Potential Scenario: You may be observing a strange effect on the output, such as small spikes that are short in duration and random in nature (on the magnitude of millivolts per volt of expected output or less). When using the clock, these spikes are synchronized with the clock, but without the clock, they seem to be random. Is this behavior normal for the MAX5891?
Answer: The MAX5891 uses an internal, automatic calibration system that does create periodic glitches on the output. The reason the glitches occur when no clock is applied is due to noise acting as a clock signal to the device. The differential clock input, self biased with an AC coupled source, will bias the two inputs to nearly the identical voltage, and thus any noise on the inputs that is not in phase can cause the internal clock circuitry to change states and increment the time counter for the calibration engine.
There are no external controls for the calibration engine. The operational mode of this calibration engine was designed to provide superior performance in the frequency domain. As such, typical time-domain specifications are often compromised, including such things as glitch energy and settling time. Frequency domain operation implies that testing is performed by generating signals and measuring the DAC output with a spectrum analyzer and comparing it against the IDEAL INPUT signal. Where as time-domain applications are more concerned with these other parameters which were not as stringently specified.