AN1058 Freescale Semiconductor / Motorola, AN1058 Datasheet - Page 14

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

Application Note
Leakage Current
on ADC Input Pin
Charge Time
on Sample
Capacitor
14
Major sources of M68HC11 ADC errors controlled by external circuit
parameters are discussed in the following paragraphs.
The electrical model of an M68HC11 ADC input pin is shown in
Figure
at this node (no p-channel devices are used here), which are worse at
high temperatures.
Consequently, the leakage current is 1) unidirectional and 2) bound by
the maximum specification of 400 nA. This leakage-induced error would
tend to only cause a static lowering of ADC results. This leakage-
induced error would tend to only cause a static lowering of ADC results.
To avoid leakage effects, the external circuit network feeding the ADC
pin(s) should maintain impedances, which in the presence of maximum
leakage, would guarantee a maximum desired error. For example, if the
maximum error (due to leakage) is desired to be 1 LSB with a 5-volt
reference voltage, then the maximum source impedance (resistance)
feeding this pin should be 50 k = ( 19.5 mV/400 nA).
By lengthening the resistance-capacitance (RC) time constant,
comprised of the source resistance feeding the ADC pin and the DAC
capacitance evidenced at the pin during the sample mode, errors may
result. However, given the size of the DAC input capacitance, the size of
the source resistance necessary to induce these RC time-constant
errors will probably be inundated by the effects of the pin leakage
described previously.
Freescale Semiconductor, Inc.
Note: This analog switch is closed only during the 12-cycle sample time.
For More Information On This Product,
Figure 6. Electrical Model of an M68HC11 ADC Input Pin
ANALOG
INPUT
6. The problem is caused by n-channel device junction leakages
PIN
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< 2 pF
+ ~20 V
– ~0.7 V
JUNCTION
LEAKAGE
400 nA
DIFFUSION AND
POLY COUPLER
4 k
SEE NOTE
V
RL
CAPACITANCE
~20 pF
MOTOROLA
DAC
AN1058