Motorola MCF5281 사용자 설명서

MOTOROLA

Chapter 27.  Queued Analog-to-Digital Converter (QADC)  

27-75

A series resistor limits the current to a signal; therefore, input leakage acting through a large
source impedance can degrade A/D accuracy. The maximum input leakage current is
specified in MCF5282 Electrical Specifications. Input leakage is greater at higher operating
temperatures. In the temperature range from 125°C to 50°C, the leakage current is halved
for every 8°C to 12°C reduction in temperature.

Assuming V

RH

–V

RL

 = 5.12 V, 1 count (with 10-bit resolution) corresponds to 5 mV of

input voltage. A typical input leakage of 200 nA acting through 10 k

Ω of external series

resistance results in an error of 0.4 count (2.0 mV). If the source impedance is 100 k

Ω and

a typical leakage of 100 nA is present, an error of 2 counts (10 mV) is introduced.

In addition to internal junction leakage, external leakage (for example, if external clamping
diodes are used) and charge sharing effects with internal capacitors also contribute to the
total leakage current. Table 27-25 illustrates the effect of different levels of total leakage on
accuracy for different values of source impedance. The error is listed in terms of 10-bit
counts.

Leakage below 200 nA is obtainable only within a limited
temperature range.

The four interrupt lines are outputs of the module and have no priority or arbitration within
the module. 

QADC inputs can be monitored by polling or by using interrupts. When interrupts are not
needed, the completion flag and the pause flag for each queue can be monitored in the status
register (QASR0). In other words, flag bits can be polled to determine when new results are
available.

Table 27-26 shows the status flag and interrupt enable bits which correspond to queue 1 and
queue 2 activity.

1 k

Ω

—

—

0.1 counts

0.2 counts

10 k

Ω

0.2 counts

0.4 counts

1 counts

2 counts

100 k

Ω

2 counts

4 count

10 counts

20 counts