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DAC80508: Output Current

Ryan.Bishop
Expert 3900 points
Part Number: DAC80508
Other Parts Discussed in Thread: OPA4388, DAC81408

Hello,

I have a customer that wants to use the DAC80508 with a load potentially as low as 66 Ohm. When the output is at full scale 2.5V, this could result in ~38mA.

I'm confused if this is within the capabilities of the part. The front page says 20mA, the EC table says short circuit current is 30mA on page 8, and this graph shows current capability up to 40mA.

  1. Would this device be able to supply around 35mA on all eight channels when the output voltage is 2.5V and supply voltage is 3.3V?
  2. If not, do we have a 16b octal DAC that could? (I wasn't able to find anything. Maybe two quads?).
  3. If still no, what is the best work around? Would we have to put a unity gain op-amp on the DAC outputs?
    1. I'm assuming we would want the offset voltage of that op-amp to be less than 1 LSB (38uV).
    2. Two pieces of OPA4388?

Thank you!

Regards,
Ryan B.

  • 0
    TI__Guru 53573 points
    Hi Ryan,

    1. The output would likely collapse and go to the 0V supply rail if the load was beyond the short-circuit current value. Short-circuit current is the value the DAC can sink/source into the supply. The graph you shared shows this behavior: as the current load gets larger, the output slowly approaches the rail, until it hits the short-circuit limit, then it goes to the rail quickly. The SSC is a typical value. You can see on the chart that it collapse when sourcing about 30mA, but sinking it can do more, about 40mA.
    2. Consider the DAC81408 (www.ti.com/.../dac81408.pdf).
    3. A buffer would be a good solution as well, but I would hope the DAC81408 is a better solution.

    Thanks,
    Paul
  • 0 in reply to Paul_Frost
    TI__Expert 3900 points

    Hi Paul,

    Thanks for the quick reply.

    For the DAC81408, I see the output current on the front page is listed as 25mA. However, I always trust the EC table more than the bullets on the front page because it lists all the test conditions applied. There is no maximum output current spec in the EC table, just a typical value for the short circuit current (40mA).

    I'm just worried that some parts will be able to source 35mA, but not all parts (assuming that a certain percentage of parts will have a short circuit current less than 40mA).

    The DAC81408 is decently bigger and pricier than the DAC80508, which I could probably justify if the part would definitely source 35mA.

    Are there any additional supporting details we could provide to prove the DAC81408 is a good fit for this application? Or should we switch to the buffer solution?

    Also, since the customer's Vcc is only 0.8V above the maximum output voltage, I think that will only reduce the available output current. Everything in the datasheet is spec'd at VCC=Vout(max)+1.5V.

    Thank you!

    Regards,

    Ryan B.

  • 0 in reply to Ryan.Bishop
    TI__Mastermind 46290 points

    Ryan,

    Generally speaking our devices are not used in high-current voltage output applications which is why a maximum specification is not provided in the datasheet. Anything we provide a maximum specification for must either be reinforced by a test limit at final test or significant statistical backing - in this case it would require a final test limit and the ROI just isn't there.

    So, the information that is typically provided is just the DC output impedance over the linear region of operation for the output stage along with the short-circuit current limit and finally a curve in the Typical Characteristics plots which shows how the source/sink current behaviors typically look near the supply rails where the output buffer is not linear.

    So, to Paul's point, since the DAC80508 output current limit is below your needs for this application we would expect the output to collapse and therefore we're suggesting the high-voltage sibling.

    I would point you to Figure 24, 25, and 26 in the DAC81408 datasheet to get more details on its capability in this regard - particularly in relation to your question concerning reduced headroom / footroom.

    Figure 25 and 26 illustrate how quickly the source / sink current capability decay with reduced supplies. So, in this case I think your customer will need to consider either an external buffer option or providing an external push-pull stage.

    The buffer option is straight forward. I put together a simple illustration previously for some sibling devices to the DAC81408 and have pasted that below. A very similar concept could work here, but it the end it may be simpler to select an amplifier.