Settling Time Requirements; Figure 6.4. Adc0 Equivalent Input Circuits - Silicon Laboratories C8051F12 Series Manual

C8051F120/1/2/3/4/5/6/7 C8051F130/1/2/3

6.2.3. 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 6.4 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 6.1. When measuring the Temperature Sensor output, R
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.
Equation 6.1. 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 is the sum of the ADC0 MUX resistance and any external source resistance.
TOTAL
n is the ADC resolution in bits (10).
Differential Mode
MUX Select
AIN0.x
R
MUX
RC = R * C
Input MUX SAMPLE
AIN0.y
R
MUX
MUX Select
n
 
2
×
------ -
t = ln
 
SA
= 5k
C = 10pF
SAMPLE
C = 10pF
SAMPLE
= 5k

Figure 6.4. ADC0 Equivalent Input Circuits

reduces to R
TOTAL
R C
TOTAL SAMPLE
Single-Ended Mode
MUX Select
AIN0.x
RC = R
Input MUX
Rev. 1.4
. An absolute
MUX
R = 5k
MUX
C = 10pF
SAMPLE
* C
SAMPLE
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