Settling Time Requirements
The absolute minimum tracking time is given in the electrical specifications tables. It may be necessary to track for longer than the mini-
mum tracking time specification, depending on the application. For example, if the ADC input is presented with a large series impe-
dance, it will take longer for the sampling cap to settle on the final value during the tracking phase. The exact amount of tracking time
required is a function of all series impedance (including the internal mux impedance and any external impedance sources), the sam-
pling capacitance, and the desired accuracy.
R
MUX
C
SAMPLE
RC
Input
= R
MUX
* C
SAMPLE
MUX Select
Input
Channel
Note: The value of
C
SAMPLE
depends on the PGA gain. See the electrical specifications for details.
Figure 12.2. ADC Equivalent Input Circuit
The required ADC0 settling time for a given settling accuracy (SA) may be approximated as follows:
t
= ln
(
2
n
SA
)
x
R
TOTAL
x
C
SAMPLE
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 ADC mux resistance and any external source resistance.
C
SAMPLE
is the size of the ADC sampling capacitor.
n is the ADC resolution in bits.
When measuring any internal source, R
TOTAL
reduces to R
MUX
. See the electrical specification tables in the datasheet for ADC mini-
mum settling time requirements as well as the mux impedance and sampling capacitor values.
EFM8UB3 Reference Manual
Analog-to-Digital Converter (ADC0)
silabs.com
| Building a more connected world.
Rev. 0.2 | 127