LTC2995 datasheet(33/90 Pages) AD

33 / 90 page

LTM2985

Rev. 0

For more information www.analog.com

RTD MEASUREMENTS

Channel Assignment – RTD

For each RTD tied to the LTM2985, a 32-bit channel as-

signment word is programmed into a memory location

corresponding to the channel the sensor is tied to (see

Table 29).Thiswordincludes(1)RTDtype,(2)senseresis-

torchannelpointer,(3)sensorconfiguration,(4)excitation

current, (5) RTD curve and (6) custom RTD data pointer.

(1) RTD Type

The RTD type is determined by the first five input bits B31

to B27 as shown in Table 30. Linearization coefficients

for RTD types PT-10, PT-50, PT-100, PT-200, PT-500,

PT-1000 and NI-120 with selectable common curves

(

α = 0.003850, α = 0.003911, α = 0.003916 and

α = 0.003926) are built into the device. If custom RTDs

are used, RTD Custom can be selected. In this case, user

specific data can be stored in the on-chip RAM starting

at the address defined by the custom RTD data pointers.

(2) Sense Resistor Channel Pointer

RTD measurements are performed ratiometrically relative

to a known RSENSE resistor. The sense resistor channel

pointer field indicates the differential channel the sense

resistor is tied to for the RTD (see Table 31). Sense resis-

tors are always measured differentially.

APPLICATIONS INFORMATION

(3) Sensor Configuration

The sensor configuration field is used to define various

RTDproperties.ConfigurationbitsB20andB21determine

if the RTD is a 2-, 3-, or 4-wire type (see Table 32).

The simplest configuration is the 2-wire configuration.

While this setup is simple, parasitic errors due to IR drops

in the leads result in systematic temperature errors. The

3-wire configuration cancels RTD lead resistance errors

(if the lines are equal resistance) by applying two matched

current sources to the RTD, one per lead. Mismatches in

the two current sources are removed through transparent

background calibration. 4-wire RTDs remove unbalanced

RTD lead resistance by measuring directly across the

sensor using a high impedance Kelvin sensing. 4-wire

measurements with Kelvin RSENSE are useful in applica-

tions where sense resistor wiring parasitics can lead to

errors; this is especially useful for low resistance PT-10

type RTDs. In this case, both the RTD and sense resistor

have Kelvin sensing connections.

The next sensor configuration bits (B18 and B19) deter-

mine the excitation current mode. These bits are used to

enable RSENSE sharing, where one sense resistor is used

for multiple 2-, 3- and/or 4-wire RTDs. In this case, the

RTD ground connection is internal and each RTD points

to the same RSENSE channel.

Bits B18 and B19 are also used to enable excitation current

rotation to automatically remove parasitic thermocouple

effects. Parasitic thermocouple effects may arise from the

physical connected between the RTD and the measure-

ment instrument. This mode is available for all 4-wire

configurations using internal current source excitation.

Table 29. RTD Channel Assignment Word

(1) RTD TYPE

(2) SENSE RESISTOR

CHANNEL POINTER

(3) SENSOR

CONFIGURATION

(4) EXCITATION

CURRENT

(5) RTD

CURVE

(6) CUSTOM RTD DATA POINTER

TABLE 30

TABLE 31

TABLE 32

TABLE 33

TABLE 34

TABLES 92 TO 94

Measurement Class 31 30 29 28 27 26 25 24 23 22 21 20 19

17 16 15 14 13

12 11 10 9 8 7 6 5 4 3 2 1 0

RTD

Type = 10 to 18

RSENSE Channel

Assignment [4:0]

2, 3, 4

Wire

Excitation

Mode

Excitation

Current [3:0]

Curve

[1:0]

Custom Address

[5:0]

Custom Length-1

[5:0]

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