DAC108S085: SPI interfacing with 3V3

jars121

Part Number: DAC108S085
Other Parts Discussed in Thread: TXB0104, , SN74AVC4T774, ADS1158

Good evening,

I'm planning on using the DAC108S085 in my design, and just wanted to confirm my understanding given some information presented in the datasheet. My DAC will have Va = 5V, to provide a 0-~5V analogue output across each of the 8 output channels. The DAC will interface with my 3V3 MCU, so I've been thinking about using a TXB0104 logic level shifter to accommodate the different logic levels. However, on closer inspection of the DAC datasheet, I've found the below:

Is my understanding correct (given the above) that 3V3 SPI signals (CLK, MOSI and CS/SYNC) will work as intended with the DAC even if it's a 5V device? My MCU's minimum HIGH output voltage is ~2.9V, and it's maximum LOW output voltage is ~0.4V. Would the reliability of this interface be improved at all by using external 3V3 pullups at the DAC?

Finally, I will not be using the DOUT functionality of the DAC; can this pin be left unconnected, or should it be pulled down to GND?

Thanks!

over 7 years ago

0 Paul_Frost over 7 years ago

TI__Guru 53773 points

Hi Jars121,

As long as your digital source is valid beyond the limits of the DAC (which based on your research, it does), then you can use it directly without a level shifter. Note that the bold values are guaranteed across the temperature range. You should verify that the MCU output is also guaranteed as well.

You can leave SDO unconnected.

Thanks!
Paul

0 jars121 over 7 years ago in reply to Paul_Frost

Intellectual 635 points

Thanks Paul, much appreciated.

Out of interest, if logic level conversion were required, can you recommend a suitable IC? I was initially looking at the TXB010x series, but I think my MCU will struggle with the 2mA drive requirement. An automatic bi-directional IC isn't required, as long as I can set the DIR on each bit individually. The SN74AVC4T774 would be perfect, except it doesn't support 5V logic levels; is there an equivalent IC that does?

0 Paul_Frost over 7 years ago in reply to jars121

TI__Guru 53773 points

Hi Jars,

I think this that shifter should be okay.

Thanks!
Paul

0 jars121 over 7 years ago in reply to Paul_Frost

Intellectual 635 points

Thanks again Paul. It seems I edited my second post as you were posting your reply. Please see above revised question.

0 Paul_Frost over 7 years ago in reply to jars121

TI__Guru 53773 points

Hi Jars,

There is not an equivalent, but as you are not using SDO, you could look at this unidirectional shifter:
www.ti.com/.../sn74lv4t125.pdf

Thanks

0 jars121 over 7 years ago in reply to Paul_Frost

Intellectual 635 points

Thanks Paul, that looks like a great option for unidirectional shifting. Final question, if I were using SDO, which bidirectional shifter would you recommend? I'm using a pair of ADS1158 ADCs at 5V, so this will be an issue I need to tackle shortly.

0 Paul_Frost over 7 years ago in reply to jars121

TI__Guru 53773 points

I would probably just use two unidirectional level shifters.

Thanks!

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DAC108S085: SPI interfacing with 3V3