Re: trying to understand the REF pin on a DAC

kasterborus@xxxxxxxxx wrote:
Wow thanks - that makes sense...

I was reading the data*** for the chip and I have a question...
http://focus.ti.com/lit/ds/symlink/tlv5618a.pdf

At one point it says that the maximum ref voltage is Vdd + 0.3, then
later is says the maximum recommended Vref is Vdd-1.5.
Would it be OK just to tie this pin to the 5v I'm using to power the
chip?

I want the output to be 0-5v.

You should have supplied the link to the data *** the first time. ;-)

See the note under the recommended operating conditions block on page 3.

This beast has times 2 amplifiers between the ref input and the DAC, and, evidently (though they don't seem so proud of the fact, hence the fine print) those amplifiers cannot swing anywhere near Vdd, so the maximum ref input that will not cause clipping the high end digital values is Vdd-0.4/2.

For a 5 volt Vdd, that is 2.3 volts, for an internal reference voltage of 4.6 volts, and the output cannot exceed that.

You would need to power this chip from at least 5.4 volts to get a 5 volt, full scale output, with a 2.5 volt reference input.

This is the first DAC I have seen with this internal reference multiplier "feature". If they were proud of this feature they might have written a 2X inside the pair of triangles on the block diagram at the top of page 2, with the note about the output limitation of the amplifiers, there, also.
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