Overview
To achieve the performance of high-speed digital-to-analog converters (DACs), the timing requirements of digital signals must be strictly met. As clock frequency increases, the setup and hold times of digital interfaces become key parameters for system designers. This note explains setup and hold time in detail and how these parameters relate to Maxim data-conversion implementations.
Definitions of Setup and Hold Time
Setup time (tS) is the interval relative to a DAC clock transition during which the data must have reached a valid logic level. Hold time (tH) is the interval after the device captures or samples the data during which the data is allowed to change. A device's active clock edge may be rising, falling, or selectable by the user, depending on the device, for example MAX58?5, a 16-bit, 500Msps interpolating and modulating dual-channel DAC with CMOS inputs.
Digital circuits designed with CMOS typically use the midrail of the supply swing as the switching threshold. Therefore, the timing reference point is the midpoint of the signal edge. Note that both parameters are defined as positive values here; negative setup or hold times can be confusing when they occur.
Example: MAX58-1
The MAX58?1, a 600Msps, 16-bit DAC, provides a clear example of this midrail definition. Its setup time is -1.5 ns, and its hold time is 2.6 ns. In practical systems, data often changes after the sampling clock transition.
Timing Budget: Propagation Delay and Jitter
To meet these timing requirements, designers must analyze the data source's propagation delay and jitter. Propagation delay determines the nominal timing relative to the clock, while jitter determines the allowable tolerance. For example, consider a logic gate chain with 1.5 ns propagation delay driven by the same clock. In that case there is no design margin to accommodate temperature drift, clock or data jitter, or device-to-device variation.
Mitigation Strategies
Two common methods can improve setup and hold margins: adding clock delay and maintaining consistent trace lengths. Introducing clock delay between the data source and the DAC can compensate for propagation delay examples described above. Keeping trace lengths from the data source to the DAC inputs consistent ensures that skew, drift, or jitter will not cause any single bit to slip into the next clock period. Note that a high-speed data bus contains many data lines, and all bits must meet timing requirements simultaneously.
Conclusion
High-frequency data timing presents multiple challenges. Solving them requires designers and system engineers to fully understand the specifications of every device in the signal chain. If any device in the link fails to meet its required specifications, system performance will degrade, manifested as reduced DAC output performance or limits on the allowable clock frequency.
