PXS20 Datasheet, PDF (88/119 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Electrical characteristics
3.14 16 MHz RC oscillator electrical characteristics
Table 22. RC oscillator electrical characteristics
Symbol
Parameter
Conditions
f_RC
_RCMVAR
P RC oscillator frequency
T_J = 25 °C
P Fast internal RC oscillator variation with
—
respect to f_RC.
Min Typ Max Unit
— 16 — MHz
— — ±5 %
3.15 ADC electrical characteristics
The device provides a 12-bit Successive Approximation Register (SAR) Analog-to-Digital Converter.
Offset Error OSE Gain Error GE
4095
4094
4093
4092
4091
4090
code out
7
6
5
4
3
2
( 2)
(5)
(4)
(3)
1 LSB ideal =(VrefH-VrefL)/ 4096 =
3.3V/ 4096 = 0.806 mV
Total Unadjusted Error
TUE = +/- 6 LSB = +/- 4.84mV
( 1)
(1) Example of an actual transfer curve
(2) The ideal transfer curve
(3) Differential non-linearity error (DNL)
(4) Integral non-linearity error (INL)
(5) Center of a step of the actual transfer
curve
1
1 LSB (ideal)
0
1 23 45 67
Offset Error OSE
4089 4090 4091 4092 4093 4094 4095
V_in(A) (LSB_ideal)
Figure 8. ADC characteristics and error definitions
3.15.1 Input Impedance and ADC Accuracy
To preserve the accuracy of the A/D converter, it is necessary that analog input pins have low AC impedance. Placing a capacitor
with good high frequency characteristics at the input pin of the device can be effective: the capacitor should be as large as
possible, ideally infinite. This capacitor contributes to attenuating the noise present on the input pin; further, it sources charge
during the sampling phase, when the analog signal source is a high-impedance source.
A real filter can typically be obtained by using a series resistance with a capacitor on the input pin (simple RC filter). The RC
filtering may be limited according to the value of source impedance of the transducer or circuit supplying the analog signal to
PXS20 Microcontroller Data Sheet, Rev. 1
88
Freescale Semiconductor
Preliminary—Subject to Change Without Notice