Data Acquisition

Settling timeIn a practical D/A converter, there is a limit to the rate at which the converter can acquire newanalog output values, because the analog output signal produced takes a finite time to settle toa new value, in response to a change in digital input. The settling time is defined as the timerequired for the output to reach, and remain, within a given error margin of the final value,usually a percentage of full-scale or ±1/2 LSB, following a prescribed change at the input(usually a full-scale change). This figure takes into account all internal factors affecting thesettling time, i.e. turning the switches on and off; current changes within the resistor network,and the time required by the op-amp or buffer outputs to settle within their error bands.The settling time of the D/A converter, especially of high speed DACs, is prolonged by theoccurrence of sometimes-large transient errors in the output. Glitches are spikes in the outputof a D/A converter that may result when, due to the occurrence of an intermediate state, theoutput is driven toward a value opposite to its final value. An intermediate state is the resultof one or more switches in the DAC being faster than the others are. As an example, considerthe most major transition of a DAC, when the input changes from 100…000 to 011...111. Ifthe MSB switch changes faster, an intermediate state of 000...000 could occur, momentarilydriving the output of the DAC to 0 V before returning the correct value. This is shown inFigure 5.28.1000000001111111GlitchFigure 5.28Glitch occurring at the output of a DAC during the major transitionWith Ideal De-glitcherThe better matched the switching times and the faster the switches, the smaller will be theenergy contained in the glitch. As the size of the glitch is not proportional to the signalchange, linear filtering may be unsuccessful and can in fact make matters worse. Deglitchers,in the form of a sample and hold circuitry, are often included as part of the D/Aconverter, holding the outputs constant at the previous value until the switches reachequilibrium, then sampling and holding the new value. The de-glitcher circuitry, thoughcleaning up the output, will result in a reduction of the update rate.Slew rateThe slew rate is the maximum rate of change that the DAC can produce on the output signal,usually limited by the slew rate of the amplifier used at its output.Update rateThe speed or update rate of a DAC is a function of both the settling time and the slew rateand is critical in determining the maximum frequency of an output waveform that can beproduced. Therefore, a DAC with a small settling time and high slew rate can generate highfrequency signals, because little time is needed to accurately change the output to a newvoltage level.