Question

Why does the EFM8LB1 temperature sensor calibration use the 1.65 V internal voltage reference?

Answer

There are multiple reasons for using the 1.65V reference.

 

The three internal references all have similar accuracy, since the accuracy is dominated by the bandgap reference that is used for all three of them. However, there is no way to directly measure the 1.65V reference; it must be inferred by applying a precise input voltage to the ADC and measuring the output codes using the 1.65V reference.  This adds some additional uncertainty to the measurement.  The 1.65V reference is also a little lower, so its measurement is more affected by ADC noise.  Finally, there is some rounding error in the reported accuracy because we round the result to the nearest 10 mV.  For these reasons, the specifications on the 1.65V internal reference are slightly relaxed compared to the other two references.

 

The 1.65V reference has essentially the same temperature performance as the other two references, it settles quickly, and it does not require an external pin. These attributes make it more desirable for use with the temperature sensor.  We wanted the temp sensor to be completely self-contained on the die to eliminate any interference from other blocks on the chip or from the PCB.  The 1.2V and 2.4V references can be used for biasing the comparators or the DACs on LB1, which could have resulted in comparator or DAC noise being coupled into the temperature measurement.  Also, in some applications, the firmware may put the device into a low-power state when the temp sensor is not being used, and the 1.2V/2.4V reference has a very long startup time.  This will substantially increase the power associated with one set of temperature measurements, since the reference will need to be powered up for several ms before its output is stable.  More likely, the 1.2V/2.4V reference would just be left on all the time.

 

The accuracy of the 1.65V reference is built in to the accuracy for the temperature sensor, and it was exclusively used for all of the temperature sensor characterization.  This reference can only be used by the ADC.  By using the 1.65V reference for the temperature sensor, we free up the 1.2V/2.4V references for use by other resources, or we give the system the option to avoid using that reference at all, and using the pin for something else.

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