C8051F321-GMR Datasheet, PDF (45/250 Pages) Silicon Laboratories – Full Speed USB, 16 k ISP FLASH MCU Family

C8051F320/1
5.3.3. Settling Time Requirements
When the ADC0 input configuration is changed (i.e., a different AMUX0 selection is made), a minimum
tracking time is required before an accurate conversion can be performed. This tracking time is determined
by the AMUX0 resistance, the ADC0 sampling capacitance, any external source resistance, and the accu-
racy 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 most applications, these three SAR clocks will meet the mini-
mum tracking time requirements.
Figure 5.5 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 5.1. When measuring the Temperature
Sensor output or VDD with respect to GND, R_TOTAL reduces to R_MUX. See Table 5.1 for ADC0 minimum
settling time requirements.
Equation 5.1. ADC0 Settling Time Requirements
t
=
ln


S^-2---A^{n--}

R_TOTALC_SAMPLE
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
MUX
Select
Single-Ended Mode
MUX Select
Px.x
R_MUX = 5k
C_SAMPLE = 5pF
Px.x
R_MUX = 5k
RC_Input= R_MUX * C_SAMPLE
RC_Input= R_MUX * C_SAMPLE
C_SAMPLE = 5pF
Px.x
R_MUX = 5k
MUX Select
Figure 5.5. ADC0 Equivalent Input Circuits
C_SAMPLE = 5pF
Rev. 1.4
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