LTC2757 update rate specifications

Question

Hi, 
 A few questions about this DAC: 
 1) Based on the timing characteristics of the digital interface for Vdd = 3.3V, it should be feasible to update the LTC2757 at 16.66 MHz. However, if the output settling time specification in the datasheet is 2.1 us, does this mean that I should not being updating any faster than 476 kHz (or 555 kHz if optimizing settling time to 1.8us per Note 7)? 
 2) Is the settling time dependent on the step size? The datasheet states conditions for a span code = 000, which is 0-5V, but also states for a 10V step. In my experiments on the eval board, the size of the step didn't seem to affect the settling time. 
 
 3) I am having trouble understanding the graph of multiplying frequency response vs digital code. Why is there an individual line per bit rather than a single line for attenuation vs frequency? 
 
 Thanks, 
 Jonny

ADIApproved · Answer

1. You are talking about two different things, you can update the DAC at 16.66MHz because that is how fast the SPI lines and registers can go. The DAC settles in 1.8us so that is how fast it takes for the signal to settle. If the DAC isn't fully settled before you change the code, then you will get an error. 
 2. Yes the settling time is code dependent. A full scale step will take longer to settle than a small code change. A 0-5V step will be quicker than a 0-10V step. It has to do with the slew rate of the final internal amplifier. 
 3. If you want the frequency response you can use the All bits on plot. The other plots are there to show the response with different multiplying factors.

ADIApproved · Answer

The update rate is limited by the digital circuitry in the DAC. By going code by code or going into smaller steps the settling time might improve, but if you need to have a full scale step you would need to step through so many codes you would lose the advantage of the decrease settling time. Once again, there is no free lunch.