Specifications

C8051F330/1
40 Rev. 1.1
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 accuracy required for the conver
-
sion. 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 minimum tracking time requirements.
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 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.
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).
Equation 5.1. ADC0 Settling Time Requirements
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
Px.x
Px.x
R
MUX
= 5k
C
SAMPLE
= 5pF
RC
Input
= R
MUX
* C
SAMPLE
MUX Select
Single-Ended Mode
Px.x
Figure 5.4. ADC0 Equivalent Input Circuits