C8051F120/1/2/3/4/5/6/7
88
Rev. 1.2
7.2.3. Settling Time Requirements
When the ADC2 input configuration is changed (i.e., a different MUX or PGA selection), a minimum tracking time is
required before an accurate conversion can be performed. This tracking time is determined by the ADC2 MUX resis-
tance, the ADC2 sampling capacitance, any external source resistance, and the accuracy required for the conversion.
Figure 7.3 shows the equivalent ADC2 input circuit. The required ADC2 settling time for a given settling accuracy
(SA) may be approximated by Equation 7.1. Note: An absolute minimum settling time of 800 ns required after any
MUX selection. Note that in low-power tracking mode, three SAR2 clocks are used for tracking at the start of every
conversion. For most applications, these three SAR2 clocks will meet the tracking 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 ADC2 MUX resistance and any external source resistance.
n
is the ADC resolution in bits (8).
Equation 7.1. ADC2 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
AIN2.x
AIN2.y
R
MUX
= 5k
C
SAMPLE
= 5pF
RC
Input
= R
MUX
* C
SAMPLE
MUX Select
Single-Ended Mode
AIN2.x
Note: When the PGA gain is set to 0.5,
C
SAMPLE
= 3pF
Figure 7.3. ADC2 Equivalent Input Circuit
Summary of Contents for C8051F120
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Page 8: ...C8051F120 1 2 3 4 5 6 7 8 Rev 1 2 26 2 Flash Programming Commands 318 26 3 Debug Support 321 ...
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