Datasheet

1068
SAM4S [DATASHEET]
11100E–ATARM–24-Jul-13
43.7.1.1 Track and Hold Time versus Source Output Impedance
The following figure shows a simplified acquisition path.
Figure 43-13.Simplified Acquisition Path
During the tracking phase, the ADC needs to track the input signal during the tracking time shown below:
 10-bit mode: t
TRACK
= 0.042 x Zsource + 160
 12-bit mode: t
TRACK
= 0.054 x Zsource + 205
with t
TRACK
expressed in ns and Z
SOURCE
expressed in Ohms.
Two cases must be considered:
1. The calculated tracking time (t
TRACK
) is lower than 15 t
CP_ADC
.
Set TRANSFER = 1 and TRACTIM = 0.
In this case, the allowed Zsource can be computed versus the ADC frequency with the hypothesis of:
t
TRACK
= 15 × t
CP_ADC
:
Where t
CP_ADC
= 1/f
ADC
.
Table 43-38. Zsource Versus ADC Frequency
f
ADC
= ADC Clock (MHz) Z
SOURCE
(kOhms) for 12 bits Z
SOURCE
(kOhms) for 10 bits
20.00 10 14
16.00 14 19
10.67 22 30
8.00 31 41
6.40 40 52
5.33 48 63
4.57 57 74
4.00 66 85
3.56 74 97
3.20 83 108
2.91 92 119
2.67 100 130
2.46 109 141
2.29 118 152
2.13 126 164
2.00 135 175
1.00 274 353
Sample & HoldMux.
Zsource Ron
Csample
ADC
Input
12-bit
ADC
Core