Kinetis M series VREF module trim function linearity Analysis

Document created by Zhe Tian Employee on Mar 20, 2015Last modified by Zhe Tian Employee on Mar 20, 2015
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VREF module provide one input pin VREF, this pin can provide reference voltage for both external circuit but also for internal Sigma Delta ADCs, AFE and DAC.  Also it includes trim function in it. In this document, I would like to use my testing to do a linearity analysis.

From figure1 and 2, you can find how to make VREF as reference voltage of different analog module. For example, you can find when S1 switches down, S0 closes and S2 select VREF, VREF will be voltage resource of SAR ADC.



                                    Figure 1: Voltage reference function configuration


                                  Figure 2: Voltage reference module diagramming


I do a testing for VREF trim value linear analysis. The process is connecting the voltmeter to VREFH pin, then record measure result for each setting of VREF trim. There will be 64 records for every different setting. Testing chip is MKM34Z256. Please find my testing result from testing data table. From testing result, you can get the formula that y = kx + x0, where k is about 0.005 (5mV) and x0 is about 1.1881, x is trim value.  You can find there is only a little offset from offset curve.


                                        Table 1: Trim voltage testing date



                                                           Figure 3: Offset curve