Abstract: Using resistor dividers to scale the differential inputs and a stable voltage reference to offset the inputs, this circuit enables an ADC with 0V to 5V input range (MAX1402) to accept inputs in the range +10.5V to -10.5V. This design idea appeared in the August 5, 2008 issue of EE Times magazine. Many high-accuracy analog-digital converters require input levels between 0.0V and 5.0V. As an example, the MAX1402 (an 18-bit multi-channel sigma-delta ADC) measures the difference between two inputs. In a typical single-ended application, it compares an input voltage with a fixed reference voltage such as 2.500V: for ADCIN = 0V the digital output represents 0V - 2.5V = -2.5V, for ADCIN = 2.5V the output represents 2.5V - 2.5V = 0V, and for ADCIN = 5V the output represents 5V - 2.5V = 2.5V. Thus, the digital output range corresponding to ADCIN values between 0V and 5V is ±2.5V. The circuit of Figure 1 converts an input signal in the range ±10.5V to the input range of the MAX1402 ADC (0V to 5V). Two of the ADC channels (IN1 and IN2 in this case) are configured for either full differential or precision single-ended measurements. Resistor dividers R1 and R2 scale the inputs, and a stable source of 3.28V offsets the inputs. As a result, the ADC input is centered at 2.50V when the measurement inputs are grounded. (That is, the ADC digital output is zero when VIN = 0V.) Precision component values maintain the ADC's 16-bit accuracy. Figure 1. This circuit enables an ADC with input range of 0V to 5V (single-ended or differential) to accept inputs in the range ±10.5V. Configuring the MAX1402 for differential measurements allows it to measure the voltage difference between IN1 and IN2. These inputs accept voltages anywhere in the ±10.5V range, and the internal programmable-gain amplifier (PGA) is available to increase the resolution for low-level signals. A gain of four, for example, enables the ADC to resolve a ±2.625V input signal with 16-bit resolution. To make single-ended measurements you can configure the inputs as two separate channels, and compare them to a 2.50V reference voltage attached to IN6. Or, for higher precision, you can configure the ADC for differential inputs, with one channel acting as a ground-sense input. The resistor divider ratio can be altered to accommodate different input ranges, but the same ratio is needed in the circuit that generates the offset voltage. A ratio of 5:1, for example, would yield an input range of ±15.0V and an offset voltage of 3.00V. To calibrate the system, simply record the output value with inputs grounded and with a known input voltage. Those two values let you calculate the offset and gain factors for each input range. SPI is a trademark of Motorola, Inc. 相关型号   APP 4617: Nov 02, 2009 MAX1402 +5V、18位、低功耗、多通道、过采样(Σ-Δ) ADC 完整的数据资料 (PDF, 564kB) 免费样品 MAX4238 超低失调/漂移、低噪声、精密的SOT23封装放大器 完整的数据资料 (PDF, 336kB) 免费样品 MAX5491 精密匹配的电阻分压器，SOT23封装 完整的数据资料 (PDF, 168kB) 免费样品 MAX6225 低噪声、精密的、+2.5V/+4.096V/+5V电压基准 完整的数据资料 (PDF, 904kB) 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 (45kB)  AN4617, AN 4617, APP4617, Appnote4617, Appnote 4617