DAC Essentials: Understanding your DAC's speed limit

Other Parts Discussed in Post: DAC8568

In the last two “DAC Essentials” posts, Tony Calabria introduced a digital-to-analog converter (DAC) dynamic specification called glitch and discussed common techniques to “de-glitch” your DAC.

Today, we’ll look at two related dynamic specifications – slew rate and settling time. To learn more about how static and dynamic specifications differ, refer to this post.

What is slew rate?

Retired TIer, and analog guru, Bruce Trump may have summed up slew rate best in one of his final blog posts on The Signal, when he described it as an op amp’s speed limit. DAC slew rate specs match 1:1 with op amp slew rate specs.

Basically, when a sufficiently large change in the input voltage occurs, like when a new DAC code is latched that is several codes away from the current code, the output amplifier will begin to slew, or increase the output voltage as quickly as it can. It does this until it gets close to the intended value, and the output begins to settle within a specified error-band.

 The datasheet specification tells you the maximum rate of change you can expect to see at the output of the DAC when it is slewing, typically in microvolts per second.

Note: This figure isn’t drawn to scale for any real device; it is exaggerated to show each region

 What is settling time?

DAC settling time also bears some striking similarities to op amp settling time. The chief difference, though, is that DAC settling time also includes a figure called dead time. Dead time is the time the DAC spends latching, or updating, its output. This latching action is typically triggered by the falling edge of a digital signal, called LDAC. The LDAC and DAC output interaction is illustrated in the figure below, taken from the DAC8568 datasheet.

If a large input step occurs, the DAC will enter the slew region, which appears in both of the figures above. In the slew region, the DAC’s progress is limited by the slew-rate specification. If the DAC does have to slew, the next phase of setting time will be an overload recovery condition, followed by linear settling time into a specified error band. This error band is typically specified within 1 LSB for the DAC.

The datasheet specification for settling time will be given for a relatively large step-size. The DAC8568, for example, specifies settling time as 5us typical for a change from ¼ full-scale output to ¾ full-scale output.

Keep in mind that slew time can dominate your overall settling time figure, so if your output step-size is smaller than the step-size for the settling time spec in the datasheet, it will take less time for your system to settle. In most high-accuracy applications, settling time is the effective update rate for the DAC.

My next post will be about total unadjusted error, or TUE, a handy way of succinctly describing DAC accuracy. That post will conclude the “DAC Essentials” series on Analog Wire. But Tony and I are not going away. You’ll find us contributing to the TI Precision Designs Hub (“The Hub”), a new blog from Texas Instruments providing precision analog tips, tricks and techniques – from how to read data sheet specs and test conditions to how to optimize the external reference for analog-to-digital (ADC) performance.

An added bonus this week: be sure to check out a new “Engineer It” video about multiplying DACs (MDACs) from my colleague Rahul Prakash.

As always, leave your comments below if you’d like to hear more about anything mentioned in this post, or if there is something you would like to see included in a future post here or on The Hub.