The last blog post covered ADC selection tools. While the ADC Driver Tool and Noise Tool are useful for the selection and evaluation of Analog-to-Digital Converters, they do not work with Digital-to-Analog converters because of the differing requirements of DACs. This post will discuss a tool for DAC selection.
The drive-side precision signal chain elements are often chosen by optimizing Total Uncorrected Error (TUE). This can be a time-consuming and annoying task, as an engineer must design a circuit, and then look up errors for amplifiers, Voltage references, and DACs, to find the system TUE. Once these errors are found, the system error must be calculated. There are several types of error, including DC error at 25C, Temperature Coefficient (TC), Line and Load regulation error, Bias Current error, Component Tolerance Error, Linearity Error, and on and on. This is further complicated because some errors may be factory-calibrated, while others drift over temperature or time. The DAC Error Budget Tool can grab all of these errors from the Product Selection Tables (PST) and sum them automatically. The ability to recalculate the error on the fly makes the DAC Error Budget Tool indispensable to an iterative selection process.
Figure 1: Inputs to TUE Calculation
Setting up the tool is simple. First, choose the Type of DAC to be used. Analog Devices has several types of precision DACs, including Voltage Output, Multiplying, 4-20mA Current Loop, iDAC (Current-Mode DAC), and Fast Precision. Individual products may be chosen by clicking on the selection link in the upper-left of the Application Parameters window. It is similarly easy to choose amplifiers and voltage references. Various circuit topologies are available via the Circuit Design tab, as each DAC product is built for a specific topology or topologies. The graphical point-and-click environment is friendly and easy to use, allowing extensive exploration of possible solutions with little effort. The tool is effective for learning about system design as well as for optimization.
Figure 2: DAC Error Tool GUI
Once the products, topology, and parameters are chosen, the DAC Error Budget Tool does the rest. It outputs a total system error and lists error by source, allowing the user to pinpoint the cause of total error and trade-off errors in design. Both Root-Sum-Square (RSS) and Worst-Case errors are presented. This is helpful, as Worst-Case error can become extremely rare in practice, especially when several or many uncorrelated error sources contribute. Figure 3 shows a typical breakdown by error source.
Figure 3: Error Tool Results
Specific characteristics of the digital-to-analog converter and additional components may also be used in selection. For example, if a DAC is to be used in a loop, such that speed or latency are important, then the DAC could be chosen from the appropriate category. In this case, fast-precision. Despite the use of what might be considered a “high-speed” converter, fast-precision lives up to its name: DC errors and linearity are still high-quality. In this way, a DAC and amplifier may be chosen for speed performance, but still optimized for DC performance, preserving the overall precision of the system. Alternatively, error may be difficult to calculate due to changes between current and voltage modes. Here the DAC Error Budget Tool simplifies analysis by providing the Full-Scale output available for a circuit and the actual output for the given conditions, including the error range, in appropriate units. Figure 4 shows an example in mA. Output may also be chosen as parts-per-million or % of full-scale.
Figure 4: Adjustable Results Display
It cannot be overstated how much exploration is facilitated by this tool. For brute-force calculations, it is a huge effort to look at alternatives. With this limitation, the first result that is near-ish the mark is liable to be chosen. Using a spreadsheet to make the calculations reduces the effort, and can allow the comparison of alternatives, but does nothing to simplify or shorten the task of getting the error terms from datasheets or the PST. Having the tool pull all relevant error terms automatically means that several, tens, or more than a hundred calculations could be made in a relatively short time. Any possible topology that might solve a problem could be chosen and parameterized. This then becomes an educational tool, fostering an intuitive understanding of DAC circuits as a whole, rather than as a matter of repetition. Further, the tool gives helpful advice when pairing devices that may not be a good match, as in Figure 5.
Figure 5: Example Tool Warning
The DAC Error Tool gives an easy way to produce a complete estimation of system performance during product selection. The next blog will discuss a more in-depth converter evaluation using Active Functional Models (AFM).