This next example shows how a resistance temperature detector (RTD) can be read by an ADC. Note that instead of driving the excitation current through the RTD only, a reference resistor is added in series with the RTD in order to derive the ADC’s reference voltage. In such a configuration, the ADC conversion result is essentially VRTD over VREF, which is exactly equal to RRTD over RREF and is completely independent of the value of the excitation current. As long as RREF remains constant, the ADC result will directly correlate with the RTD resistance, even in the face of changing excitation current. As already stated, this is only the case as long as RREF remains constant since any variation in RREF will directly result in a measurement error. This means that a low-drift resistor is typically necessary for this component, but that’s still a lot cheaper than a current source of similar precision. Note that, with the exception of the reference resistor and the RTD itself, everything in this diagram is fully contained in the ADuC7060, or any other sigma-delta MicroConverter product for that matter.