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Theory of Operation
10MΩ
HI
INPUT
R334
LO
K307
S
K301
S
A. VOLTS
10MΩ
HI
INPUT
R334
LO
K307
S
K302
S
B. OHMS
Figure 6-9
Zero check configuration in volts and ohms
10MΩ
HI
INPUT
LO
S
Figure 6-10
Zero check configuration in amps and coulombs
6.4
Additional signal conditioning
Before the signal can be applied to the A/D converter for dig-
itization, it must be further scaled as described in the follow-
ing paragraphs.
6.4.1 Ranging amplifier
The ranging amplifier provides inverting gain values of X10,
X1, X0.1, or X0.01. The actual gain value will depend on the
selected range and function.
6-6
PREAMP
OUTPUT
Z IN = 10MΩ
R F
900kΩ
1V
PREAMP
OUTPUT
Z IN = 10MΩ || R F + 900kΩ
Z F
K302
K301
K307
PREAMP
OUTPUT
Z IN = 10MΩ || Z F
S
A simplified schematic of the ranging amplifier is shown in
Figure 6-11. The ranging amplifier itself is U130, while gain
is set by feedback resistors R142, R143, R144, R145, and in-
put resistor R128. The gain is selected by switching these re-
sistors in and out of the circuit with FETs.
FROM FET
CONTROLS
R128
SIGNAL INPUT
FROM PREAMP
200kΩ
Figure 6-11
Simplified schematic of ranging amplifier
6.4.2 Multiplexer and buffer amplifier
The multiplexer selects among the three signals that are part of
the Model 6512 measurement cycle. During the common phase,
the multiplexer selects signal common. During the reference
phase, the -2V reference voltage is selected, while the signal
from the ranging amplifier is selected during the signal phase.
Figure 6-12 shows a simplified schematic of the multiplexer
and buffer amplifier. U145 is the multiplexer IC, which con-
tains CMOS devices that act as analog switches to select
among the three input signals. The multiplexer IC is controlled
by digital signals that are generated by the microprocessor.
Figure 6-13 shows the general signal switching phases for
the three signals. During each phase, an integration is per-
formed by the A/D converter, and the resulting data is then
used by the microprocessor to calculate the final reading.
R145
2kΩ
R144
20kΩ
R143
220kΩ
R142
2MΩ
U130
SIGNAL OUTPUT
TO MULTIPLEXER
S
A=X10, X1, X0.1 OR X0.01
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