AN929 Datasheet, PDF (6/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

AN929
SYSTEM INTEGRATION ISSUES
Local versus Remote Sensing
The location of the sensor relative to the conditioning
circuit, as shown in Figure 10, plays a key role in
selecting the appropriate interface and noise reduction
circuit. Local sensors are located relatively close to
their signal conditioning circuits. Therefore, the noise
environment usually is not as severe as remote
sensors. In contrast, remote sensors are connected to
the amplifier via long wires that often introduce noise
into the electronics. A non-inverting amplifier circuit is a
good choice for a local sensor, while a remote sensor
requires a differential measurement in order to cancel
noise. All sensors should be considered as remote sen-
sors in high-noise environments or precision
applications to take advantage of the high CMRR and
noise reduction of a differential amplifier.
Local
Sensing
Amplifier
Sensor
PCB
Remote Sensing
Sensor
Amplifier
PCB
FIGURE 10:
Sensing.
Local versus Remote
Noise Reduction Techniques
Accurate temperature measurements require careful
attention to noise reduction techniques. The high
CMRR of the differential amplifier reduces noise.
However, grounding, shielded cables and Electromag-
netic Interference/Electro-Static Discharge (EMI/ESD)
filters are also required to prevent noise from degrading
the accuracy of the measurement.
Grounding
Figure 11 shows four basic methods of grounding a
sensor. A grounded source has its negative terminal
connected to ground at the sensor, often by virtue of
the mechanical mounting of the sensor. In contrast, a
floating source connects the sensor’s negative terminal
to ground at the amplifier.
The preferred grounding configuration for a remote
sensor is shown in circuits B and D. These circuits
provide for a two-wire differential measurement that
can be implemented with either a differential or
instrumentation amplifier. A differential measurement
requires that the common mode voltage level of the
signal source does not exceed the amplifier’s
maximum input voltage specification. As shown in
circuit D, adding bias resistors to reference the input
signal to a known voltage can solve this problem and
the resistors will not affect the measurement, if they are
relatively large.
The grounding methods of circuits A and C provide a
single-ended input measurement that should only be
used with a local sensor. The separate sensor and
amplifier grounds of circuit A can produce an offset
voltage due to the difference in the voltage potentials of
the two grounds. In local sensor applications, the
magnitude of ∆V_GND is small and either an inverting or
non-inverting op amp can be used. The configuration of
circuit C should be used with caution in low signal
output sensors such as thermocouples. Noise can be
induced into the measurement via the thermocouple
wires and the magnitude of the sensor voltage will be
affected by ground bounce or switching noise at the
amplifier’s ground.
DS00929A-page 6
 2004 Microchip Technology Inc.