TC14433/A
DS21394D-page 8 © 2008 Microchip Technology Inc.
4.0 DETAILED DESCRIPTION
The TC14433 CMOS IC becomes a modified dual-
slope A/D with a minimum of external components.
This IC has the customary CMOS digital logic circuitry,
as well as CMOS analog circuitry. It provides the user
with digital functions such as (counters, latches,
multiplexers), and analog functions such as
(operational amplifiers and comparators) on a single
chip. Refer to the Functional Block diagram, Figure 4-3.
Features of the TC14433/A include auto-zero, high
input impedances and auto-polarity. Low power
consumption and a wide range of power supply
voltages are also advantages of this CMOS device.
The system’s auto-zero function compensates for the
offset voltage of the internal amplifiers and compara-
tors. In this “ratiometric system,” the output reading is
the ratio of the unknown voltage to the reference
voltage, where a ratio of 1 is equal to the maximum
count of 1999. It takes approximately 16,000 clock
periods to complete one conversion cycle. Each
conversion cycle may be divided into 6 segments.
Figure 4-1 shows the conversion cycle in 6 segments
for both positive and negative inputs.
i
FIGURE 4-1: Integrator Waveforms at
Pin 6.
Segment 1 – The offset capacitor (C
O
), which compen-
sates for the input offset voltages of the buffer and
integrator amplifiers, is charged during this period.
However, the integrator capacitor is shorted. This
segment requires 4000 clock periods.
Segment 2 – During this segment, the integrator output
decreases to the comparator threshold voltage. At this
time, a number of counts equivalent to the input offset
voltage of the comparator is stored in the offset latches
for later use in the auto-zero process. The time for this
segment is variable and less than 800 clock periods.
Segment 3 – This segment of the conversion cycle is
the same as Segment 1.
Segment 4 – Segment 4 is an up going ramp cycle with
the unknown input voltage (V
X
as the input to the
integrator. Figure 4-2 shows the equivalent
configuration of the analog section of the TC14433.
The actual configuration of the analog section is
dependent upon the polarity of the input voltage during
the previous conversion cycle.
FIGURE 4-2: Equivalent Circuit Diagrams
of the Analog Section During Segment 4 of the
Timing Cycle
Segment 5 – This segment is a down-going ramp
period with the reference voltage as the input to the
integrator. Segment 5 of the conversion cycle has a
time equal to the number of counts stored in the offset
storage latches during Segment 2. As a result, the
system zeros automatically.
Segment 6 – This is an extension of Segment 5. The
time period for this portion is 4000 clock periods. The
results of the A/D conversion cycle are determined in
this portion of the conversion cycle.
Start
1
2
3
4
5
6
Typical
Positive
Input Voltage
Typical
Negative
Input Voltage
Time
Segment
Number
End
V
X
V
X
C
1
Comparator
R
1
Buffer
Integrator
+
–
+
–
+
–
V
X
