C8051F300/1/2/3/4/5
Rev. 2.9
41
5.3.3. Settling Time Requirements
When the ADC0 input configuration is changed (i.e., a different AMUX0 or PGA selection is made), a mini-
mum tracking time is required before an accurate conversion can be performed. This tracking time is deter-
mined by the AMUX0 resistance, the ADC0 sampling capacitance, any external source resistance, 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 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 V
DD
with respect to GND,
R
TOTAL
reduces to
R
MUX
. See Table 5.1 for ADC0 minimum
settling time (track/hold time) requirements.
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 AMUX0 resistance and any external source resistance.
n
is the ADC resolution in bits (8).
Figure 5.5. ADC0 Equivalent Input Circuits
t
2
n
SA
-------
R
TOTAL
C
SAMPLE
×
ln
=
R
MUX
= 5k
RC
Input
= R
MUX
* C
SAMPLE
R
MUX
= 5k
C
SAMPLE
= 5pF
C
SAMPLE
= 5pF
MUX Select
MUX Select
Differential Mode
P0.x
P0.y
R
MUX
= 5k
C
SAMPLE
= 5pF
RC
Input
= R
MUX
* C
SAMPLE
MUX Select
Single-Ended Mode
P0.x
Note: When the PGA gain is set to 0.5,
C
SAMPLE
= 3pF
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