Defining the Error Band for a DAC Settling Measurement

A DAC output that has completed most of its voltage transition may still be settling outside the accuracy required by the load. Define the error band and update reference event before comparing settling times. Rise time alone does not establish when the output becomes usable.

Read the specified transition

The AD5686/AD5684 data sheet defines its output-settling measurement using a specified code transition and timing reference. Those conditions matter when comparing the number with a board measurement using another step size or update mechanism.

Record the initial and final codes, reference voltage, supply, load resistance and capacitance. Identify the event that updates the analog output, which may differ from the start or end of a software write. Include any external output buffer or filter in the system-level requirement.

Set the accuracy criterion

Define how close the output must be to its final value and require it to remain within that band. For an illustrative 4 V full-scale range, one part in 65,536 is approximately 61 microvolts. Measuring settlement to that scale requires more care than observing a transition on a full-scale oscilloscope display.

Use the tolerance required by the application rather than automatically adopting one least significant bit. Keep the final-value error and the time-dependent settling error distinct. A DAC can settle quickly to a value that still has an offset or gain error.

Choose revealing code changes

Test the transitions the product actually uses, including large positive and negative steps and important code boundaries. A single midrange step does not characterize every possible output sequence. Check whether the external load or buffer behaves differently near its operating limits.

Account for repeated updates. If a new code arrives before the previous output has settled, define whether that behavior is permitted and how the downstream circuit interprets it. A data-interface throughput specification does not guarantee analog settling at that update rate.

Measure without obscuring the tail

Use an observation method with sufficient resolution and recovery behavior for the selected error band. Check that the measurement instrument is not recovering from its own overload after the large transition. Record probe loading and the timing reference used for each trace.

Keep the code sequence, load and measured settling interval with the interface specification. Revisit the result when changing the output filter, cable or reference. This gives firmware a justified delay and makes the claimed analog update rate traceable to the accuracy actually required at the receiving circuit.