C8051F336 Datasheet, PDF (47/234 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F336/7/8/9
7.2.3. Settling Time Requirements
A minimum tracking time is required before each conversion to ensure that an accurate conversion is per-
formed. This tracking time is determined by any series impedance, including the AMUX0 resistance, the
the ADC0 sampling capacitance, and the accuracy required for the conversion. Note that in low-power
tracking mode, three SAR clocks are used for tracking at the start of every conversion. For many applica-
tions, these three SAR clocks will meet the minimum tracking time requirements.
Figure 7.3 shows the equivalent ADC0 input circuits for both Differential and Single-ended modes. Notice
that the equivalent time constant for both input circuits is the same. The required ADC0 settling time for a
given settling accuracy (SA) may be approximated by Equation 7.1. When measuring the Temperature
Sensor output or V_DD with respect to GND, R_TOTAL reduces to R_MUX. See Table 6.8 for ADC0 minimum
settling time requirements as well as the mux impedance and sampling capacitor values.
t
=
ln
⎛
⎝
S^-2---A^{n--⎠⎞}
×
R_TOTALC_SAMPLE
Equation 7.1. ADC0 Settling Time Requirements
Where:
SA is the settling accuracy, given as a fraction of an LSB (for example, 0.25 to settle within 1/4 LSB)
t is the required settling time in seconds
R_TOTAL is the sum of the AMUX0 resistance and any external source resistance.
n is the ADC resolution in bits (10).
Differential Mode
Single-Ended Mode
MUX Select
MUX Select
Px.x
RMUX
CSAMPLE
Px.x
RMUX
RCInput= RMUX * CSAMPLE
RCInput= RMUX * CSAMPLE
CSAMPLE
Px.x
RMUX
MUX Select
Figure 7.3. ADC0 Equivalent Input Circuits
CSAMPLE
Rev. 0.2
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