Theory of Operation
Microcomputer IC (U2)
2
2-3
Finally, digital circuitry interfaces with U2 via a parallel address and bidirectional data
bus.
Voltage is measured using a ratio comparison of the unknown voltage to the reference
voltage (REFI pin). Resistance is calculated using a ratio comparison of the voltage
across the unknown resistor to the voltage across a precision reference resistor, with the
same current in both. Capacitance is measured by determining the amount of charge
added for a given dc voltage change.
Microcomputer IC (U2)
U2 writes range settings and a/d converter information to U1. U2 reads a/d converter
results and status information. This includes the low battery check, slide-switch position,
continuity check, and Automatic Selection data. The microcomputer performs math
operations on the raw data from U1 and configures it for the LCD. U2 also reads push-
button inputs. Finally, the 2.1 MHz clock signal at U2 is divided down to 131 kHz and
sent to U1 (CLK pin) for the counter.
Automatic Selection Input Resistance
As shown in Figure 2-1, the input resistance for the Automatic Selection function is non-
linear. The data in the graph are for the volts dc function (any range) and are the steady
state values obtained after the PTC thermistor (RT1) has stabilized. The data also apply
for the volts ac function.
500
100
50
10
5
1
0.1 0.5 1.0 5.0 10.0 50.0 500.0
Small Signal Input Tesistance (kΩ)
|Vin| (Vdc)
Positive DCV Input
Negative DCV Input
as01f.eps
Figure 2-1. Typical Automatic Selection Input Resistance with DC Volts Function Selected
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