Theory of Operation
Introduction
2
2-3
Introduction 2-1.
This chapter describes the functional blocks shown in Figure 2-1. Overall descriptions
are broken down into Power Supply, Analog Circuitry, and Digital Circuitry.
For all measurements, inputs (e.g., Ë and â) are applied through overvoltage
(and overcurrent) protection circuits, switched to an appropriate range, and branched into
two signal paths. One path leads first to a Fast A/D Converter (8-bit, 4.8-MHz sample
rate) that digitizes the data, then to a digital gate array that stores and processes the data.
The other path leads to a Slow A/D Converter (high accuracy, dual slope) in the U30
custom IC. The microprocessor takes data from both the Slow A/D Converter (U30) and
the custom digital IC (U24) storage to simultaneously display a waveform and a 4-1/2
digit meter reading. Power supplies include 5.2V dc and -5.2V dc for analog circuitry,
3.3V dc for digital circuits, and VEE (nominally -23V dc) for the LCD module.
Start-Up Sequence 2-2.
The GMM sequences through the following steps when power is applied:
• The 3.3V power supply comes up.
• The reset pin on the microprocessor (U25-1) goes high.
• The microprocessor (U25) begins executing the program stored in EEPROMs U11
and U19.
• LCD controller (U13) data is initialized.
• VEE is turned on (-20V dc), and the display comes on.
Function Selection 2-3.
When the selector is turned to a new function, a rotary switch wiper sets up a resistor
divider by grounding the selected resistor in series with R170. The resulting voltage is
read by the microprocessor (U25-83).
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