input current is ac and the AC function is selected, the
I
-
R drop is rectified by the ac converter before going to
the low
-
pass filter. In either event the a/d converter
receives a dc input voltage proportional to the current
passing through the selected shunt.
3
-
24. RESISTANCE MEASUREMENTS
3
-
25. Resistance measurements are made using a
ratio technique as shown in Figure 3
-
3C. When the kfl
function is selected a simple series circuit is formed by
the internal reference voltage, a reference resistor from
the voltage divider (selected by range switches), and the
external unknown resistor. The ratio of the two resistors
is equal to the ratio of their respective voltage drops.
Therefore, since the value of one resistor is known, the
value of the second can be determined by using the
voltage drop across the known resistor as a reference.
This determination is made directly by the
aid
converter.
present during a voltage measurement is replaced by the
voltage drop across the reference resistor. This allows
the voltage across the unknown resistor to be read
during the integrate period, and compared against the
reference resistor during the read period. As before, the
length of the read period is a direct indication of the
value of the unknown.
3
-
27. CONDUCTANCE MEASUREMENTS
3
-
28.
Conductance measurements are made using a
ratio technique similar to that used in making resistance
measurements. See Figure 3
-
3C. The main difference is
that only two ranges are provided
(200
nS and
2
mS),
and the function of the range and unknown resistors in
the measurement cycle is reversed. That is, the voltage
drop across the range resistor is used as the unknown
input during the integrate period, and the voltage across
the unknown resistor is used for the reference input
during the read period. As a result the display provides a
3
-
26.
Overall operation of the
ald converter during a reading that is the reciprocal (1
/
fl) of the unknown
resistance measurement is basically as described earlier
input resistance,
i.e., the higher the input resistance the
in this section, with one exception. The reference voltage
lower the display reading.
3
-
4
Flgure
3-3.
Input Slgnal Condltloners
VOLTAGE
VIK~
=
?
DIVIDER
-
DC
-
-
HI
T
-
INPUT
-
TERMINALS
-
-
)
UNKNO'JVN
TO AID
FILTER
AC
CONVERTER
CONVERTER
LOW
PASS
COMM
=
1
1
-
LO
-
A. VOLTAGE MEASUREMENTS
SELECTED
CUl3REN.T
VIK~
SHUNTS
-
INPUT
TERMINALS
HI
T
-
UNKNOWN
AC
LOW
TO AID
CONVERTER
CONVERTER
COMM
-
I
LO
B
CURRENTMEASUREMENTS
SELECTED REFERENCE RESISTOR
K
-
HI
vIK~I~s
.
/
-
fl
RANGES
/\/
1
1
/
PASS
FILTER
UNKNOWNIV REF.
TO AID
CONVERTER
COMM
+
-
I
I
1
I
I
-
d
LO
C
RESISTANCEICONDUCTANCE MEASUREMENTS
LOW
PASS
FILTER
-
INPUT TERMINALS
VSOURCE
V REF
TO
AID
A
CONVERTER
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