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Theory of Operation (2635A)
Detailed Circuit Description
2A
2A-27
OHMS
VOLTAGE
SOURCE
A/D
INTEGRATE
REFERENCE
LOW
HIGH
A3R34
A3Z4
REFERENCE
RESISTOR
R
REF
+
–
VR
REF
A3K16
A3RT1 & A3R10
A3K17
UNKNOWN
RESISTOR
PASSIVE
FILTER
A/D
INTEGRATE
UNKNOWN
HIGH
LOW
IX
A3R11
R
X
VR
X
VR
X
VR
REF
=
I
X•RX
IX•RREF
R
REF
R
X
=
+
-
LO
HI
A3R42
s15f.eps
Figure 2A-5. Ohms Simplified Schematic (2635A)
For the RTD, 300Ω, 3-kΩ, and 30-kΩ ranges, the ratio technique is implemented by
integrating the voltage across the unknown resistance for a fixed period of time and then
integrating the negative of the voltage across the reference resistance for a variable time
period. In this way, each minor cycle result gives the ratio directly.
For the 300-kΩ, 3-MΩ, and 10-MΩ ranges, the ratio is determined by performing two
separate voltage measurements in order to improve noise rejection. One fixed-period
integration is performed on the voltage across the unknown resistance, and the second
integration is performed on the voltage across the reference resistance. The ratio of the
two fixed-period voltge measurements is then computed by Microcontroller A3U9. The
resistance measurement result is determined when A3U9 multiplies the ratio by the
reference resistance value.
When an input is switched in for a measurement, the ohms source in Analog Processor
A3U8 is set to the correct voltage for the range selected and is connected to the
appropriate reference resistor in network A3Z4. A measurement current then flows
through A3Z4, relay A3K16, thermistor A3RT1, resistor A3R10, the unknown
resistance, A3R43, ground, and the ohms source.
The resulting voltage across the unknown resistance is integrated for a fixed period of
time by the A/D Converter through the HI SENSE path of A3R11, A3K17, A3R42 and