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Section IV
Model 3465B
4-6. SIMPLIFIED THEORY.
4-7. A simplified theory of operation of the Multimeter is
presented in the following paragraphs. The simplified
theory describes each section of the functional block
diagram, Figure 7-1. These sections are the signal condi
tioning section, analog—to-digital section, logic section and
the display section. Also presented is a simplified descrip
tion of the power supply. Refer to Figure 7-1, Functional
Block Diagram, and Figure 4-1, Basic Block Diagram and
Measurement Sequence, for this discussion.
4-8. Signal Conditioning.
4-9. Signal conditioning consists of attenuating and/or
converting the input signal to a dc voltage within the
working limits of the input amplifier. For full-scale inputs,
this voltage can vary from 20 mV dc to 2 V dc depending
on the function and range.
4-10. The signal conditioning section consists of current
shunts, an input attenuator, ohms converter and an
ac—to—dc converter. The output from the signal condi
tioning section is applied to the input amplifier during the
run—up interval of the measurement sequence. The Input
Amplifier Gain Table located on Figure 7-3 indicates the
full-scale input level applied to the input amplifier for each
function and range. This signal is the output of the signal
conditioning section.
4-11. Ohms Converter. The ohms converter is a high gain
integrating amplifier. A simplified diagram of the ohms
converter is presented in Figure 4-2. The blocks of the
ohms converter are the integrating amplifier, protection
diodes, over-voltage protection circuit and the overload
loop. An integrating amplifier is used because this type of
amplifier is less susceptible to oscillations. The protection
diodes clamp the Q terminal to a voltage of about 1.2 V
in the positive direction or 0.7 V in the negative direction.
With the
terminal clamped, protection against excessive
voltages applied to the ohms terminals is provided by an
over-voltage protection circuit located between the ohms
amplifier and the terminal. For excessive voltages, this
circuit isolates the COM terminal from the ohms amplifier.
4-12. Figure 4-2 shows two outputs of the ohms converter
being applied to the input amplifier. The ohms output is
the ohms converter measurement signal and the auto-zero
output is the ohms amplifier dc offset signal which is called
the auto-zero (AZ) signal. This AZ signal is applied to the
input amplifier during the auto-zero interval of the mea
surement sequence and establishes the reference for the
analog—to—digital converter. An AZ signal greater than
± 1 mV causes the instrument readings to be invalid. This
condition (AZ signal > ± 1 mV) is present when the
unknown resistance, R^, is removed and an open loop is
present on the ohms amplifier. To maintain the AZ signal at
< ± 1 mV when an open loop is present, an overload
feedback circuit is used.
4-13. The ohms output, (LO terminal of the ohms con
verter) is applied to the input amplifier. This output is a dc
voltage, the level of which is dependent on the ratio of the
unknown resistance, R^, to the variable resistance, 10", and
the ohms reference supply. The variable resistance, 10", is a
resistor string located in the precision resistor pack R75.
The value of 10" is selected by the range switches shorting
those resistors in the string that are not required. The value
of 10" can range from 10 kn to 10 Mf2. A discussion of the
precision resistor pack R75 can be found in the detailed
theory.
4-14. The formula for the ohms converter output voltage
is:
Ohms
Output '
Voltage
10"
Reference Supply Voltage -t Voffset
*
10"
i-lOV
vw-
*+IV FOR
lOM RANGE
029
-{(r-
f1
PROTECTION
DIODES
OVERLOAD
PROTECTION
OVER-
VOLTAGE
PROTECTION
CIRCUIT
AUTO-ZERO
OUTPUT
AV IS
PROPORTIONAL
TO Rx
\ TO
>INPUT
AMP
OHMS
'OUTPUT
3465-B-4167
V
Figure 4-2. Simplified Diagram, Ohms Converter.
4-2
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