7106_15 Datasheet, PDF (12/18 Pages) Unisonic Technologies – CONVERTERS

7106
CMOS IC
„ DETAILED DESCRIPTION(Cont.)
SYSTEM TIMING
Fig.6 shows the clocking arrangement used in the UTC 7106. Two basic clocking arrangements can be used:
1. Fig.6A. An external oscillator connected to pin 40.
2. Fig.6B. 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 ~ 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 1/3kHz, etc. should be selected. For 50Hz
rejection, Oscillator frequencies of 200kHz, 100kHz, 66 2/3kHz, 50kHz, 40kHz, etc. would be suitable. Note that
40kHz (2.5 readings/second) will reject both 50Hz and 60Hz (also 400Hz and 440Hz).
INTERNAL TO PART
÷4
CLOCK
INTERNAL TO PART
÷ 4 CLOCK
40
39
38
TEST
FIGURE 6A
40
Fig.6 Clock Circuits
39
38
R
C
RC OSCILLATOR
FIGURE 6B
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 4μA 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, 470kΩ is near optimum and similarly a 47kΩ 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 UTC 7106, when the analog
COMMON is used as a reference, a nominaul+2V full scale integrator swing is fine. For three readings/second
(48kHz clock) nominal values for C_INT are 0.22μF and 0.10μF, 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.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
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QW-R502-018.E