88
Service Manual
2-12. AC Buffer
On the Meter, R14, R15, C5, and C27 are part of the ac buffer circuit that drives the ac
converter and guard. The Meter's ac buffer can be configured for an ac gain of 1 or 10.
2-13. AC Converter
The Meter's average ac converter uses off-chip components R30, R31, R34, R40, C24,
C28, and C29. This ac converter is a full-wave rectifying converter with a differential
output and gain selected to give a dc output equal to the rms value for a sine wave input.
Filtering is provided by C28 and C29.
2-14. Active Filter
The active filter uses off-chip passive components R17, R18, C8, and C9. It is a second
order, low pass filter with two poles at 5.9 Hz in normal mode that 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.
2-15. A/D Converter
Pins 8, 9, and 10 on precision resistor network Z1 connect to the three a/d converter gain
resistors. Pin 8 connects to 190 kW for a gain of 0.87. Pin 9 connects to 160 kW for a
gain of 1, and pin 10 connects to 16 kW for a gain of 10. Pin 11 is the common
connection. The autozero capacitor (C10) stores op amp and comparator offsets. The
integration capacitor is C11.
2-16. Beeper
Devices Q9, Q11, R56, R57 and CMOS hex inverter U6 make up the double-ended
beeper drive circuit. Transistor Q12 is connected to provide quick microcomputer power
down when the Meter is turned off.
2-17. Reference Voltage
The system reference voltage (1.235V) 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 addition to generating the a/d
reference, the VR1 voltage is used for power supply reference, voltage comparator
selectable offset generation, and the ohms source level.
2-18. Power Supply
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 6.7V. The shunt (common)
regulator sets |VDGND - VSS| (VDGND = COM = 0V) and consists of an
uncompensated op amp and large 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, uncompensated 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 power supply
decoupling for the shunt and series regulators, respectively. Voltage level information is
presented in Table 2-1. See Figure 3-7 for a simplified schematic of the power supply.
2-10 |
ACEC
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