LTC2995 datasheet(61/90 Pages) AD

61 / 90 page

LTM2985

Rev. 0

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APPLICATIONS INFORMATION

GLOBAL CONFIGURATION REGISTER

Table 67 shows a summary of the global configuration

and is at memory location 0x0F0. Bits G0 – G1 set the

notch frequency of the on chip digital filter, Bit G2 sets the

temperature results unit (°C or °F), Bits G3 and G7 are

reserved and should be set low, Bits G4 – G6 determine

the Kelvin current excitation mode.

Most RTD sensor elements are electrically isolated from

the sensor leads either through a non-conductive encap-

sulation or a separate grounding shield. While these types

of sensors may not require input overvoltage protection,

certain applications may require current limiting resistors

between the RTD and the LTM2985. One such application

is a universal input device where an input terminal can

see either an RTD or a thermocouple. Other applications

mayrequireprotectionagainsterroneousconnections,for

example, connecting a voltage source accidentally to the

RTD input terminals. The protection circuits implemented

for RTDs should accommodate 2-wire, 3-wire and 4-wire

configurations.

Thermistors are 2-wire resistance to temperature sensors

withanon-conductiveencapsulationenclosingthesensor

element. Similar to the RTD, overvoltage protection for

thermistors may be required for both universal sensor

inputapplicationsandinadvertentuserappliedovervoltage.

The LTM2985 offers several current excitation modes for

eliminating errors due to resistive overvoltage protection

circuits. The following sections describe overvoltage

protection circuits for thermocouples, 2-, 3- and 4-wire

RTDs and thermistors with an emphasis on universality

(sharing the same protection scheme for all sensor types).

Input Overvoltage Protection – Resistor Value Selection

The maximum continuous current the LTM2985 can sus-

tain without damage is ±15mA. In order to determine the

value of the overvoltage protection resistor and its power

rating, the maximum voltage is required. This voltage is

application specific and depends on the maximum antici-

pated overvoltage. For example, a system with possible

overvoltage of 40V would require a resistance R > 2.7kΩ

and a power rating (see Figure 34 and Figure 35):

P > 600mW, where R >

VMAX – VCC2

15mA

Table 67. Global Configuration Register

BIT #

FIELD NAME

DESCRIPTION

Filter Frequency Select [1:0]

00 = 55Hz

01 = 60Hz

10 = 50Hz

Temperature Result Format

0 = Celsius

1 = Fahrenheit

Reserved

Set to 0

3-Wire RTD Kelvin Current

Excitation Mode

Excitation Current on

Adjacent Channels

2-Wire RTD Kelvin Current

Excitation Mode

Excitation Current on

Adjacent Channels

Thermistor Kelvin Current

Excitation Mode

Excitation Current on

Adjacent Channels

Reserved

Set to 0

INPUT OVERVOLTAGE PROTECTION – OVERVIEW

Temperature sensors are often used in harsh environ-

ments. The sensors or leads can short to high voltages

or each other. Resistive circuits can protect the LTM2985

from these fault conditions. These external resistors can

potentially introduce measurement errors; however, the

LTM2985includesspecialmodesandfeaturesthatreduce

these effects

The tip of a thermocouple is often unshielded, creating a

low impedance conductive path to the input of the mea-

surement device. In order to protect the LTM2985 from

damage due to overvoltage conditions, current-limiting

resistorscanbeplacedbetweentheinputchannelsandthe

thermocouple sensor. The value of this resistor is chosen

such that at the maximum overvoltage, the current enter-

ing the LTM2985 is less than ±15mA. Errors due to these

protection resistors are minimal during normal operation

due to the very low input leakage (1nA) specifications of

the LTM2985.

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