ICL7104
10-50K
R
C
INT
INT
100kΩ
+15V -15V
-BUF IN BUF OUT -INT IN INT OUT
REF
OUT
8
7
1
10
BUFFER
9
11
INTEG.
14
COMP.
6
3
INT.
REF.
COMP
OUT
300pF
-
-
A1
+
A2
-
+5V
+
A3
5kΩ
8068
2
+
-1.2V
5
+BUF IN 13
+INT IN 12
10kΩ
-15V
TO ICL7104
FIGURE 7. ADDING BUFFER GAIN TO ICL8068
COUNTS
PHASE I
32768
8192
PHASE II
32768
PHASE III
65536
-16
-14
8192
16384
POLARITY
DETECTED
ZERO CROSSING
OCCURS
INTEGRATOR
OUTPUT
ZERO CROSSING
DETECTED
AZ PHASE I
INT PHASE II
DEINT PHASE III
AZ
INTERNAL CLOCK
INTERNAL LATCH
STATUS OUTPUT
AFTER ZERO CROSSING,
ANALOG SECTION WILL
BE IN AUTOZERO
NUMBER OF COUNTS TO ZERO CROSSING
PROPORTIONAL TO V
IN
CONFIGURATION
FIGURE 8. CONVERSION TIMING
Integrating Resistor
to give 9 volt swing for full scale inputs. This is a compromise
between possibly saturating the integrator (at +14 volts) due
to tolerance build-up between the resistor, capacitor and
clock and the errors a lower voltage swing could induce due
to offsets referred to the output of the comparator. In
The integrating resistor is determined by the full scale input
voltage and the output current of the buffer used to charge
the integrator capacitor. This current should be small
compared to the output short circuit current such that
thermal effects are kept to a minimum and linearity is not
affected. Values of 5 to 40µA give good results with a
nominal of 20µA. The exact value may be chosen by:
general, the value of C
is given by:
INT
(32768 for - 16)
(8192 for -14)
× 20µA × clock period
= ----------------------------------------------------------------------------------------------------------
C
INT
Integrator Output Voltage Swing
A very important characteristic of the integrating capacitor is
that it have low dielectric absorption to prevent roll-over or
ratiometric errors. A good test for dielectric absorption is to
use the capacitor with the input tied to the reference.
full scale voltage (see note)
R
= ------------------------------------------------------------------------
INT
20µA
NOTE: If gain is used in the buffer amplifier then
(BufferGain) (full scale voltage)
R
= ---------------------------------------------------------------------------------------
INT
This ratiometric condition should read half scale (100...000)
and any deviation is probably due to dielectric absorption.
Polypropylene capacitors give undetectable errors at rea-
sonable cost. Polystyrene and polycarbonate capacitors
may also be used in less critical applications.
20µA
Integrating Capacitor
The product of integrating resistor and capacitor is selected
11
ETC [ ETC ]
