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RE: What is the problem? DAC8760 Defective parts?

Alexis CROS
Prodigy 40 points
Other Parts Discussed in Thread: DAC8760

Hello,


I have a similar problem than Um : I have 8 DAC8760 daisy chained. All voltage outputs work well, but the behaviour of curent output is strange:

Between 65535 and aprox 40000 (16 bits version), the data is well converted into a current. Under 40000, the curent output is blocked and the result remains as if I would like to convert 40000.  When I send 0 for instance, I obtain a 12mA current in 0-20mA mode. I really don't understand. I hope I explain sufficiently, let me know if you need more clues. Are you familiar with this issue?

Thanks,

Alexis

  • 0
    Prodigy 40 points
    I would append that I use boost configuration with 2N2222 transistor. The current output works well between 12-24mA, but not under 12mA : it blocks at 12mA regardless the code, provided that it's under approx 40000.
  • 0 in reply to Alexis CROS
    TI__Mastermind 46290 points
    Alexis,

    I have split your thread from the previous thread.

    This is a very odd issue. Can you provide a schematic?

    How are you measuring the current output? Have you verified there are no oscillations in the system?
  • 0 in reply to Kevin Duke
    Prodigy 40 points

    Hi Kevin, thanks for your answer,

    I'm mesuring the current with a 250 ohm resistor. I share the schematic of one of the 8 DAC chained.

    Here is a capture of the resistor voltage. On ch1 it's with the boost configuration and for ch2 I disconnected it and take the current directly on Iout of the chip.

    If I use an other value of resistor, the voltage still blocks at approx 3V (small variations according to the R value), which makes me think it's not a soft problem.

  • 0 in reply to Alexis CROS
    Prodigy 40 points
    Hey,
    I solve the problem by changing the transistor reference. I try with a BC109 and now it works well. Maybe the gain of the 2N2222 is too low (approx 50) whereas the one of BC109 is between 200 and 800. If this is the reason, maybe TI could precise it in the datasheet.
    Regards