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In DCV the properly
scaled signal is applied
directly
to Ql IO
through
R107 and CllO.
In the AC V mode the scaled analog
signal is applied
to the AC converter
for transformation
to a
DC signal that is applied to QllO.
Resistance
measurements
are made using
the ratiometric
technique
(see Figure 4-5). When the resistance
function
is
selected a series circuit is formed
between
the ohms source,
reference
resistor and the external
unknown
resistance.
I
RX = VZ.RREF
"1
Ri06ARREF
hM
>TOQ113OF
1 OMR
T
MULTIPLEXER
RX
Figure
4.5.
Resistance
Measurements-Ratiometric
Technique
Three reference
resistors are used on the ohms ranges RlOl,
R102 and R106A.
RlOl is used for the 200R Et 2kR ranges,
Figure
4-6. Simplified
Schematics
of Ohms
Circuitry
R102 for the 20kR and 200kcl ranges and R106A for 2MR.
20Mn & 200Mn.
Drive for the ohms ranges is ultimately
con-
By measuring
the four
inputs
to the AID
converter
the
trolled by the microprocessor
through
Ulll
and U112.
unknown
resistance
can be computed
by the microprocessor
using this equation:
Switching
for the ohms ranges is done using low leakage
base to collector
diodes of 0102,
0104 and Q105. The ap-
propriate
transistor
is turned
on by driving
the base high
Osense HI - &~nse
LO
RX =
nref HI--nref
LO
(+ 5V). The simplified
schematics
for the ohms circuitry
is
shown
is Figure 4-6.
For the 200%
20k0,
2Ml7 ranges
fI sense HI is actually
multiplied
by a factor of 10 in the buffer circuit:
I3101 RRs~
-:'
TO 0112 OF
lkn
FIT101
MULTIPLEXER
Protection
on the ohms ranges is accomplished
by Q103,
4.15kn
RTIOI,
0101, R103, Q107and
Q108. Foran inputvoltageap-
RX
plied to the R input terminals,
QlOl clamps the voltage across
a1 04
RIO1 to a safe level. RTIOI
limits current
to Ql03
which
clamps the voltage
at 0104 to a safe limit (<12V).
For the 20k0 and 200kfl
ranges protection
is provided
by
R102, R104 and R105. Rl06A
provides
protection
for the
2MiI. 20Mn and 200MR ranges by limiting
current.
4.3.6 AC Converter
A. 2OOn
and 2kn Ranges
All AC voltage
inputs pass through
UIOI for a x2.5 voltage
4-4
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