M68HC11
REFERENCE MANUAL
PINS AND CONNECTIONS
MOTOROLA
2-27
Figure 2-16 Internal Circuitry — Analog Input-Only Pin
If the pin voltage is driven low enough relative to the gate voltage of the analog multi-
plexer device, this N-channel device can turn on. A conductive path between the neg-
ative pin and the A/D capacitor array may discharge the capacitors and disrupt any A/
D conversion in progress. The thick-field protection device and other circuit and layout
measures around the N-channel multiplexer device are intended to prevent the pin
voltage from becoming negative enough to turn on the multiplexer device. Even with
these internal protective measures, a cautious user should avoid negative levels on
any A/D pin because a large negative transient could still disrupt an A/D conversion.
An A/D conversion can be disrupted in this manner if any A/D pin experiences a seri-
ous negative transient; the transient need not be on the pin associated with the con-
version.
External diode clamps to V
DD
are not necessarily a good idea on the analog inputs.
Leakage through an external diode would be significant in relationship to the pin leak-
age current; thus, this extra leakage could affect the accuracy of analog conversion
results. Analog input pins can usually be protected by a low-pass filter with enough se-
ries impedance to limit the pin voltage. The amount of series resistance is a trade-off
between a high enough value to limit pin voltage and a low enough value to prevent
pin leakage current from adversely affecting conversion results. Conversion accuracy
is specified for a maximum external series resistance of 10 k
. The worst-case spec-
ified leakage current at the pin is 400 nA (at room temperature, leakage is typically
much less). The 400 nA acting through 10 k
causes an absolute conversion error of
minus one-fifth of a least significant bit (LSB) when V
REF
is 5.12 V, leaving only about
one-quarter of an LSB for actual A/D circuit errors before the results would be out of
specified limits. Using a larger external resistance in series with an A/D pin may cause
some inaccuracy due to the leakage current acting through this resistance, but the A/
D will still respond in a predictable manner. There may be valid system design reasons
for choosing a high external series resistance (e.g., to minimize power consumption in
P
N
VDD
T
P
PIN
N
[1]
[2]
[3]
INPUT
BUFFER
N
[4]
[5]
ANALOG
MULTIPLEXER
N