76
Service
2-10
shifting circuits are needed for the beeper drive. A voltage reference is generated separate
from U4, and some discrete resistors and transistors support the power supply.
AC Buffer 2-16.
The ac buffer can be configured for an ac gain of 1 or 10. R14 and R15 provide for the
X10 gain. C5 blocks any dc gain and provides a driven guard voltage. The ac buffer
drives the frequency comparator and the ac converter (U2).
AC Converter 2-17.
The ac converter (U2) is a monolithic true rms-to-dc converter. The signal to the
converter comes from the ac buffer. The signal is first attenuated by the voltage divider
(R10 and R11). This provides an input signal range of zero to 200 mV ac. The converter's
output range is zero to -200 mV DC. R34 is used to calibrate the ac gain of the Fluke
Model 76. The converter output goes to the active filter in U4. The microprocessor in U4
multiplies the A-D reading by two to give a zero to 4000 count display.
Active Filter 2-18.
The active filter uses components R17, R18, C8, and C9. The active filter is a second
order low-pass filter with two poles at 5.9 Hz in normal mode. It filters input signal noise
and ac ripple from the ac converter, yielding stable a/d converter readings. The
microcomputer can disable the filter completely or enable the filter fast response mode by
shorting R17 and R18 with internal IC switches.
A/D Converter 2-19.
Precision resistor network Z1-8, 9, and 10 connects to the three a/d buffer/integrator
range resistors. Z1-8 connects to 190 kΩ for the 1V range (de-integrate). Z1-9 connects
to 166 kΩ for the 400 mV range, and Z1-10 connects to 16 kΩ for the 40 mV range. Z1-
11 is the summing node of the integrator circuit. The autozero capacitor (C10) stores op
amp and comparator offsets. The integrator capacitor is C11.
The system reference voltage (1.23V) is generated by VR1 and R44. The 1.000V
reference voltage for the a/d converter is supplied through U4-1 (REFI). This voltage is
adjusted by R21, the dc calibration potentiometer in conjunction with R19 and R20. In
addition to generating the a/d reference, the VR1 voltage is used for power supply
reference, voltage comparator offset generation, and the Ohms and capacitance source
voltage.
Beeper 2-20.
Devices Q9, Q11, R57, R63, and R64 make up the beeper drive circuit.
Power Supply 2-21.
The power supply consists of two regulators, one shunt and one series, which set Vdd at
+3.0V and Vss at -3.2V for all battery voltages down to 6.5V. The shunt (common)
regulator sets |Vdgnd - Vss| (Vdgnd = COM = 0V) and consists of an op amp and current
shunt devices integrated on U4. Resistors R37 and R38 provide voltage division. The
series (Vdd) regulator, which sets |Vdd-Vdgnd|, is made up of another on-chip op amp,
along with devices Q3, R24, and R25. Q3 is the series regulator element, and R24 and
R25 are for voltage sensing. Capacitors C14 and C21 provide circuit compensation and
To Next Page
Loading...
