Feature: System Design


Figure 1. AD7606B in a typical power line monitoring application


(current transformer shown in Figure 1) allows users to detect when the sensor disconnects by monitoring if an ADC output code lower than 20 LSBs repeats for a number of samples (N). It is recommended to have an RPD


much larger than the source impedance of the sensor in order to minimise the error that this parallel resistor may introduce. However, the larger the RPD


, the larger the ADC output


code generated when the sensor disconnects, which is not desired. A large ADC output code may lead to unnoticed sensor disconnection.


Because the AD7606B has larger RIN than the AD7606, for a given RPD


, the


ADC output code is lower if the sensor disconnects, as shown in Figure 3, reducing the risk of false alarms. When entering software mode for


the AD7606B, there is an open-circuit detection feature, eliminating the burden on the back-end software that detects the sensor disconnection. After programming the number of samples N (N = 3 on the example of Figure 4), if the analogue input remains for several samples reporting a small dc value, the algorithm will automatically run and


Figure 2. Gain error introduced by a series resistor


assert a flag if the analogue input signal has been disconnected.


System-Level Performance System Offset Calibration When using a pair of external resistors, as seen in Figure 1, any mismatch between them will cause an offset. This


offset can be measured as the ADC output code when the sensor is shortcut to ground. An offset from –128 LSBs to +127 LSBs can be then added to or subtracted from the conversion result by programming the corresponding channel offset register in order to compensate for that system offset.


www.electronicsworld.co.uk November 2024 23


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