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DAC8218: offset DAC usage in asymmetrical bipolar mode

Part Number: DAC8218
Other Parts Discussed in Thread: DAC81408, DAC71408

Hello,

I need to generate several analog output signals with voltage range -0.5V/+28V. DAC8218 seems the only available option, but what is written at page 39 of the datasheet about offset DAC usage is not clear to me.

Could the following setup work?

  • Bipolar operation (but with ASYMMETRIC supplies)
  • AVdd = 30V (< +38V)
  • AVss = -3V  (>  -19V) or larger, if required (e.g. -5V)
  • AVdd-AVss = 33V (< +38V)
  • Gain = 6 (table 5 at page 39)
  • Offset DAC voltage = 2V, so that output range becomes -2V/+28V (anyway, I need only -0.5V)

I don't want to use unipolar mode with 0V-30V range because I really need to reach 0V with several mA sink current (-0.5V is a plus, but not strictly required).

Thanks for any help

LR

  • Hi Luca,

    You can use the unipolar 30V range and then apply the offset.  You could also consider the DAC81408, which has a REF-GND feature that lets you offset the output by ±300mV.  Look at the figure 67.

    Thanks,

    Paul

  • Hi,

    These settings should work. Let me know if you need more help. Headroom and footroom requirements are +/-1.5V from either rails. 

    So if you can have -3.5V as AVSS it will be ideal for -2V to 28V.

    Regards,

    AK

  • Hello Paul,

    thanks for your answer. I read on the DAC8218 datasheet (p. 39) that, for single supply operations, the offset DAC is turned off. Therefore I think that no offset can be applied in unipolar 30V range, do you confirm?

    Nevertheless, let's suppose that I can accept a 0V minimum (instead of -0.5V), so that I need no offset: can I still work in unipolar mode, applying a (small) negative supply AVss = -5V (larger than the footroom value), and still get the maximum 9 mA output current with 0V output voltage? This is important: can you please confirm that the DAC can operate full-load in unipolar mode with AVdd=30V and AVss=-5V? Sorry but this was not apparent to me.

    Also thanks for suggesting DAC81408 (for me it is enough DAC71408), but I would prefer avoiding the external reference circuitry. Anyway, It could be an interesting solution without offset because it could deliver larger currents (20 mA): can you confirm that it is possible to work like described above, that is in unipolar mode and a slightly negative AVss=-5V?

    A last question: I see that DAC71408 is recent, while DAC8218 is more than 10 years old: do you think that, in the future (let's say in 5-10 years) there might be an end of life issue for the latter?

    Thanks for your support.

    Luca

  • Hi Luca,

    The DAC8218 is still very popular and for the precision DAC product line, 10 years is still pretty young!  We have no plans to retire the DAC8218.

    If you provide a negative supply (-3 to -5V) you will have excellent current drive capability from either device. Both support it.

    Thanks,
    Paul

  • Ok,

    thanks everyone for supporting.

    Luca