AN929 Datasheet, PDF (7/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

+
V_S
-
V_MEAS = V_S +∆V_GND
∆V_GND
A. Grounded Source - Ground Loop
+
V_S
-
V_MEAS = V_S
AN929
+
V_S
-
V_MEAS = V_S
C. Floating Source/Common Ground
- Use Only with Local Sensors
+
V_S
-
V_MEAS = V_S
B. Grounded Source - Preferred Connection
D. Floating Source - Preferred Connection
FIGURE 11:
Accurate Measurements Require Careful Grounding.
Shielded Cables
A shielded cable is an effective tool to prevent radiated
interference from introducing a noise voltage on the
signal wires. Shielded twisted-pair cables reduce the
loop area of the victim signal and minimize the voltage
induced on the sensor signal lines. The noise signals
on each wire in the cable will be essentially equal,
which is the assumption needed for the high CMRR
feature of a differential amplifier to cancel the
interference. For most applications, it is recommended
that the shield be connected to ground only at one
point. In applications using a metal enclosure, the
shield’s connection to chassis ground can occur
through the mechanical connection of the cable
connector to the box. Figure 12 provides an example of
a remote RTD measurement using a shielded cable.
RTD
Connector
Shielded
Cable
V_REF
EMI Filter
EMI Filter
PCB
FIGURE 12:
Use Shielded Cables and EMI Filters for Remote Measurements.
 2004 Microchip Technology Inc.
DS00929A-page 7