Linearity of Digital-to-Analog Converter (DAC)
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In selecting a digital-to-analog converter (DAC) one of the most common criteria you look at is linearity. Two tests have evolved as the most popular linearity tests: differential nonlinearity (DNL) and integral nonlinearity (INL.) If a DAC were ideal, each output step would be exactly the same size; meaning when supplied with equally increasing increments of supply voltage, the output would ramp up in equal increments. DNL is the degree to which each output step (or code width) varies from the ideal step. DNL is generally more critical when outputting small signals. INL measures the deviation of the entire transfer function from the ideal function and is generally more critical for outputting large signals.
To test the linearity of a DAC, you need to generate the digital stimulus and capture the analog response. Usually you would sweep the DAC from 0 to its maximum output step, and compare the response to what it should be at every output step. When most manufacturers specify DNL and INL on a data sheet, they specify the worst cases - the maximum and minimum over the entire transfer functions. You might only be interested in the minimum and maximum, or you might be interested in the measurements for all code widths. LabVIEW provides native array functions to search the data array for the max and min or other points of interest, and you can graph the overall transfer functions for trending information.
To generate the digital stimulus and capture the analog response, you need a high-speed digital source and high-speed digitizer. In hardware, you need to control the timing between the acquisition and generation modules, so you know which acquisition sample should correspond to which generation sample. Platforms such as PXI and VXI are ideal for such applications These platforms are designed with timing and synchronization resources built-in, so modules can share reference clocks, sample clocks, triggers, and events.
Figure 1. Differential Nonlinearity of RGB Video DAC
Related Links:
Mixed-Signal Stimulus-Response
Video DAC Linearity Test
View an online demo of INL and DNL testing of a digital-to-analog converter (DAC)
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