Hi Chase,

    I will take a look at this with my team and get back to you after the weekend.

    Best,

    Katlynne Jones 

  • 0 in reply to Katlynne Jones
    TI__Guru 53773 points

    Hi Chase,

    This plot information is pretty hard to interpret.  What are they using to measure the noise? Could this abnormal noise be present in the ADC rather than the DAC?  This looks like intense flicker noise, but that should not really be possible unless there is some kind of reference noise as well.

    Thanks,

    Paul