tc7135 Microchip Technology Inc., tc7135 Datasheet - Page 6

• Manufacturer Part Number: tc7135
• Description: 4-1/2 Digit A/d Converter
• Manufacturer: Microchip Technology Inc.

TC7135
3.0
All pin designations refer to the 28-pin PDIP package.
3.1
The TC7135 is a dual-slope, integrating A/D converter.
An understanding of the dual-slope conversion
technique will aid in following the detailed TC7135
operational theory.
The conventional dual-slope converter measurement
cycle has two distinct phases:
1.
2.
The input signal being converted is integrated for a
fixed time period. Time is measured by counting clock
pulses. An opposite polarity constant reference voltage
is then integrated until the integrator output voltage
returns to zero. The reference integration time is
directly proportional to the input signal.
In a simple dual-slope converter, a complete
conversion requires the integrator output to “ramp-up”
and “ramp-down”.
A simple mathematical equation relates the input
signal, reference voltage and integration time:
EQUATION 3-1:
For a constant V
EQUATION 3-2:
TABLE 3-1:
DS21460D-page 6
Conversion Cycle Phase
System Zero
Input Signal Integration
Reference Voltage Integration
Integrator Output Zero
* Assumes a positive polarity input signal. SW
Where:
T
Input signal integration.
Reference voltage integration (de-integration).
DEINT
V
T
REF
----------------------- -
R
INT
INT
DETAILED DESCRIPTION
Dual-Slope Conversion Principles
1
C
= Reference voltage
= Signal integration time (fixed)
= Reference voltage integration time
INT
IN
(variable)
∫
V
T
0
INTERNAL ANALOG GATE STATUS
:
INT
IN
V
=
IN
V
------------------------------- -
T ( )DT
REF
T
T
INT
DEINT
=
Closed
V
------------------------------- -
SW
—
—
—
REF
R
INT
I
T
C
DEINT
RI
INT
Closed*
SW
would be closed for a negative input signal.
—
—
—
RI
+
SW
—
—
—
—
RI
-
Closed
SW
—
—
—
The dual-slope converter accuracy is unrelated to the
integrating resistor and capacitor values, as long as
they are stable during a measurement cycle. An
inherent benefit is noise immunity. Noise spikes are
integrated, or averaged, to zero during the integration
periods.
Integrated ADCs are immune to the large conversion
errors that plague successive approximation converters
in high-noise environments (see
FIGURE 3-1:
3.2
The TC7135 incorporates a system zero phase and
integrator output voltage zero phase to the normal two-
phase dual-slope measurement cycle. Reduced
system errors, fewer calibration steps and a shorter
overrange recovery time result.
The TC7135 measurement cycle contains four phases:
1.
2.
3.
4.
Internal analog gate status for each phase is shown in
Figure
Analog Input
Z
Signal
Voltage
System zero.
Analog input signal integration.
Reference voltage integration.
Integrator output zero.
REF
Integrate
Closed
3-1.
Signal
SW
Fixed
Time
Operational Theory
—
—
—
R
Polarity Control
Variable
Reference
Integrate
Time
Closed
Closed
Closed
SW
—
Display
Switch
Drive
1
Integrator
-
+
Basic Dual-Slope Converter.
© 2007 Microchip Technology Inc.
V
V
Closed
IN
IN
SW
Control
—
—
—
Phase
≈
≈
IZ
V
1/2 V
REF
Figure
+
-
Comparator
REF
Reference Figures
Control
Logic
3-1).
Figure 3-2
Figure 3-3
Figure 3-4
Figure 3-5
Counter
Clock