LTC2995 datasheet(53/90 Pages) AD

53 / 90 page

LTM2985

Rev. 0

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APPLICATIONS INFORMATION

(3) Sensor Configuration

The sensor configuration field is used to define various

thermistor properties. Configuration bit B21 is set high

for single-ended (measurement relative to COM) and low

for differential (see Table 56).

(4) Excitation Current

Thenextfieldinthechannelassignmentword(B18toB15)

controls the magnitude of the excitation current applied to

the thermistor (see Table 57). In order to prevent hard or

soft faults, select a current such that the maximum volt-

age drop across the sensor or sense resistor is nominally

1.0V. The LTM2985 has no special requirements related

to the ratio between the voltage drop across the sense

resistor and the sensor. Consequently, it is possible to

have a sense resistor several orders of magnitude smaller

than the maximum sensor value. For optimal performance

over the full thermistor temperature range, auto ranged

current can be selected. In this case, the LTM2985 conver-

sion is performed in three cycles (instead of the standard

two cycles) (see Table 83). The first cycle determines the

optimal excitation current for the sensor resistance value

and RSENSE value. The following two cycles use that cur-

rent to measure the thermistor temperature.

(5) Steinhart-Hart Address/Custom Table Address

See Custom Thermistors section near the end of this data

sheet for more information.

Table 56. Sensor Configuration Data

(3) SENSOR

CONFIGURATION

SGL

EXCITATION

MODE

SINGLE-ENDED/

DIFFERENTIAL

RSENSE

ROTATE

B21

B20

B19

Differential

Yes

Differential

Yes

Reserved

Single-Ended

Reserved

Table 57. Excitation Current for Thermistors

(4) EXCITATION CURRENT

B18

B17

B16

B15

CURRENT

Reserved

250nA

500nA

1µA

5µA

10µA

25µA

50µA

100µA

250µA

500µA

1mA

Auto Range*

Invalid

External

*Auto Range not allowed for custom sensors.

The next sensor configuration bits (B19 and B20) deter-

mine the excitation current mode. These bits are used to

enable RSENSE sharing, where one sense resistor is used

formultiplethermistors.Inthiscase,thethermistorground

connection is internal and each thermistor points to the

same RSENSE channel.

Bits B19 and B20 are also used to enable excitation current

rotation to automatically remove parasitic thermocouple

effects. Parasitic thermocouple effects may arise from

the physical connection between the thermistor and the

measurement instrument. This mode is available for dif-

ferential thermistor configurations using internal current

source excitation.

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