AN2727 Freescale Semiconductor / Motorola, AN2727 Datasheet - Page 30

• Manufacturer Part Number: AN2727
• Description: Designing Hardware for the HCS12 D-Family
• Manufacturer: Freescale Semiconductor / Motorola

Connecting Capacitors Directly to Output Pins
channels (coupling ratio K), generating an error voltage proportional to the source resistance of the input
being converted.
The additional input voltage error on the converted channel can be calculated as V
with I
There are two coupling ratios specified:
Connecting Capacitors Directly to Output Pins
80-Pin Package Considerations
The DB128 80QFP pinout differs from all other 80-pin variants.
30
1. Avoid connecting capacitors directly to output pins as in Figure X in an attempt to prevent system
2. Connecting a large capacitor directly to an output pin (to create a long time delay, for example) can
•
•
INJ
Kp = 10
Kn = 10
noise reaching the MCU pin.
The capacitor will appear as low impedance to transitions of the output, resulting in fast rising
pulses of current and EMC noise.
A simple solution is to add some series resistance to the MCU side of the filter. This will increase
the rise time of the signal on the capacitor (due to the time constant of the increased effective O/P
impedance and the filter capacitor) and reduce the EMC generated.
result in exceeding the max IDD spec for the pin and damage to the MCU. Ensure that there is
enough series resistance from the MCU to any capacitor to limit the peak current to << 25 mA.
being the sum of the currents injected into the two pins adjacent to the converted channel.
-4
-2
for positive current injection, where the input is taken above VDDA
for negative current injection, where the input is taken below VSSA.
Designing Hardware for the HCS12 D-Family, Rev. 0
MCU
Output pin
Low pass filter
intended to keep
application noise
from reaching
the MCU
Freescale Semiconductor
ERR
= K * R
S
* I
INJ
,