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Use only 1 channel on DAC8228?

Laurent Sommervogel
Prodigy 80 points
Other Parts Discussed in Thread: DAC8228, DAC8871, OPA211

Hello,

For my application I need to send a 5 MHz digital signal with a +/- 15V magnitude.

In my opinion, DAC8228 seems to be a perfect match for this.

1) In the datasheet I can see that this is an "octal" converter, meaning it has 8-channel output. I do not require the 7 other channels. What should I do?


- leave the corresponding pins unconnected?

- tie all outputs together?

2) Can I drive a 50-ohm load directly?

Thanks a lot for any tips about this.

  • 0
    TI__Mastermind 46290 points

    Laurent,

    There are a lot of things to consider here...

    I'm not sure exactly what got you to look at the DAC8228 to understand if other parts might be acceptable as well. There is a single channel device that is also capable of bipolar high-voltage outputs called the DAC8871. DAC8871 is an unbuffered DAC while DAC8228 includes an internal buffer, which may seem bad but...

    The DAC8228 datasheet indicates 3mA "load current" and 10mA short-circuit current for the output. In order to drive a 50Ω load at +/-15V you need something with a couple of orders of magnitude more source/sink current capability, 300mA or more (probably more to minimize drooping depending on how much DC accuracy is important to you).

    The other thing to think about is settling time / slew rate as you're trying to generate a fairly fast signal for a precision DAC (not sure exactly what this digital signal looks like or what the load capacitance might load look like). The DAC8228 slew rate is 6V/us and the settling time specifications (depending on exactly what you consider a settled output) are on the order of 10us for fairly large steps (again, not sure what the digital waveform looks like). That means for DAC8228 about the fastest thing you can generate would be at 100kHz update rate depending on what you call settled and what the waveform looks like.

    DAC8871 is a bit faster as it is an unbuffered DAC at 40V/us and 1us settling time (to 1LSB) so you'd get an order of magnitude better performance at 1MHz update rate.

    Once again, it depends on what this waveform looks like and what you'd call settled but it doesn't look to me like either DAC will be fast enough for you.

    In either buffered or unbuffered DAC cases you'd need to include a power op-amp, or maybe a precision op-amp with an external push-pull stage, in between the DAC and the load in order to source the currents you need.

  • 0 in reply to Kevin Duke
    Prodigy 80 points

    Thank you very much Kevin for your clear and comprehensive answer!

    What got me looking into DAC8228 was because of the bipolar high voltage output and the fast parallel interface. I also saw the DAC8871 as well but I figured that the serial interface would be too slow for sending 16-bit 5 MHz waveform. But as you mentioned, slew rate will prevent me from doing this in both cases.

    As far as the load current is concerned, it seems like I would need something like 600 mA, so even if I use op-amp like OPA211 (recommended in DAC8871 datasheet), it is not going to be enough.

    Thanks again and I'll continue searching paying particular attention to slew rate and load current...

    Laurent.