Fluke 2180A - Manual (Page 25)

2180A
Section 3
Theory of Operation
3-1. INTRODUCTION
3-2. This section of the manual contains an overall
functional description and a brief circuit analysis of the
2180A Digital Thermometer. Simplified circuit diagrams
are provided, as necessary, to supplement the text.
Detailed schematics are given in Section 8 of this manual.
3-3. The Model 2180A is a single point RTD Digital
Thermometer with 0.1° and .01° resolution for either
degrees F or C. This instrument features dual slope A/D
conversion, microcomputer control logic, and a 5-digit
display with temperature scale indicator. Various RTD
types can be used over a temperature range of -394° F to
1435°F (-219°C to 664°C). Refer to Figure 3-1 for the
following functional description. Mnemonic definitions
are provided in Table 3-1.
3-4. FUNCTIONAL DESCRIPTION
3-5. The Model 2180A executes a continuous series of
measurement cycles. These cycles, controlled entirely by a
microcomputer, include three major periods: the Auto-
Zero, the Integrate, and the Read periods. During each
period, digital controls are applied to the analog section
of a dual slope converter. The converter in turn generates
a compare output. The configuration of the analog
section during each phase of the measurement cycle is
established by the condition of microcomputer controlled
FET switches.
3-6. The measurement cycle begins with the Auto-Zero
period. During this period, the input to the Buffer
Amplifier is connected to ground through an FET switch
and the accumulated dc offset voltages present in the
analog section are sampled and held by the Auto-Zero
capacitor. This voltage is used later in the measurement
cycle to cancel measurement errors introduced by offset
voltages in the analog circuitry. The final measurement is
therefore proportional to the RTD probe output voltage
and does not include offset errors.
3-7. During the Integrate period, the RTD input voltage
read across the RTD, is applied to the integrator. The
algebraic sum of the AZ and RTD input voltages is
integrated over a 100 ms period. At the end of this period,
the RTD input voltage is removed from the integrator,
and the Read period is started.
3-8. A reference voltage is applied to the integrator
during the Read period, causing the integrator capacitor
to be discharged at a linear rate. When the integrator
output reaches zero, a compare signal is generated to end
the Read period. The duration of the Read period is
translated by the microcomputer, to provide a digital
indication of the RTD temperature.
RTD
DUAL SLOPE
ANALOG CIRCUIT
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MICROCOMPUTER
CONTROL & LINEARIZATION
DATA
DISPLAY
Figure 3-1. 2180A Simplified Block Diagram
3-1