Microcontrollers often comprise analog to digital converters (ADCs) which convert an analog voltage applied to their input pin to a digital value. In particular, a successive approximation (SAR) ADC comprises an input multiplexer that allows selection of an analog input channel from a plurality of analog input channels connected to the input pins of the ADC.
In many applications, an analog input signal of an input channel may be measured by two different ADCs depending on respective application requirements. In this case, however, the measurement result will fall off in quality.
On the one hand, an ADC input itself represents a load for the analog signal source and, on the other hand, the multiplexer itself represents a capacitance whose charge has to be changed when changing between different input signals. Additionally, leakage current may degrade the measurement result.
An additional error may occur when two ADCs simultaneously measure the same analog signal, e.g. when the two measurements temporarily overlap. Then, the signal source will “see” a doubled load during the overlap, that will degrade the measurement result.
In some specific applications, a large number of analog inputs, e.g. 50 analog inputs, have to be measured by ADCs. In such cases, the large number of inputs is divided between several ADCs to keep the capacitance of the multiplexer small, which also allows to measure more than one signal at a time.
Such applications also often require capability of a background scan, i.e. capability to separately test or scan all or some of the plurality of analog channels. In case this background scan is carried out by means of an additional ADC, the individual analog inputs are connected to two multiplexers (of the two different ADCs) which leads to a degradation of conversion quality. Additionally, a rather complex synchronization of the ADCs would be required to avoid overlap of sampling of the ADCs, i.e. to avoid two ADCs sampling the same signal at the same time. Furthermore, an analog multiplexer having many inputs is rather large and may lead to substantial leakage current.
Therefore, there e.g. exists a need for an apparatus and a method that provide the functionality of an additional ADC while avoiding degradation of conversion quality.