MC68HC16Z1CAG16 Datasheet, PDF (204/500 Pages) Freescale Semiconductor, Inc – M68HC16Z Series users manual

Freescale Semiconductor, Inc.
NOTE
This star-point scheme still requires adequate grounding for digital
and analog subsystems in addition to the star-point ground.
Other suggestions for PCB layout in which the ADC is employed include the following:
• The analog ground must be low impedance to all analog ground points in the cir-
cuit.
• Bypass capacitors should be as close to the power pins as possible.
• The analog ground should be isolated from the digital ground. This can be done
by cutting a separate ground plane for the analog ground.
• Non-minimum traces should be utilized for connecting bypass capacitors and
filters to their corresponding ground/power points.
• Minimum distance for trace runs when possible.
8.8.4 Accommodating Positive/Negative Stress Conditions
Positive or negative stress refers to conditions which exceed nominally defined oper-
ating limits. Examples include applying a voltage exceeding the normal limit on an
input (for example, voltages outside of the suggested supply/reference ranges) or
causing currents into or out of the pin which exceed normal limits. ADC specific con-
siderations are voltages greater than V_DDA, V_RH or less than V_SSA applied to an analog
input which cause excessive currents into or out of the input. Refer to APPENDIX A
ELECTRICAL CHARACTERISTICS on exact magnitudes.
Both stress conditions can potentially disrupt conversion results on neighboring inputs.
Parasitic devices, associated with CMOS processes, can cause an immediate disrup-
tive influence on neighboring pins. Common examples of parasitic devices are diodes
to substrate and bipolar devices with the base terminal tied to substrate (V_SSI/V_SSA
ground). Under stress conditions, current introduced on an adjacent pin can cause er-
rors on adjacent channels by developing a voltage drop across the adjacent external
channel source impedances.
Figure 8-7 shows an active parasitic bipolar when an input pin is subjected to negative
stress conditions. Positive stress conditions do not activate a similar parasitic device.
NEGATIVE
STRESS
VOLTAGE
R_STRESS ^IOUT
PIN UNDER
STRESS
+
10K
PARASITIC
VDD
I_IN ^DEVICE
R_ADJACENT
ADJACENT
PINS
Figure 8-7 Input Pin Subjected to Negative Stress
ADC PAR STRESS CONN
8-18
ANALOG-TO-DIGITAL CONVERTER
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M68HC16 Z SERIES
USER’S MANUAL