DAC7678: Addressing problem

Part Number: DAC7678

I am using eight DAC7678 in a system. The addresses are set for each one by jumpers on the card. All addresses for which the address pins are pulled high or low work properly. All addresses for which one pin is left open do not. They behave as if the pin is pulled high. What is the expected input voltage on a pin which is left floating? How can I fix this issue?

10 Replies

  • Hi David,

    Welcome to E2E and thank you for your query. Floating means, the voltage when easured on these pins should be < VIH-min and > VIL-max, so that it doesn't get detected as HIGH or LOW. Could you please measure the voltage at these pins?

    Regards,
    Uttam Sahu
    Applications Engineer, Precision DACs
  • In reply to Uttam Sahu:

    They all measure 5V when open.

  • In reply to David Siddons:

    On further investigation I discovered that the address pins had been installed with pull-ups to 5V, which explains that. Sorry for that misinformation. However, when I removed the pull-ups, the "floating" pins now measure 0V, and the addresses follow that.

    I also tried adding a 10k resistor divider to the address pins, and verified that when left open they sit at 2.5V. Under these conditions, the addresses look like zeros when left floating at 2.5V.

    Has anyone else tried to use these "floating" addresses for this device?

  • In reply to David Siddons:

    Hi David,

    I am checking with the internal team. I will get back by EO this week. Thank you for your patience.

    Regards,
    Uttam
  • In reply to Uttam Sahu:

    Hi David,

    Could you please check the impedance between these pins w.r.t. VDD and GND without connecting ?

    Regards,
    Uttam
  • In reply to Uttam Sahu:

    I see two values, depending on the polarity of the multimeter. WIth the multimeter red probe on the pin, I see 560kohm to both GND and VDD. With reversed probes I see 270kohm to both GND and VDD.

  • In reply to David Siddons:

    Hi David,

    I got some response from the design team and it is as follows:

    1. The address pin gets sampled at device start-up, and then is ignored after that, until there is another power-on-reset event. This sampling also relies on a programmed OTP bit to get read correctly at power-up, only then the pin float detection circuit is operational
    2. The internal design doesn't seem to have a method by which the device would be pulling the pin low internally, unless the device was already damaged somehow

    So, we can debug the issue as follows:
    1. Remove the DAC device and measure the “floating” address pin(s) voltage, and resistance to VDD/GND
    2. Truly float the address pin(s), and then power-cycle the part with as fast a ramp rate as possible (less than ~1ms if possible) from 0V to 5.5V, and then see if the proper address is working
    3. Are you seeing this issue across multiple devices and boards or is it specific to one board or device?

    The designers think that perhaps either the OTP bit isn’t getting latched by the time the power-on-reset signal expires, or some resistive path is pulling the pin low. This effect is worst at slow supply ramp rates

    Hope that provides enough probe points to debug further. Do share the results with us.

    Regards,
    Uttam
  • In reply to Uttam Sahu:

    Hi Uttam,

       That is quite a lot of stuff ! It will be a while before I can get to it. The project consists of eight daughter cards supervised by a ZynQ-based controller. I have attached a picture of the daughter board. Conn6 and Conn7 are the DAC address jumpers, and the chip is just to the right of them. R28 & R29 were the pull-ups which I have removed. Luckily for me, I had provided for standalone operation of the board by providing trimmers for each of the values to be controlled by the DAC, so I can work with them, however painfully!

    I have no control over the power supply turn-on rate. It is a small 5V switcher module. I can easily imagine it takes longer than 1ms for it to come up, so to make the test you propose will be a project for me. I will ask around to see if something is available which could test the hypothesis.

    Pete.

  • In reply to David Siddons:

    Hi David,

    I understand your point that the power-supply ramp control is a difficult task. You can try de-soldering the DAC's on which you are using the floating option and measure the voltage and resistance at that point.

    Regards,
    Uttam
  • In reply to Uttam Sahu:

    HI Uttam,

    OK, I removed the chip. Both inputs now measure open circuit.

    Before I removed it, I re-checked the voltage using the 'scope probe rather than the multimeter. Still shows 0V.

    The power-on takes about 4ms.

    Pete.