DAC908 Questions - What is output compliance voltage with 3.3V power? Can VD and VA be different voltages?

Hi Mark,

I'm trying to track this information down. Unfortunately I probably won't be able to get the compliance voltage at 3V, unless the test program provides some clues. My thoughts on the answers are below.

1. It should scale down a bit. 0.75V is a conservative estimate for the max compliance voltage. If the compliance range is exceeded you may experience degraded distortion performance. If 0.75V is too low for your application my thought is that you can implement the circuit with a max compliance voltage of 1 V and if distortion is too large then you'll need to back it off.

2. The datasheet is a little contradictory on this, but my feeling is that they should match. I don't see any references to using separate supply voltages. However, I'm checking the test program to see if we do test different supply voltages.

Regards,
Matt Guibord

Hi Matt,

Thanks for the quick follow up!  If you could find any further information it wold be appreciated.  Your information below is already helpful.  I also think that from the way the specifications are given that VA should be the same voltage as VD, but it would be nice if VD could be 3.3V and VA 5V.

Hi Mark,

I don't know if this helps or if it adds more confusion, but we do perform a test where we vary the digital and analog supply voltages while holding the data constant to check for voltage variations. We use all iterations of 2.7V to 5.5V between the two supplies. This seems to indicate that we tolerate different voltage levels, however I would be more comfortable saying so if we were outputting a dynamic pattern to check timing.

Regards,
Matt Guibord

Hi Matt,

Thanks for the extra information above. I have implemented 5V for the analog VA power through a ferrite bead and have another in parallel to 3.3V so I can power VA from either 5V or 3.3V. I need VD to be 3.3V, as that if the IO levels from the FPGA that will drive the DAC908. If there is some anomaly, I will switch VA to 3.3V and also lower the load resistor 50 ohms to 25 ohms if necessary, to maintain compliance on output swing for 3.3V VA rather than 5V, as we are planning to use ~20mA output current.

May i ask if the distortion specs use an anti-alias filter or are they just the direct output of DAC through a transformer?

Hi Mark,

That sounds like a good plan. Please report back on your findings!

Distortion specs are through transformer without filter. Using a filter would be cheating from a spec standpoint :)

Regards,
Matt Guibord