LTC2995 datasheet(64/90 Pages) AD

64 / 90 page

LTM2985

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 41. 2-wire RTD with Protection Resistors

2985 F41

CH2

CH1

CH3

CH4

CH5

RP1

RP2

= EXTERNAL TERMINAL CAN BE

EXPOSED TO FAULT VOLTAGE.

RP3

RSENSE

EXCITATION

CURRENT

FLOW

2985 F42

CH2

CH1

CH3

CH4

CH5

RP1

RP2

= EXTERNAL TERMINAL CAN BE

EXPOSED TO FAULT VOLTAGE.

RP3

RP4

RSENSE

EXCITATION

CURRENT

FLOW

ILEAK

Figure 42. 2-Wire RTD Kelvin Current Mode (G5 = 1)

2-Wire RTDs

2-wire RTDs are difficult to protect because the protection

resistor (RP3) is in series with the RTD (see Figure 41).

Every 1Ω of protection resistance adds 1Ω measurement

error to the RTD.

The LTM2985 offers a 2-wire Kelvin current source mode

in order to remove the errors associated with protection

resistors. This feature is enabled by setting the global

2-wire RTD Kelvin current excitation mode bit (G5 = 1,

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

current excitation mode uses the adjacent channel (CH5

for this example) for the internal ground connection (see

Figure 42). One additional protection resistor is added

betweentheRTDandCH5.Theexcitationcurrentnolonger

flows through RP3, removing the voltage drop across it.

Figure 43 shows the channel assignment convention for

this mode.

Thermistors

Similar to the 2-wire RTD, thermistors are difficult to

protect because the protection resistor RP3 is in series

with the sensor (see Figure 44). Every 1Ω of protection

resistance adds 1Ω measurement error to the thermistor.

Figure 43. 2-Wire Kelvin Current Mode Channel

Assignment Convention (G5 = 1)

2985 F43

EXCITATION

CURRENT

FLOW

CHRTD–1

CHRTD

CHRTD+1

= CHRTD (2 ≤ RTD ≤ 9)

GLOBAL REGISTER G5 = 1

CHANNEL

ASSIGNMENT

2ND TERMINAL TIES TO SENSE RESISTOR (CHRSENSE)

Figure 44. Thermistor with Protection Resistors

2985 F44

CH2

CH1

CH3

CH4

RP1

RP2

= EXTERNAL TERMINAL CAN BE

EXPOSED TO FAULT VOLTAGE.

RP3

RSENSE

EXCITATION

CURRENT

FLOW

The LTM2985 offers a thermistor Kelvin current source

modeinordertoremovetheerrorsassociatedwithprotec-

tion resistors. This feature is enabled by setting the global

thermistor Kelvin current excitation mode bit (G6 = 1,

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

current excitation mode uses the adjacent channel (CH5

for this example) for the internal ground connection (see

Figure 45). One additional protection resistor is added

between the thermistor and CH5. The excitation current

no longer flows through RP3, removing the voltage drop

across it. Figure 46 shows the channel assignment con-

vention for this mode.

LTC2995 Datasheet(PDF) 64 Page - Analog Devices

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