Matched offsets put signals into ADC's range
Michael Petersen, Iowa State University, Ames, IA -- EDN, December 9, 1999
The circuit in Figure 1 measures a voltage that is negative with respect to the system's ground point. The constraint is that the a single supply powers the ADC, and its ground point is fixed relative to the measured voltage. A simple current source and matched resistor set provide the necessary offset to bring the voltage within the ADC's common-mode range.
This circuit was designed for measuring the current from a set of power trackers on the Iowa State University Solar Car, the PrISUm Phoenix. The power trackers provide impedance matching between the solar array and the main electrical bus on the car. Because the current shunt for the power trackers is on the low side and the ground reference point is on the electrical-system side of the shunt, the voltage is normally negative with respect to the system ground.
The current to be measured flows through the 0.01W shunt. Because the voltage is negative with respect to the system ground, you must add an offset to place the signal within the common-mode range of the ADC. The REF200 matched current sources and the 10-kW matched resistors create this offset. This scheme provides a positive offset of approximately 1V with a reasonable temperature coefficient. Because the ADC measures a differential voltage, the exact offset is unimportant as long as both legs have the same offset and remain matched. For this application, 1V is an adequate offset because the maximum current expected to flow through the shunt is only 10A, which equates to a maximum differential voltage of only 0.1V. You can easily modify this offset for other current-and-shunt-resistor combinations by selecting different resistor values for the matched resistor pair. Take care not to make the resistance values too high because high values may cause settling-time errors on the ADC's input.
Suitable ADCs for this circuit are the AD7705 (Analog Devices) and the ADS121x (Burr-Brown Corp). These ADCs provide system-calibration capabilities that let you easily correct for slight mismatches in the current sources and resistors. The system-gain- and system-offset-correction features enable the circuit to maintain accuracy even in the presence of these mismatches. To adjust for offset, apply zero current and instruct the ADC to perform the system-offset calibration. To perform gain calibration, apply a current that corresponds to full scale and instruct the ADC to perform the system-gain calibration. You can obtain these calibration values, store them in the µC, and then send them to the ADC upon subsequent power-ups, eliminating the need for calibration on every power-up.
The diode across the current shunt passes any high-current transients when you apply the power source. In the original design, this feature was necessary to pass the high-current surge that charged the power tracker's output capacitors when the power trackers switch into the system. Without the diodes, this surge destroyed the current shunt. Depending on the application and the size of the current shunt, these diodes may be unnecessary. (DI #2450)
