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LTC2995 Datasheet(PDF) 11 Page - Linear Technology |
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LTC2995 Datasheet(HTML) 11 Page - Linear Technology |
11 / 20 page LTC2995 11 2995f D+ D– 470pF LTC2995 2N3904 2995 F02 GND 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). 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 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. Again, care must be taken that the leakage current of the second transistor does not degrade the measurement accuracy. Figure 4. Polarity Insensitive Remote Diode Sensor D+ D– 220pF 10Ω LTC2995 MMBT3904 PESD5Z6.0 2995 F03 GND 10Ω D+ D– LTC2995 MMBT3904 470pF 2995 F04 GND |
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