LTC2995 datasheet(63/90 Pages) AD

63 / 90 page

LTM2985

Rev. 0

For more information www.analog.com

Figure 37. 4-Wire RTD with Protection Resistors

2985 F37

CH2

CH1

CH3

CH5

CH4

RP1

RP2

ILEAK

= EXTERNAL TERMINAL CAN BE

EXPOSED TO FAULT VOLTAGE.

RP4

RP3

ILEAK

RSENSE

EXCITATION

CURRENT

FLOW

2985 F38

CH2

CH1

CH3

CH4

RP1

RP2

= EXTERNAL TERMINAL CAN BE

EXPOSED TO FAULT VOLTAGE.

RP3

RSENSE

EXCITATION

CURRENT

FLOW

Figure 38. 3-Wire RTD with Protection Resistors

3-Wire RTDs

3-wireRTDsaremoredifficulttoprotectthan4-wireRTDs.

Normally, protection resistors are tied to each of the 3

RTD terminals (see Figure 38). The LTM2985 provides

two matched excitation currents, I1 and I2. These currents

flow from CH3 and CH4 through RP2 and RP3 into the

RTD. The resulting voltage is measured between CH3 and

CH4. Assuming RP2 = RP3 and I1 = I2, the errors result-

ing from the protection resistors are cancelled. While the

LTM2985 provides matched current source excitation,

external protection resistors may be difficult to match.

Every 1Ω mismatch in RP2 and RP3 translates to a 1Ω

error in the RTD measurement.

The LTM2985 offers a 3-wire RTD Kelvin current source

modeinordertoremoveerrorsduetomismatchedprotec-

tion resistors. This feature is enabled by setting the global

3-wire RTD Kelvin current excitation mode bit (G4 = 1, see

Table 67) prior to initiating a conversion start. This mode

uses the adjacent channels (in this example CH5 and CH6)

for the current source excitation and performs the mea-

surement on CH3 and CH4 (see Figure 39). Two additional

resistors are placed between the RTD and the channels

tied to the excitation current sources, but the protection

resistor matching constraint is removed for all resistors.

The excitation current no longer flows through RP2 or

RP3, removing the voltage drop across them. Figure 40

shows the channel assignment convention for this mode.

Figure 39. 3-Wire RTD Kelvin Current Mode (G4 = 1)

Figure 40. 3-Wire RTD Kelvin Current Mode Channel

Assignment Convention (G4 = 1)

2985 F39

CH2

CH1

CH3

CH4

RP1

RP2

= EXTERNAL TERMINAL CAN BE

EXPOSED TO FAULT VOLTAGE.

RP3

CH5

RP4

CH6

RP5

RSENSE

EXCITATION

CURRENT

FLOW

ILEAK

2985 F40

EXCITATION

CURRENT

FLOW

CHRSENSE

CHRTD–1

CHRTD+1

CHRTD

CHRTD+2

= CHRTD (2 ≤ RTD ≤ 8)

GLOBAL REGISTER G4 = 1

CHANNEL

ASSIGNMENT

APPLICATIONS INFORMATION

LTC2995 Datasheet(PDF) 63 Page - Analog Devices

LTC2995 Datasheet(HTML) 63 Page - Analog Devices

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