LTC2995 datasheet(62/90 Pages) AD

62 / 90 page

LTM2985

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 36. Thermocouple with Protection Resistors

2985 F36

CH1

COM

ERROR = 4V

–

1nA (MAX)

= EXTERNAL TERMINAL CAN BE

EXPOSED TO FAULT VOLTAGE.

Input Overvoltage Protection – Thermocouples

Thermocouples are low impedance devices that generate

voltage as a function of temperature differences. Since

the LTM2985 input impedance is very high (input leak-

age < 1nA) external overvoltage protection resistors have

minimaleffectonthetemperaturemeasurementaccuracy.

For example, a 2kΩ protection resistor results in a worst-

case error of 4µV (see Figure 36). This corresponds to a

0.1°C error for a Type K thermocouple at 25°C.

In addition to the protection resistors, 100pF capacitors

should be added to each input for anti-alias filtering; these

are not shown in the following schematics for simplicity.

Figure 34. Maximum Fault Voltage vs Minimum

Protection Resistance

Figure 35. Maximum Fault Voltage vs Minimum

Protection Resistor Power Rating

MAXIMUM FAULT VOLTAGE (V)

120

160

200

1.3

2.7

4.0

5.3

6.7

8.0

9.3

10.7

12.0

13.3

vs Minimum Protection Resistance

2985 F34

MAXIMUM FAULT VOLTAGE (V)

120

160

200

0.3

0.6

0.9

1.2

1.5

1.8

2.1

2.4

2.7

3.0

Power Rating

2985 F35

Input Overvoltage Protection – RTDs

RTDs are resistive devices that require excitation current

in order to determine their temperature. The excitation is

applied to the series network consisting of the RTD and

a sense resistor in order to make a ratiometric measure-

ment. Overvoltage protection is implemented by placing

a resistor between each RTD terminal and the LTM2985

input channels.

4-Wire RTDs

ThesimplestRTDconfigurationtoprotectisthe4-wireRTD.

A protection resistor is tied to each of the 4 RTD terminals

(seeFigure 37).Excitationcurrentflowsthroughthesense

resistor (RSENSE), the RTD and protection resistors RP1

and RP4. The LTM2985 measures the voltage drop across

the RTD using CH3 and CH4 through protection resistors

RP2 and RP3. Since the excitation current does not flow

through RP2 and RP3, errors due to the protection resis-

tance are negligible. Measurement errors are dominated

by input leakage current (I < 1nA). For example, errors

due to leakage current for a PT-100 RTD with 1kΩ sense

resistor and 1kΩ protection resistors are below 0.025°C.

LTC2995 Datasheet(PDF) 62 Page - Analog Devices

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