MC68HC05L28 Datasheet, PDF (102/156 Pages) Motorola, Inc – Flexible general-purpose microcomputer

9.3 ADx analog input
The external analog voltage value to be processed by the A/D converter is sampled on an internal
capacitor through a resistive path, provided by input-selection switches and a sampling aperture
time switch, as shown in Figure 9-2. Sampling time is limited to 12 bus clock cycles. After sampling,
the analog value is stored on the capacitor and held until the end of conversion. During this hold
time, the analog input is disconnected from the internal A/D system and the external voltage
source sees a high impedance input.
The equivalent analog input during sampling is an RC low-pass filter with a minimum resistance
of 50 kΩ and a capacitance of at least 10pF (these are typical values measured at room
temperature).
Analog
input
pin
Input protection device
≥ 50kΩ
< 2pF
+ ~20V
- ~0.7V
400 nA
junction
leakage
≥ 10pF
DAC
capacitance
V_RL
9
Note:
The analog switch is closed during the 12 cycle sample time only.
Figure 9-2 Electrical model of an A/D input pin
MOTOROLA
9-6
A/D CONVERTER
TPG
MC68HC05L28