AN1812 Datasheet, PDF (22/34 Pages) Motorola, Inc – Freescale Semiconductor, Inc.

11 Result
AN1812
Figure 17 shows the relationship between measured voltages V_measured (average of the 16
readings measured by the converter) and input voltage V_in (As measured by the multimeter) for
negative voltages.
Figure 17. Measured vs input for negative voltages
Figure 18. Error vs input for negative input voltages
Note:
In Figure 18, we see that for the negative input voltages varying from 0 to -1V, the maximum
error is around -2.89mV for -1V input. The error of 0.5 mV occurs for the input value of -269 mV
and it increases gradually afterwards. The maximum difference between maximum value and
the minimum in loop is around 600 uV. So the accuracy of the average value measured is
around 3 mV, and without averaging the accuracy is around 3.6 mV.
11.3 Effect of the capacitor value
As discussed in error analysis (Section 7), reducing the R*C time constant by reducing the
value of R or C, will reduce the accuracy. We have also taken the reading for the capacitor value
of 10 uF. This shows that the accuracy of the ADC is reduced. An example is given in Figure 19
of readings with a 10 uF capacitor.
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