Fluke 77 - Manual (Page 22)

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.