在这个应用中，传感器是一个精密的测压元件，其额定负载5kg，即约11磅。在铝盘上测重大约150g的物体，即大约5盎司。由于铝盘自重，即使没有任何物体称重，仪表放大器的输出信号也不能低到0V。现在，问题是如何补偿仪表放大器的输出偏置电压和铝盘本身产生的电压值。
软件弥补系统偏置是最简单的方法。电源启动期间，铝盘上没有称重物体，系统可以获取偏移电压，并将数据记录在单片机内存中。随后，当有物体称重时，从获得的数据中减去它即可。但是，这种做法不能达到5kg满量程，仅能达到5-0.15kg或4.85kg。
本设计方案说明如何利用单片机实现硬件补偿。当电源启动后，运行软件程序复位系统偏移。解
双通道DAC（IC4）的DAC_A输出在仪表放大器参考引脚处，提供200mV的参考电压，避免放大器本身近地饱和，但传输特性不是线性关系。放大器最坏情况下输出偏移是：VREF+VPAN±VOFFSET=200 mV+125 mV±500×150 µV=325 mV±75 mV="250" mV/400 mV，这里VPAN=125 mV，是铝盘自重产生的电压值。
因此系统输出偏移是250到400mV。电源启动，微控制器运行程序设置DAC_A输出为200mV，同时，增加双通道DAC（IC4）的DAC_B输出直到等于ADC（IC3）管脚2的系统偏置，转换结果就是000h。这一结果是可能的，因为IC4包含两个12位2.5V满量程电压的DAC，最低有效位（LSB）等于0.61mV，小于IC3为1mV的分辨率。这个数字相当于该天平的分辨率：5000g/4096=1.22g。当最大负载5kg时，仪表放大器的最大输出电压是4.096V+VOUT_TOTAL_OFFSET_INA=4.346V/4.496V，低于4.62V高饱和温度的最坏情况。
IC3有一个单极差分输入，所以可以从+IN输入电压中减去一个恒定电压值等于IC4的DAC_B提供的系统偏置。在第一个半时钟周期内，ADC采样和保持正向输入电压。这阶段结束后，或在获取时间内，输入电容切换到负输入并开始转换。在IC3输入处的RC输入滤波器的时间常数为0.5µs，允许在正负输入电压利用最高为200kHz时钟频率在转换时间的第一时钟周期内达到12位精度。如果想增加时间常数，必须降低时钟频率。
此外，DAC和ADC有三线串行接口，可方便地将数据传输到最高采样率为12.5kS/s的普通微控制器。当ADC处于没有转换的时候，它会自动把功率降至1nA的电源电流，而且如果单片机通过其引脚3来关闭IC1，电路限制电源电流在最坏情况下仅为1mA，因为所有的IC集成电路都是微功耗的。
英文原文：
Circuit compensates system offset of a load-cell-based balance
A dual DAC stores the system-offset voltage, which gets determined during a power-on calibration sequence.
Luca Bruno, ITIS Hensemberger, Monza, Italy; Edited by Charles H Small and Fran Granville -- EDN, 8/16/2007
It’s a challenge to interface a resistive bridge sensor with an ADC receiving its power from a 5V single-supply power source. Some applications require output-voltage swings from 0V to a full-scale voltage, such as 4.096V, with excellent accuracy. With most single-supply instrumentation amplifiers, problems arise when the output signal approaches 0V, near the lower output-swing limit of a single-supply instrumentation amp. A good single-supply instrumentation amp may swing close to single-supply ground but does not reach ground even if it has a true rail-to-rail output
In this application, the sensor is a precision load cell with a nominal load of 5 kg, or about 11 lbs, to weigh objects on an aluminum pan weighing approximately 150g, or approximately 5 oz. Because of the pan’s weight, the instrumentation amplifier’s output signal can never go down to 0V, even if there are no objects to weigh. Now, the problem arises of how to compensate the instrumentation amp’s output-offset voltage and the voltage that the pan itself produces.
A software approach is the simplest way to compensate the system offset. During power-up, there are no objects to weigh on the pan, and the system can thus acquire the offset voltage and hold the data in the microcontroller’s memory, subsequently subtracting it from the data it acquired when there was an object to weigh. This approach, however, does not reach the 5-kg full-scale of the balance, reaching only 5?0.15 kg, or 4.85 kg.
This Design Idea shows how to achieve hardware compensation using a microcontroller that, on power-up, starts a software routine to reset the system offset. The solution is a simple circuit based on four ICs from Linear Technology in Figure 1. A precision voltage reference, IC1, has a high minimum output current of 50 mA. It provides an output voltage of 4.096V to power the load cell and to set the full-scale of the 12-bit ADC, IC3. The highly accurate LT1789-1 instrumentation amplifier, IC2, features maximum input-offset voltage of 150 µV over the temperature range of 0 to 70°C and maximum input-drift-offset voltage of 0.5 µV/°C over the temperature range of 0 to 70°C with rail-to-rail output that swings within 110 mV of ground. You set the gain through precision resistor R2 to a nominal value of 500Ω to give an output span of 4.096V when the load is 5 kg and its maximum input signal is VCC×S=4.096V×2 mV/V=8.192 mV, where S is the sensor’s sensitivity.
The output of DAC_A of dual-DAC IC4 provides a reference voltage of 200 mV at the refer
The system-output offset is thus 250 to 400 mV. On power-up, the microcontroller starts a routine that sets the output of the DAC_A equal to 200 mV, while it increases the output of the DAC_B of dual-DAC IC4 until it is equal to the system offset on Pin 2 of ADC IC3, and the result of the conversion is 000h. This result is possible because IC4 contains two 12-bit DACs with the same full-scale voltage of 2.5V, making 1 LSB equal to 0.61 mV, which is smaller than IC3’s resolution of 1 mV. This figure corresponds to the resolution of the balance: 5000g/4096=1.22g. The maximum output voltage of the instrumentation amp with a maximum load of 5 kg is 4.096V+VOUT_TOTAL_OFFSET_INA=4.346V/4.496V, which is less than the minimum worst case over temperature of 4.62V high saturation.
IC3 has a single unipolar differential input, so you can subtract from the +IN input voltage a constant voltage of value equal to the system offset that that DAC_B of IC4 provides. During the first one and a half clock cycles, the ADC samples and holds the positive input. At the end of this phase, or acquisition time, the input capacitor switches to the negative input, and the conversion starts. The RC-input filters on the inputs of IC3 have a time constant of 0.5 µsec to permit the negative and positive input voltages to settle to a 12-bit accuracy during the first clock cycle of the conversion time, using the maximum clock frequency, which is 200 kHz. If you want to increase the time constant, then you must use a lower clock frequency.
Furthermore, the DAC and ADC have a three-wire serial interface that easily permits transferring data to a wide range of microcontrollers with a maximum sampling rate of 12.5k samples/sec. When the ADC performs no conversions, it automatically powers down to 1 nA of supply current, and, if the microcontroller shuts down IC1 through its Pin 3, the circuit draws a worst-case supply current of just 1 mA, because all the ICs are micropower
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