Summing inverter aids sensor calibration
When you need to use multiple sensors with analog outputs in a measurement system, you’ll probably need to calibrate it. Each sensor may respond differently to a known stimulus. You can do that by converting the sensor’s individual voltage output to current and summing them.The figure shows a simplified version of the circuit. Each sensor has voltage output. Resistors R1, R2…Rn consist of two resistors in series (shown as one in the figure). You can adjust one of the two resistors to adjust the current from each sensor. The voltage between the sensor outputs and a reference voltage (shown as ground in the figure) in combination with the resistors, sets the current. The feedback resistor RFEEDBACK sets sets the gain of the summing amplifier.
A summing amplifier produces a voltage output that’s proportional to the sum of the sensor outputs, which allows for a system calibration. Source: IEEE. (Click on the figure to enlarge)
The calibration procedure is described in detail from a paper, “A Low-Cost Voltage-to-Current Calibration Technique for Multiple-Sensor Systems.” The paper describes the technique for calibrating a weighing system using piezoresistive sensors, but you can apply it to other measurement applications.
The calibration procedure starts with measurements on the sensor outputs under no load. Start with resistors R1, R2, etc. as single resistors and measure the each sensor’s output voltage. You can then calculate the current in each leg of the amplifier. Then, apply a known load, measure each leg’s current, fron which y0ou can calculate a correction factor and thus calculate the additional resistance you need to produce a calibrated system. The paper provides all the equations you need for the calculations.
