AN1058 Freescale Semiconductor / Motorola, AN1058 Datasheet - Page 22

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

Application Note
22
feeding the 324 buffers were slowly varying dc or signals with
frequencies below 500 Hz. Yet, the full bandwidth of signals allowed by
the buffer passed unaltered to the ADC inputs. This manipulation
violated a design guideline that urges the designer to tailor the
bandwidth of each ADC channel to the bandwidth of the input signal. By
properly filtering the input to the ADC, frequencies that may prove
troublesome if left unfiltered will not be allowed to pass to the ADC input.
To test the effect of this guideline on this specific industrial controller,
0.01 F capacitors were soldered to the ADC input pins at the
M68HC11. The measurements taken with this configuration (see
Figure
there were still occasional occurrences of out-of-spec ADC readings.
In the absence of other guidelines, the only choice left to achieve
specified ADC performance was to refine the implementation of the
existing guidelines. The second guideline, duplicate ideal operating
conditions, is usually the most likely candidate for improvement. One of
the corollaries to duplicating ideal operating conditions is reducing
unwanted interaction between adjacent circuit segments. In this case,
V
local IC regulator for just the M68HC11, an alternative method of V
isolation was attempted: a series diode with V
Note: Readings are rounded to nearest 1% value. Columns with 0% and a grey bar imply >0% and <0.5%.
READING
DD
B9–C0
C1–FF
(HEX)
99–A0
A1–A8
A9–B0
B1–B8
81–98
Freescale Semiconductor, Inc.
AD C
0–80
noise had not been completely eradicated. Rather than inserting a
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11) showed significant improvements. As shown in
FREQ .
100 %
0 %
0 %
0 %
0 %
0 %
0 %
0 %
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EXPANDED SCALE
0
CO NDITIO NS: 0.01 F CAPAC ITO R AT ADC PIN
TO TAL SAM PLES = 16,384
5
Figure 11. Capacitor on ADC Pin
10
20
30
PERCENT OCCURRENCE
40
50
DD
forming a peak detector
60
M 68HC 11 IND USTRIAL
70
CO NTRO LLER
80
Figure
MOTOROLA
90
AN1058
11,
DD
100