Fluke 76 - How to Measure Resistance

Theory of Operation
Circuit Descriptions 2
2-7
Analog Section of Integrated Multimeter IC (U4) 2-11.
The a/d converter, autorange switching, frequency comparator, and most of the remaining
analog circuitry are contained in the analog section of U4. Peripherals to this U4 analog
section include the crystal clock, the system reference voltage, the ac converter, and the
filter and amplifier resistors and capacitors.
U4 uses the dual-rate, dual-slope a/d converter circuit shown in Figure 2-4, A/D
Converter. For most measurements, the basic a/d conversion cycle is 25 ms, for a rate of
40 measurements per second. A single conversion at this rate is called a minor cycle
sample. Each minor cycle sample is used to provide updates at a rate of 40 per second for
the fast response bar graph display, and fast autoranging.
Eight minor cycle samples are necessary to accumulate data for displaying a full-
resolution (4000-count full scale) measurement on the digital display. A 40 ms autozero
phase occurs following every eight-sample sequence. Therefore, each digital display
update requires 240 ms, approximating four updates per second.
Basic a/d conversion elements and waveforms are illustrated in Figure 2-4, A/D
Converter. A voltage level proportional to the unknown input signal charges integrator
capacitor C11 for an exact amount of time. This capacitor is then discharged by a
reference voltage of opposite polarity. The discharge time, which is proportional to the
level of the unknown input signal, is measured by the digital circuits in U4 and sent to the
display.
Basic timing for the a/d converter is defined as a series of eight integrate and read (de-
integrate) cycles, followed by a 40 ms autozero phase. However, the 40 MΩ, capacitance,
overload recovery, autoranging, and Touch Hold
®
modes all require variations from the
basic timing.
Capacitance Measurements 2-12.
Refer to Figure 2-5, 100 nF Range Simplified Schematic. Capacitance measurements to
10,000 μF are made by measuring the charge required to change the voltage across the
unknown capacitor from zero to the system reference voltage. This technique is referred
to as a ballistic type of measurement, the configuration of which is the same as for Ohms.
The unknown capacitor is discharged through the 1.0001k Ohm resistor of Z1, then
charged during the a/d converter integrate cycle through the appropriate Z1 resistor. The
voltage drop across the Z1 resistor is integrated by the a/d converter. During the a/d read
cycle, the charge is held on the capacitor, and a count is accumulated. The microcomputer
calculates a display value from the latched count, the capacitor is discharged, and the
cycle repeats.
Frequency Measurements 2-13.
A voltage comparator is used for both signal detection in frequency mode and threshold
detection in continuity mode. In frequency mode, digital pulses from the voltage
comparator are routed to the counter. Pressing the range push button while in frequency
mode causes a range change in the primary function (ac volts) that may change the
sensitivity.