Abstract: This application note explains how to eliminate the voltage change when a digital potentiometer is used as a voltage-divider in series with other resistors. Introduction Mechanical and electronic digital potentiometers tend to have loose end-to-end tolerances. Maxim digital pots typically have a 20% to 30% resistance tolerance. The resistance tolerance can be problematic when the digital pot is used as a voltage-divider in series with other resistors. That configuration will result in an unacceptable voltage change over tolerance. This application note discusses a ratiometric method to convert that resistance tolerance into an acceptable current change. The proposed design also effectively removes the voltage change. In the circuit presented here the voltage output depends only on the ratio of the steps of the pot. The temperature coefficient is better controlled in the design. Ratiometric Method for the Design The design challenge is straightforward: a variable voltage between 3V and 4.5V with a tolerance of 3%. Start with the schematic in Figure 1 and do the math. The digital pot is 50kΩ (25% tolerance); R1 is 16.5K (1%) and R2 is 100K (1%). The 25% tolerance of the pot's end-to-end resistance will dominate in this design. Figure 1. Basic schematic. Now consider the same design with a different pot. If the pot is 37.5kΩ, the top of the pot is 4.46V and the bottom is 3.25V. Continuing on, if the pot is 62.5kΩ, the top of the pot is 4.54V and the bottom is 2.79V. This basic approach does not solve the changing voltage problem because the pot's end-to-end tolerance is in the circuit. The next circuit in Figure 2 only uses the pot's ratio. Figure 2. Alternate design features two voltage references. By using two voltage references the tolerances and the temperature coefficient are controlled. The absolute end-to-end tolerance of the digital pot changes the current, but does not affect the voltage. The output voltage is ratiometric; the voltage out depends only on the ratio of the steps of the pot. Both references use feedback to control the output voltage. R2 (~25K to 50K) ensures that both references source current. Bypass capacitors are discussed in the data sheet for each Maxim digital pot. Some capacitors may be required, depending on the board layout. Ultimately, an application dictates the system's requirements. Device temperature coefficients can be predicted from the respective digital pot data sheets. The pots also offer a choice of noise specifications. Maxim's complete list of digital pots and voltage references is available on the website. 相关型号   APP 4290: Sep 23, 2008 DS1805 可编址数字电位器 完整的数据资料 (PDF, 260kB) 免费样品 MAX5160 低功耗数字电位器 完整的数据资料 (PDF, 264kB) 免费样品 MAX5161 低功耗数字电位器 完整的数据资料 (PDF, 264kB) 免费样品 MAX5400 256抽头SOTPoT、低漂移数字电位器，SOT23封装 完整的数据资料 (PDF, 264kB) 免费样品 MAX5463 32抽头FleaPoT™、2线数字电位器 完整的数据资料 (PDF, 244kB) 免费样品 MAX5464 32抽头FleaPoT™、2线数字电位器 完整的数据资料 (PDF, 244kB) 免费样品 MAX5465 32抽头FleaPoT™、2线数字电位器 完整的数据资料 (PDF, 244kB) 免费样品 MAX6023 精密的、低功耗、低压差、UCSP封装电压基准 完整的数据资料 (PDF, 276kB) MAX6029 超低功耗、高精度串联型电压基准 完整的数据资料 (PDF, 716kB) 免费样品 MAX6045A 精密的、低功耗、低压差、SOT23-3封装、电压基准 完整的数据资料 (PDF, 288kB) 免费样品 MAX6045B 精密的、低功耗、低压差、SOT23-3封装、电压基准 完整的数据资料 (PDF, 288kB) 免费样品 MAX6063A 精密的、微功耗、低压差、高输出电流、SOT23封装电压基准 完整的数据资料 (PDF, 560kB) 免费样品 MAX6063B 精密的、微功耗、低压差、高输出电流、SOT23封装电压基准 完整的数据资料 (PDF, 560kB) MAX6067B 精密的、微功耗、低压差、高输出电流、SOT23封装电压基准 完整的数据资料 (PDF, 560kB) 免费样品 MAX6103 低成本、微功耗、低压差、高输出电流、SOT23封装的电压基准 完整的数据资料 (PDF, 336kB) 免费样品 MAX6107 低成本、微功耗、低压差、高输出电流、SOT23封装的电压基准 完整的数据资料 (PDF, 336kB) 免费样品 MAX6145 SOT23封装、低成本、低压差、三端电压基准 完整的数据资料 (PDF, 728kB) 免费样品 自动更新 需要自动接收最新发布的应用笔记吗？请订阅EE-Mail™ (English only)。 我们期待您的反馈！ 喜欢？不喜欢？有待改善？或为我们提供建议？请与我们联系 — 我们将根据您的意见或建议改善我们的工作。 网页评价或提供建议   下载，PDF格式 (38kB)  AN4290, AN 4290, APP4290, Appnote4290, Appnote 4290