Fluke NetDAQ 2640A, Service Manual

Page: 78 / 260

NetDAQ
Service Manual
2-36
Ohms and RTD Measurement Circuitry 2-62.
Resistance measurements are made by sourcing dc current through the unknown resistor
and measuring the resultant dc voltage (see Table 2-9). The current source consists of
operational amplifier A1U31, FET A3Q19, and switches internal to the Stallion.
four-wire measurements use separate source and sense signal paths to the point of the
unknown resistance. This technique eliminates lead wire resistance errors. Figure 2-9
shows a simplified signal path for an RTD four-wire measurement.
Table 2-9. Measurement Matrix for Ohms
Ohms
Range
Current
Used
Full-Scale
Input to
Stallion
(dc volts)
Full-Scale
Output of
Stallion
(dc volts)
Gain of DC
Buffer
Amplifier
Full-Scale DC
Volts input to
Multislope A/D
Buffer
Range
Control
Signal
300 Ohm 1 mA 300 mV 300 mV 10 3V BR3
3k Ohm 100
µ
A 300 mV 300 mV 10 3V BR3
30k Ohm 10
µ
A 300 mV 300 mV 10 3V BR3
300k Ohm 10
µ
A 3V 3V 1 3V BR3
3M Ohm 1
µ
A 3V 3V 1 3V BR3
CHANNEL 1
CHANNEL II
A3K1 A3K21
HI
Sense
A3R111
1K Fusible A3K26 A3R130
HI
S23 S64 S44
to A/D
Buffer
A3K11 A3K27
HI
A3R110
1K Fusible
A3K24
A3R138
1K
VDD
A3Z7
1K
A3Q19
A3U31
+3.45V Ref
+
_
7
A3R128
3.45K
A3U31
+
_
1
S19
S14
S15
S13
S12
A3K27
VDD
A3Z7
10K
A3Z7
1K
A3Z7
100K
A3Z7
1M
S9 S10 S8 S7
I
OUT
HI
CHANNEL 1
CHANNEL II
LO
LO
3K
Ω
Range
300
Ω
Range
-30K & 300K Range
-3M
Ω
Range
RESISTOR
BEING
MEASURED
I
S
Figure 2-9. RTD Measurement Simplified Schematic