
M68HC11
REFERENCE MANUAL
ANALOG-TO-DIGITAL CONVERTER SYSTEM
MOTOROLA
12-21
Figure 12-6 Graphic Estimation of Analog Sample Level (Case 2)
Waveform [3] is the familiar RC exponential decay through the external network, and
it is assumed that the external capacitance is very large compared to the 20 pF DAC
capacitance. The time, t
1
, marks the end of the 12-cycle sample period, and level [5]
is the level that will be captured, held, and converted even though the pin voltage con-
tinues to decay through the external network.
The third case arises when the external time constant is very long compared to the
time between samples. In this case, the residual charge redistributed during the first
sample is not completely dissipated before the next sample; thus, there is an accumu-
lation of charge. This accumulation causes increasing errors on successive samples
until equilibrium is reached between the charge added during a sample and the charge
dissipated between samples. Errors caused by cases of this third type are often mis-
interpreted as leakage between adjacent channels. The magnitude of this type error
is estimated by developing equations for voltage change caused by charge added dur-
ing a sample time and voltage change caused by the action of the external RC be-
tween successive samples. These two equations are set equal to each other, and the
equilibrium voltage can then be resolved.
The voltage change during the sample is controlled by the ratio of the internal DAC
capacitance to the external capacitance (as in the previous case), but the external RC
is assumed to be so long that it produces no significant effect until after the sample
time ends. The voltage change between samples results from a simple RC exponen-
tial decay.
To illustrate the estimation of errors resulting from a case-three situation, consider this
example. The E-clock rate equals 2 MHz, V
RH
equals 5.12 V, V
RL
equals 0.0 V, chan-
nel one is connected to a 5.12-V analog level, and channel 2 is connected to a 0-V
[1]
[2]
[3]
[4]
[5]
t
1
t
0
0
.
0 V
TIME
P