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LTC2995C

Temperature Sensor and Dual Voltage Monitor with Alert Outputs

Linear Technology 

LTC2995

APPLICATIONS INFORMATION

Leakage currents at D+ affect the precision of the remote

temperature measurements. 100nA leakage current leads

to an additional error of 2°C (see Typical Performance

Characteristics).

Note that bypass capacitors greater than 1nF will cause

settling time errors in the different measurement cur-

rents and therefore introduce an error in the temperature

measurement (see Typical Performance Characteristics).

The LTC2995 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 LTC2995 is only limited

by the mutual capacitance introduced between D+ and

D– which degrades measurement accuracy (see Typical

Performance Characteristics).

For example an AT6 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.

components. Noise around odd multiples of 6kHz (±20%)is

amplified by the measurement algorithm and converted at

a DC offset in the temperature measurement (see Typical

Performance Characteristics).

The LTC2995 can withstand up to ±4kV of electrostatic

discharge (ESD, human body). 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).

MMBT3904

PESD5Z6.0

10Ω

LTC2995

D+

10Ω

220pF

D–

GND

2995 F03

2N3904

470pF

D+ LTC2995

D– GND

2995 F02

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.

The temperature measurement of the LTC2995 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 content DC but also AC

MMBT3904

LTC2995

D+

470pF

D– GND

2995 F04

Figure 4. Polarity Insensitive Remote Diode Sensor

Again, care must be taken that the leakage current of the

second transistor does not degrade the measurement

accuracy.

2995f

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