Datasheet

LTC2996
10
2996f
D
+
D
–
470pF
LTC2996
2N3904
2996 F02
GND
applicaTions inForMaTion
Note that bypass capacitors greater than 1nF will cause
settling time errors of the different measurement cur-
rents and therefore introduce an error in the temperature
measurement (see Typical Performance Characteristics).
The LTC2996 compensates series resistance in the
measurement path and thereby allows accurate remote
temperature measurements even with several meters of
distance between the sensor and the device. The cable
length between the sensor and the LTC2996 is only limited
by the mutual capacitance introduced between D
+
and
D
–
which degrades measurement accuracy (see Typical
Performance Characteristics).
For example, a CAT6 cable with 50pF/m should be kept
shorter than ~20m to keep the capacitance less than 1nF.
To save wiring, the cathode of the remote sensor can
also be connected to remote GND and D
–
to local GND
as shown below.
The LTC2996 can withstand up to ±4kV of electrostatic
discharge (ESD, human body model). ESD beyond this
voltage can damage or degrade the device including
lowering the remote sensor measurement accuracy due
to increased leakage currents on D
+
or D
–
.
To protect the sensing inputs against larger ESD strikes,
external protection can be added using TVS diodes to
ground (Figure 3). Care must be taken to choose diodes
with low capacitance and low leakage currents in order
not to degrade the external sensor measurement accuracy
(see Typical Performance Characteristics curves).
The temperature measurement of LTC2996 relies only
on differences between the diode voltage at multiple test
circuits. Therefore DC offsets smaller than 300mV between
remote and local GND do not impact the precision of the
temperature measurement. The cathode of the sensor
can accommodate modest ground shifts across a system
which is beneficial in applications where a good thermal
connectivity of the sensor to a device whose temperature
is to be monitored (shunt resistor, coil, etc.) is required.
Care must be taken if the potential difference between
the cathode and D
–
does not only contain DC but also AC
components. Noise around odd multiples of 6kHz (±20%)
is amplified by the measurement algorithm and converted
to a DC offset in the temperature measurement (see Typical
Performance Characteristics).
Figure 2. Single Wire Remote Temperature Sensing
Figure 3. Increasing ESD Robustness with TVS Diodes
To make the connection of the cable to the IC polarity
insensitive during installation, two sensor transistors
with opposite polarity at the end of a two wire cable can
be used as shown on Figure 4.
Figure 4. Polarity Insensitive Remote Diode Sensor
D
+
D
–
220pF
10Ω
LTC2996
MMBT3904
PESD5Z6.0
2996 F03
GND
10Ω
D
+
D
–
LTC2996
MMBT3904 470pF
2995 F04
GND
Again, care must be taken that the leakage current of the
second transistor does not degrade the measurement
accuracy.