10
FN3086.6
July 21, 2005
System Timing
Figure 7 shows the clocking arrangement used in the
ICL7136. Two basic clocking arrangements can be used:
1. Figure 9A, an external oscillator connected to DIP pin 40.
2. Figure 9B, an R-C oscillator using all three pins.
The oscillator frequency is divided by four before it clocks
the decade counters. It is then further divided to form the
three convert-cycle phases. These are signal integrate
(1000 counts), reference de-integrate (0 to 2000 counts) and
auto-zero (1000 to 3000 counts). For signals less than full
scale, auto-zero gets the unused portion of reference de-
integrate. This makes a complete measure cycle of 4,000
counts (16,000 clock pulses) independent of input voltage.
For three readings/second, an oscillator frequency of 48kHz
would be used.
To achieve maximum rejection of 60Hz pickup, the signal
integrate cycle should be a multiple of 60Hz. Oscillator
frequencies of 240kHz, 120kHz, 80kHz, 60kHz, 48kHz,
40kHz, 33
1
/
3
kHz, etc., should be selected. For 50Hz
rejection, Oscillator frequencies of 200kHz, 100kHz,
66
2
/
3
kHz, 50kHz, 40kHz, etc. would be suitable. Note that
40kHz (2.5 readings/sec.) will reject both 50Hz and 60Hz
(also 400Hz and 440Hz).
Component Value Selection
Integrating Resistor
Both the buffer amplifier and the integrator have a class A
output stage with 100µA of quiescent current. They can
supply 1µA of drive current with negligible nonlinearity. The
integrating resistor should be large enough to remain in this
very linear region over the input voltage range, but small
enough that undue leakage requirements are not placed on
the PC board. For 2V full scale, 1.8MΩ is near optimum and
similarly a 180kΩ for a 200mV scale.
Integrating Capacitor
The integrating capacitor should be selected to give the
maximum voltage swing that ensures tolerance buildup will
not saturate the integrator swing (approximately 0.3V from
either supply). In the ICL7136, when the analog COMMON
is used as a reference, a nominal +2V full-scale integrator
swing is fine. For three readings/second (48kHz clock)
nominal values for C
lNT
are 0.047µF and 0.5µF,
respectively. Of course, if different oscillator frequencies are
used, these values should be changed in inverse proportion
to maintain the same output swing.
An additional requirement of the integrating capacitor is that
it must have a low dielectric absorption to prevent roll-over
errors. While other types of capacitors are adequate for this
application, polypropylene capacitors give undetectable
errors at reasonable cost.
Auto-Zero Capacitor
The size of the auto-zero capacitor has some influence on
the noise of the system. For 200mV full scale where noise is
very important, a 0.47µF capacitor is recommended. On the
2V scale, a 0.047µF capacitor increases the speed of
recovery from overload and is adequate for noise on this
scale.
Reference Capacitor
A 0.1µF capacitor gives good results in most applications.
However, where a large common mode voltage exists (i.e.,
the REF LO pin is not at analog COMMON) and a 200mV
scale is used, a larger value is required to prevent roll-over
error. Generally 1µF will hold the roll-over error to 0.5 count
in this instance.
Oscillator Components
For all ranges of frequency a 180kΩ resistor is
recommended and the capacitor is selected from the
equation:
CLOCK
INTERNAL TO PART
40 39
38
³4
TEST
CLOCK
INTERNAL TO PART
40 39
38
³4
R
C
FIGURE 7B. RC OSCILLATOR
FIGURE 7. CLOCK CIRCUITS
FIGURE 7A. EXTERNAL OSCILLATOR
f
0.45
RC
-------------
For 48kHz Clock (3 Readings/s.),=
C50pF.=
ICL7136
