Data Sheet

960
Atmel | SMART SAM D21 [DATASHEET]
Atmel-42181G–SAM-D21_Datasheet–09/2015
36.9.4.3 Inputs and Sample and Hold Acquisition Times
The analog voltage source must be able to charge the sample and hold (S/H) capacitor in the ADC in order to achieve
maximum accuracy. Seen externally the ADC input consists of a resistor ( ) and a capacitor ( ). In
addition, the source resistance ( ) must be taken into account when calculating the required sample and hold
time. Figure 36-5 shows the ADC input channel equivalent circuit.
Figure 36-5. ADC Input
To achieve n bits of accuracy, the capacitor must be charged at least to a voltage of
The minimum sampling time for a given can be found using this formula:
for a 12 bits accuracy:
where
R
SAMPLE
C
SAMPLE
R
SOURCE
R
SOURCE
R
SAMPLE
Analog Input
AINx
C
SAMPLE
V
IN
VDDANA/2
C
SAMPLE
V
CSAMPLE
V≥
IN
12
n 1+()–
–()×
t
SAMPLEHOLD
R
SOURCE
t
SAMPLEHOLD
R
SAMPLE
R+
SOURCE
()C
SAMPLE
()× n 1+() 2()ln××≥
t
SAMPLEHOLD
R
SAMPLE
R+
SOURCE
()C
SAMPLE
()× 9.02×≥
t
SAMPLEHOLD
1
2 f
ADC
×
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=