LTC2984 datasheet(63/78 Pages) AD

63 / 78 page

LTC2984

2984fb

For more information www.linear.com/LTC2984

FAULT PROTECTION AND ANTI-ALIASING

The LTC2984 analog input channels draw a maximum of

1nA DC. As a result, it is possible to add anti-aliasing and

faultprotectioncircuitrydirectlytotheinputoftheLTC2984.

The most common input circuitry is a low pass filter with

1k to 10k resistance (limited by excitation current for

RTDs and thermistors) and a capacitor with 100pF – 0.1µf

capacitance. This circuit can be placed directly between

the thermocouples and 4-wire RTDs and the LTC2984.

In the case of 3-wire RTDs, mismatch errors between

the protection resistors can degrade the performance.

Thermistors requiring input projection should be tied to

the LTC2984 through a Kelvin type connection in order to

avoid errors due to the fault protection resistors.

2- AND 3-CYCLE CONVERSION MODES

The LTC2984 performs multiple internal conversions in

order to determine the sensor temperature. Normally, two

internal conversion cycles are required for each tempera-

ture result providing a maximum output time of 167.2ms.

The LTC2984 uses these two cycles to automatically

remove offset/offset drift errors, reduce 1/f noise, auto-

calibrate matched internal current sources, and provide

simultaneous 50/60Hz noise rejection.

Inadditiontoperformingtwoconversioncyclesperresult,

theLTC2984alsooffersseveraluniquefeaturesbyutilizing

a 3rd conversion cycle. In this case, the maximum output

time is 251ms and all the benefits of the 2-cycle modes

are present (see Table 68).

Onefeatureutilizingthethreeconversioncyclemodeisthe

internal open circuit detect mode. Typically, thermocouple

open circuit detection is performed by adding a high re-

sistance pull-up between the thermocouple and VDD. This

method can be used with the LTC2984 while operating

in the two conversion cycle mode (OC=0). This external

pull-up can interact with the input protection circuitry and

lead to temperature measurement errors and increased

noise. These problems are eliminated by selecting the

internal open circuit detection mode (OC=1). In this case,

a current is pulsed for 8ms and allowed to settle during

one conversion cycle. This is followed by the normal two

SUPPLEMENTAL INFORMATION

conversion cycle measurement of the thermocouple. If

the thermocouple is broken, the current pulse will result

in an open circuit fault.

A second feature taking advantage of the 3rd conversion

cycle is thermistor excitation current auto ranging. Since

a thermistor’s resistance varies many orders of magni-

tude, the performance in the low resistance regions are

compromised by the small currents required by the high

resistance regions of operation. The auto ranging mode

applies a test current during the first conversion cycle in

order to determine the optimum current for the resistance

state of the thermistor. It then uses that current to perform

the thermistor measurement using the normal 2-cycle

measurement.Ifa3-cyclethermistormeasurementisused

as the cold junction sensor for a 2-cycle thermocouple

measurement, the thermocouple conversion result is

ready after three cycles.

A third feature requiring a 3rd conversion cycle is the

three current diode measurement. In this mode, three

ratioed currents are applied to the external diode in order

to cancel parasitic lead resistance effects. This is useful

in applications where the diode is remotely located and

significant, unknown parasitic lead resistance requires

cancellation. If a 3-cycle diode or thermistor measure-

ment is used as the cold junction sensor for a 2-cycle

thermocouplemeasurement,thethermocoupleconversion

result is ready after three cycles.

Table 68. 2- and 3-Cycles Conversion Modes

TYPE OF SENSOR

CONFIGURATION

NUMBER OF

CONVERSION

CYCLES

MAXIMUM

OUTPUT TIME

Thermocouple

OC = 0

167.2ms

RTD

All

167.2ms

Thermistor

Non-Autorange

Current

167.2ms

Diode

Two Readings

167.2ms

Thermocouple

OC = 1

251ms

Thermocouple

OC = 0, 3-Cycle

Cold Junction

251ms

Thermistor

Autorange

Current

251ms

Diode

Three Readings

251ms

LTC2984 Datasheet(PDF) 63 Page - Analog Devices

LTC2984 Datasheet(HTML) 63 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78

Datasheet Download

Link URL