Datasheet LTC1392 (Analog Devices) - 9

LTC1392
U U W U APPLICATIONS INFORMATION Differential Voltage Conversion Thermal Coupling/Airflow
The LTC1392 measures the differential input voltage The supply current of the LTC1392 is 700µA typically through pins + VIN and –VIN. Input ranges of 0.5V or 1V when running at the maximum conversion rate. The equiva- full scale are available for differential voltage measure- lent power dissipation of 3.5mW causes a temperature ment with resolutions of 10 bits. Tables 4a and 4b describe rise of 0.455°C in the SO8 and 0.35°C in PDIP packages the exact relationship of output data to measured differen- due to self-heating effects. At sampling rates less than 400 tial input voltage in the 1V and 0.5V input range. Equations samples per second, less than 20µA current is drawn from (3) and (4) can be used to calculate the differential voltage the supply (see Typical Performance Characteristics) and in the 1V and 0.5V input voltage range respectively. The the die self-heating effect is negligible. This LTC1392 can output code is in unipolar format. be attached to a surface (such as microprocessor chip or a heat sink) for precision temperature monitoring. The Differential Voltage = 1V • (10-bit code)/1024 (3) package leads are the principal path to carry the heat into Differential Voltage = 0.5V • (10-bit code)/1024 (4) the device; thus any wiring leaving the device should be
Table 4a. Codes for 1V Differential Voltage Range
held at the same temperature as the surface. The easiest
OUTPUT INPUT INPUT
way to do this is to cover up the wires with a bead of epoxy
CODE VOLTAGE RANGE = 1V REMARKS
which will ensure that the leads and wires are at the same 1111111111 1V – 1LSB 999.0mV temperature as the surface. The thermal time constant of 1111111110 1V – 2LSB 998.0mV the LTC1392 in still air is about 22 seconds (see the graph ... ... ... in the Typical Performance Charateristics section). At- 0000000001 1LSB 0.977mV 1LSB = 1/1024 taching an LTC1392 to a small metal fin (which also 0000000000 0LSB 0.00mV provides a small thermal mass) will help reduce thermal time constant, speed up the response and give the steadi-
Table 4b. Codes for 0.5V Differential Voltage Range
est reading in slow moving air.
OUTPUT INPUT INPUT CODE VOLTAGE RANGE = 0.5V REMARKS
1111111111 0.5V – 1LSB 499.5mV 1111111110 0.5V – 2LSB 499.0mV ... ... ... 0000000001 1LSB 0.488mV 1LSB = 0.5/1024 0000000000 0LSB 0.00mV 9