AN1058 Freescale Semiconductor / Motorola, AN1058 Datasheet - Page 17

• Manufacturer Part Number: AN1058
• Description: Reducing A/D Errors in Microcontroller Applications
• Manufacturer: Freescale Semiconductor / Motorola

Real-World Example
The Problem
AN1058
MOTOROLA
When discussing the mechanics of noise phenomena in MCU/ADC
systems, it is very difficult to understand how large the noise problem is,
how well it is expected to respond to corrective action, and how closely
the analysis matches the real world. To help resolve these problems, an
actual troubleshooting session involving an M68HC11-based assembly
is presented.
The subject assembly, an industrial controller, is a typical MCU/CPU
installation utilizing the M68HC11 in expanded multiplexed (CPU) mode.
The customer designed the program memory to expand to 32 K x 8,
RAM to 2 K x 8, an external address decoder, some additional digital I/O
lines, and analog buffers feeding the ADC inputs. Built on a 6-layer PCB,
the assembly had the benefit of separate ground and voltage planes and
was designed to be placed in a Faraday shield providing
electromagnetic compatibility. This assembly was designed without the
aid of any of the concepts presented in this application note.
Understandably, the customer was having difficulty with ADC
performance.
Functionally, the ADC noise problem manifested itself as an extreme
shutdown condition in the final product. Since this assembly provides
control to industrial equipment, conditions sensed by this controller could
indicate dangerous conditions, which must be dealt with by severe and
swift action, including functional shutdown of the controlled equipment.
To achieve the safest response times and largest safety margins to such
stimuli, the software designers of this system required 64 s continuous
conversions (> 15-kHz sampling frequency). Once they were run
through part of the designer’s algorithm, the conversion results could not
deviate more than 2 counts from the actual system ADC
measurements. The M68HC11 was selected for this application
because of its high level of integration as well as the 1 least significant
bit (LSB) 8-bit ADC performance.
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
Real-World Example
Application Note
17