Theory of Operation
Circuit Descriptions 2
2-9
The Touch Hold mode is a secondary software function. This means that the
microcomputer performs a different control algorithm on the data. When the Touch Hold
function is selected, the microcomputer does not allow a full resolution conversion to be
completed unless the input signal is stable. When a stable reading occurs, the conversion
is completed, and the microcomputer generates and freezes the corresponding display.
The microcomputer now waits for a change in the signal to exceed a certain threshold,
then begins watching for a stable reading again. Note that a reading is forced when the
Touch Hold function is first selected. Also, open test lead signals generally do not update
the display.
The SMOOTHING™ mode is another secondary software function. In the Smoothing
function, the equivalent of eight readings are averaged, resulting in a more stable display
of noisy input signals.
OHMS AND
CAPACITANCE
VOLTAGE
SOURCE
A/D
INTEGRATE
LOW
HIGH
R13
+
–
S1
R1, R8,
R12, RTI
A/D
MEASURE
VC
X
HIGH
LOW
I
CHARGE
R2
C
X
UNKNOWN
CAPACITOR
V
CHARGE
3
7
Z1
1.1111M
REFERENCE
RESISTOR
S1
Z1
9.996M
2
1
6
7
9
7
DISCHARGE C
X
7
6
Z1
1.0001k
CHARGE C
X
VC
X
+
–
l5f.eps
Figure 2-5. 100 nF Range Simplified Schematic
Peripherals to U4 2-15.
In addition to input overload protection and input signal conditioning circuits, other
devices peripheral to U4 are needed to support the meter's features. The active filter and
a/d converter circuits require off-chip resistors and capacitors. Digital drive and level-
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