Siemens SAK-C167CR-L25M, Manual

Page: 9 / 11

Semiconductor Group Errata Sheet, C167CR-LM, ES-DB, DB, 1.1, Mh - 9 of 11 -
6. Effects on the Conversion Result and TUE
The effect on the conversion result and the TUE has to be calculated based on the current I
ANx
and
the impedace of the analog source R
ASRC
. I
ANn
causes an external voltage U
∆
n at the analog channel
ANn which is the reason for an additional unadjusted error AUE of the conversion result. This AUE
can increase the specified total unadjusted error TUE (Specified value: TUE =
±
2 LSB). The
voltage U
∆
n is nearly independent on the voltage value of the analog source.
U
∆
n = I
ANn
* R
ASRC
AUE = U
∆
n / 1 LSB [U
∆
n in mV and LSB in mV]
TUE = (
±
2 LSB) - AUE
Note:
The overload current crosstalk effect at the analog inputs decreases the analog signal voltage
supplied by the analog signal source (the crosstalk current flows into the µC). In case of a crosstalk
current at V
AGND
in combination with R
AGND
> 40 Ohm the analog signal voltage is increased.
5. Calculation Example
Assumed system values:
I
OV4
= -300 µA negative overload current at P5.4
R
ASRC
= 20 kOhm resistance of the external sensor at P5.5
V
AREF
= 5 V 1 LSB = 4.9 mV
R
AGND
= 0.1 Ohm
Ð
GND shift error is neglictable
I
AN5
= ovf1 * I
OV4
I
AN5
= (-4.7*E-3) * (- 300 µA)
I
AN5
= 1.41 µA
U
∆
5 = I
AN5
* R
ASRC
U
∆
5 = 1.41 µA * 20 kOhm
U
∆
5 = 28.2 mV
AUE = U
∆
n / 1 LSB
AUE = 28.2 mV / 4.9 mV
AUE = 5.8 LSB
Result:
The negative overload current I
OV4
of this system example can distort the real result of AN3 and AN4
by an additional unadjusted error, AUE = 5.8 LSB. The TUE is in the range of –7.8LSB to 3.8LSB.