
M68HC11
REFERENCE MANUAL
ANALOG-TO-DIGITAL CONVERTER SYSTEM
MOTOROLA
12-1
SECTION 12
ANALOG-TO-DIGITAL CONVERTER SYSTEM
The MC68HC11A8 analog-to-digital (A/D) converter system uses an all-capacitive
charge-redistribution technique for conversions. The A/D system is an 8-channel, 8-
bit, successive approximation converter with
±
1/2 least significant bit (LSB) accuracy
over the complete operating temperature range. Because of the charge-redistribution
technique, no external sample and hold circuits are required.
This section first discusses the charge-redistribution technique, which is useful in un-
derstanding subtle details concerning the application of the MC68HC11A8 A/D sys-
tem. The second part of this section includes a detailed description of the
MC68HC11A8 A/D system and features. The section concludes with a discussion of
how external circuitry can influence A/D accuracy.
12.1 Charge-Redistribution A/D
Figure 12-1
shows a simplified circuit to perform a 4-bit, successive-approximation A/
D conversion using charge redistribution. The actual circuit used in the MC68HC11A8
includes several additions to this simple circuit to improve quality and simplify manu-
facturing. After an initial introduction to the charge-redistribution technique, some of
these additions are discussed. Since the capacitive charge redistribution technique
depends upon capacitance ratios rather than absolute capacitance values, the capac-
itors in
Figure 12-1
are marked in units.
During the sample time (
Figure 12-1
(a)), the top plate of all capacitors is switched to
V
L
(0.0 V), and the bottom plates are connected to the unknown analog input, V
X
. Us-
ing the simple relationship Q
S
equals CV, the total charge may be calculated by
Q
S
= 16(V
X
– V
L
)
Unless otherwise stated, it is assumed V
equals 0.0 V; therefore,
Q
S
= 16 V
X
Next, the circuit is changed to the hold state by logically controlled analog switches
(
Figure 12-1
(b)). In this state, the top plates are disconnected from V
L
, and the bottom
plates are switched from V
X
to V
L
. The charge is now written as
Q
H
= (V
L
– V
i
) 16
Since V
L
= 0
Q
H
= – 16 V
i