Theory of Operation
Detailed Description
2
2-5
2-9. Active Filter
An active filter that includes R9, R10, C5, and C6 is located in U1.
Conditioned input signals are passed through the active filter in route to the
a/d converter section of U1. (See Figure 2-2, AC and A/D Converter.)
The clock frequency for the digital portion of the circuit is a function of
32.768-kHz crystal Y1. Y1, C12, C15, and amplifiers in U1 make up the
oscillator circuit.
2-10. Rotary Switch
Rotary switch S1 FRONT selects and routes the input signals. Function codes
for switch S1 REAR are shown in Table 2-1. Range switch S2 signals the
microcomputer U2 for the manual ranging and automatic Touch Hold
function.
CR1 acts as protection for U1 if the battery is installed backwards. C2 is part
of the power-on reset for microcomputer U2.
2-11. A/D Conversion
Analog-to-digital (a/d) conversion is accomplished within U1 using a
modified dualslope a/d converter circuit. (See Figure 2-2, AC and A/D
Converter.)
Since the a/d conversion process is essentially a dual slope method, two
voltages are required to complete a measurement cycle. One is the unknown
input and the other is the reference voltage.
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