C8051F040/1/2/3/4/5/6/7
56 Rev. 1.5
5.3.3. 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 5.5 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 settling time for a given settling accuracy (SA) may be approximated by
Equation 5.2. When measuring the Temperature Sensor output, R
TOTAL
reduces to R
MUX
. 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 tracking requirements. See Table 5.2 for absolute
minimum settling/tracking time requirements.
Equation 5.2. 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 r
equired settling time in seconds
R
TOTAL
is the sum of the ADC0 MUX resistance and any external source resistance.
n is the AD
C resolution in bits (12).
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
= 10pF
C
SAMPLE
= 10pF
MUX Select
MUX Select
Differential Mode
AIN0.x
AIN0.y
R
MUX
= 5k
C
SAMPLE
= 10pF
RC
Input
= R
MUX
* C
SAMPLE
MUX Select
Single-Ended Mode
AIN0.x
