Device User Guide — 9S12DT128DGV2/D V02.16
108
Freescale Semiconductor
specifies results in an error of less than 1/2 LSB (2.5mV) at the maximum leakage current. If device or
operating conditions are less than worst case or leakage-induced error is acceptable, larger values of source
resistance is allowed.
A.2.2.2 Source capacitance
When sampling an additional internal capacitor is switched to the input. This can cause a voltage drop due
to charge sharing with the external and the pin capacitance. For a maximum sampling error of the input
voltage
≤
1LSB, then the external filter capacitor, C
f
≥
1024 * (C
INS
– C
INN
).
A.2.2.3 Current injection
There are two cases to consider.
1.
A current is injected into the channel being converted. The channel being stressed has conversion
values of $3FF ($FF in 8-bit mode) for analog inputs greater than V
RH
and $000 for values less than
V
RL
unless the current is higher than specified as disruptive conditions.
2.
Current is injected into pins in the neighborhood of the channel being converted. A portion of this
current is picked up by the channel (coupling ratio K), This additional current impacts the accuracy
of the conversion depending on the source resistance.
The additional input voltage error on the converted channel can be calculated as V
ERR
= K * R
S
*
I
INJ
, with I
INJ
being the sum of the currents injected into the two pins adjacent to the converted
channel.
Table A-9 ATD Electrical Characteristics
Conditions are shown in (Table A-4) unless otherwise noted
Num C
Rating
Symbol
Min
Typ
Max
Unit
1
C Max input Source Resistance
R
S
-
-
1
K
Ω
2
T
Total Input Capacitance
Non Sampling
Sampling
C
INN
C
INS
10
22
pF
3
C Disruptive Analog Input Current
I
NA
-2.5
2.5
mA
4
C Coupling Ratio positive current injection
K
p
10
-4
A/A
5
C Coupling Ratio negative current injection
K
n
10
-2
A/A
Summary of Contents for MC9S12A128
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