9

LTC1392

Differential Voltage Conversion

The LTC1392 measures the differential input voltage

full scale are available for differential voltage measure-

ment with resolutions of 10 bits. Tables 4a and 4b describe

the exact relationship of output data to measured differen-

tial input voltage in the 1V and 0.5V input range. Equations

(3) and (4) can be used to calculate the differential voltage

in the 1V and 0.5V input voltage range respectively. The

output code is in unipolar format.

Differential Voltage = 1V • (10-bit code)/1024

(3)

Differential Voltage = 0.5V • (10-bit code)/1024

(4)

Table 4a. Codes for 1V Differential Voltage Range

OUTPUT

INPUT

INPUT

CODE

VOLTAGE

RANGE = 1V

REMARKS

1111111111

1V – 1LSB

999.0mV

1111111110

1V – 2LSB

998.0mV

...

...

...

0000000001

1LSB

0.977mV

1LSB = 1/1024

0000000000

0LSB

0.00mV

Table 4b. Codes for 0.5V Differential Voltage Range

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

Thermal Coupling/Airflow

The supply current of the LTC1392 is 700

µA typically

when running at the maximum conversion rate. The equiva-

lent power dissipation of 3.5mW causes a temperature

rise of 0.455

°C in the SO8 and 0.35°C in PDIP packages

due to self-heating effects. At sampling rates less than 400

samples per second, less than 20

µA current is drawn from

the supply (see Typical Performance Characteristics) and

the die self-heating effect is negligible. This LTC1392 can

be attached to a surface (such as microprocessor chip or

a heat sink) for precision temperature monitoring. The

package leads are the principal path to carry the heat into

the device; thus any wiring leaving the device should be

held at the same temperature as the surface. The easiest

way to do this is to cover up the wires with a bead of epoxy

which will ensure that the leads and wires are at the same

temperature as the surface. The thermal time constant of

the LTC1392 in still air is about 22 seconds (see the graph

in the Typical Performance Charateristics section). At-

taching an LTC1392 to a small metal fin (which also

provides a small thermal mass) will help reduce thermal

time constant, speed up the response and give the steadi-

est reading in slow moving air.