8060A
Instruction Manual
4-8
The integrate period begins at the end of the autozero period. As the period
begins, the AZ switches open and the INTEG switches close. This applies
the unknown input voltage to the input of the converter. The voltage is
buffered and then begins charging the INTEG capacitor. The waveform at
the INTEG capacitor is a ramp from near zero to some maximum value
determined by the amplitude and polarity of the unknown input voltage.
As the read period begins, the INTEG switches opens and the READ
switches close. This applies the known reference voltage from a “flying”
capacitor whose polarity is chosen by the a/d converter to be the opposite of
the polarity of the unknown input voltage. The INTEG capacitor begins
discharging at a fixed rate while a counter begins counting. The counter
stops counting when the INTEG capacitor voltage equals the initial autozero
voltage. The count is proportional to the unknown input voltage, and is
placed on the display by the microcomputer.
If during the read period the counter counts up to the maximum number of
counts for a full-scale reading (19999 counts) and the INTEG capacitor
charge has not yet reached the initial autozero voltage, the microcomputer
knows an overrange reading has been taken. The microcomputer places
“OL” on the display and commands the a/d converter to go into the overload
(OL) period which rapidly slews the integrator voltage back to the initial
autozero voltage.
The measurement cycle ends at the end of the read period for an on-scale
reading, or at the end of the overload period for an overrange reading. A new
measurement cycle then begins with the autozero period. The display update
rate for measurement functions that use the a/d converter is approximately
0.4s, or about 2-1/2 readings per second.
4-6. Voltage Measurement
Both the ac and dc voltage ranges use an over-voltage protected 10 MΩ
input divider as shown in Figure 4-4. The over-voltage protection includes
two 2-watt fusible resistors and four metal-oxide varistors for high voltage
clamping. Depending on the range selected, lower leg resistors of the divider
are connected to ground to perform the input signal division.
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