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Original question:

DAC8871: Reference input

DAC8871: SCLK is kept low

Dukel
Intellectual 610 points
Part Number: DAC8871

Hi,

I'm not sure why, but my clock is pulled low when connected to the SCLK of the DAC

Image 1 shows the clock while connected to the DAC, image2 enlarges the clock and image3 shows while the clock is not connected to the SCLK of the DAC.

What am I doing wrong?

Connections are as followed:

Vout -> scope                    DGND -> ground
VCC -> +15V                        LDAC -> ground
Vss -> -15V                          SDI -> MOSI
AGND -> ground               SCLK -> SCLK
VREFH-F -> =15V               CS -> GPIO
VREFH-S -> nothing         RST -> 3.3V
VREFL-S -> nothing          RSTSEL -> VDD
VREFL-F -> -15V                 VDD -> 3.3V

(Clock should give 3.3V for high)

  • 0
    TI__Genius 14220 points
    Hi Dukel,

    You can check the impedance of the SCLK pin of the DAC to ground when the system is off.

    I see that you have connected -15V to VREFL-S. It should be connected to VREFL-F. Please correct that although it is not related to the SCLK pin.

    Please also check whether the SCLK source is proper or not by putting a dummy load.

    Regards,
    Uttam
  • 0 in reply to Uttam Sahu
    Intellectual 610 points
    Hi Uttam,

    It was a mistake, -15 was connected to VREFL-F. Because my power supply is nog working well, powering both the Vcc-Vss and the Reference is chose to change to VREFH-F 5V and VREFL-F 0V

    How can I check the impedance of the SCLK when the system is off?

    Thanks for your help again!
  • 0 in reply to Dukel
    TI__Genius 14220 points
    You can use a multimeter in resistance mode to do that.

    Regards,
    Uttam
  • 0 in reply to Uttam Sahu
    Intellectual 610 points
    OK, but that will give me the DC resistance. I tought you where talking about an AC impedance.

    The resistance is 3.1Ohm
  • 0 in reply to Dukel
    TI__Genius 14220 points
    Dukel,

    The impedance is too low. Looks like the pin is shorted on your board. You can either re-solder or replace the device and check again.

    Regards,
    Uttam
  • 0 in reply to Uttam Sahu
    Intellectual 610 points

    What whould a normal impedance be?

    When I set the multimeter to continuity it doesn't say it is connected to something else, is that because of internal components. (otherwise it means that is is not shorted)

  • 0 in reply to Dukel
    TI__Genius 14220 points
    It's a high-Z input. Hence, the impedance must be pretty high. At least > 10k Ohms. You can check it in normal resistance mode. Please note that it shouldn't be of the order of 3 Ohms, as you mentioned in the last post.

    Regards,
    Uttam
  • 0 in reply to Uttam Sahu
    Intellectual 610 points

    I disconnected it completely from the system, now the resistance is around 14k Ohms. There must of been still some electroncs active yesteraday, because now I measure around 9.8k Ohms while system is connected but no power is supplied. (between SCLK and AGND/DGND)

    It seems that a shortcircuit is not the problem probably . Any idea what this could be?

    Best regards,
    Dukel

  • 0 in reply to Dukel
    TI__Genius 14220 points
    Do you see still the clock voltage drop? If you are measuring around 9.8k, you should not see it. If you see it, please check the drive capability of the SCLK source by connecting a 9.8k or similar value resistor.

    If the resistance was coming to be 3.1 Ohms while the system connected last time, that means there was something coming in parallel between SCLK-GND. It may not be a hard short, but can be a similar situation.

    Regards,
    Uttam
  • 0 in reply to Uttam Sahu
    Intellectual 610 points
    Hi Uttam,

    I de-soldered the DAC and re-soldered again. The SCLK-ground resitance is now 100k ohms and the clock is going up as it should do. I let it run for a minute or sow and than it started make little sparks somewhere on the left side of the DAC. My guess is between port 4 and 5 :(

    Even while the clock was running ok, the DAC was not putting out a signal at Vout.
    Any idea why it was not giving a Vout?

    Regards,
    Dukel
  • 0 in reply to Dukel
    TI__Genius 14220 points
    Hi Dukel,

    By any chance, was there an accidental short-circuit or something? You can see if the device is damaged. If so, you will need to test with another device.

    Regards,
    Uttam
  • 0 in reply to Uttam Sahu
    Intellectual 610 points
    Yes, I think so. I Already bought a new DAC.
    I will relate a question to this one in case there is a problem.

    Regards,
    Dukel