Datasheet

Appendix A Electrical Characteristics
MC9S12E128 Data Sheet, Rev. 1.07
Freescale Semiconductor 591
A.6.3.2 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.6.3.3 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 0x3FF (0xFF in 8-bit mode) for analog inputs greater than VRH and 0x000 for values
less than VRL 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-21. 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 KΩ
2 T Total Input Capacitance
Non Sampling
Sampling
C
INN
C
INS
—
—
—
—
10
15
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