C8051F124-GQR Datasheet, PDF (59/350 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F120/1/2/3/4/5/6/7
C8051F130/1/2/3
5.2.3. Settling Time Requirements
A minimum tracking time is required before an accurate conversion can be performed. This tracking time is
determined by the ADC0 MUX resistance, the ADC0 sampling capacitance, any external source resis-
tance, and the accuracy required for the conversion. Figure 5.4 shows the equivalent ADC0 input circuits
for both Differential and Single-ended modes. Notice that the equivalent time constant for both input cir-
cuits is the same. The required settling time for a given settling accuracy (SA) may be approximated by
Equation 5.1. When measuring the Temperature Sensor output, R_TOTAL reduces to R_MUX. An absolute
minimum settling time of 1.5 µs is required after any MUX or PGA selection. Note that in low-power track-
ing mode, three SAR clocks are used for tracking at the start of every conversion. For most applications,
these three SAR clocks will meet the tracking requirements.
t
=
ln


S^-2---A^{n--}

R_TOTALC_SAMPLE
Equation 5.1. 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 ADC0 MUX resistance and any external source resistance.
n is the ADC resolution in bits (12).
Differential Mode
MUX Select
Single-Ended Mode
MUX Select
AIN0.x
R_MUX = 5k
C_SAMPLE = 10pF
AIN0.x
R_MUX = 5k
RC_Input= R_MUX * C_SAMPLE
RC_Input= R_MUX * C_SAMPLE
C_SAMPLE = 10pF
AIN0.y
R_MUX = 5k
MUX Select
Figure 5.4. ADC0 Equivalent Input Circuits
C_SAMPLE = 10pF
Rev. 1.4
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