LTC2984 datasheet(28/78 Pages) AD

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LTC2984

2984fb

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DIODE MEASUREMENTS

Channel Assignment – Diode

For each diode tied to the LTC2984, a 32-bit channel as-

signment word is programmed into a memory location

corresponding to the channel the sensor is tied to (see

Table 21). This word includes (1) diode sensor selection,

(2) sensor configuration, (3) excitation current, and (4)

diode ideality factor.

1) Sensor Type

The diode is selected by the first five input bits B31 to

B27 (see Table 22).

(2) Sensor Configuration

The sensor configuration field (bits B26 to B24) is used to

define various diode measurement properties. Configura-

tion bit B26 is set high for single-ended (measurement

relative to COM) and low for differential.

Bit B25 sets the measurement algorithm. If B25 is low, two

conversioncycles(oneat

1Iandoneat8Icurrentexcitation)

are used to measure the diode. This is used in applications

where parasitic resistance between the LTC2984 and the

diode is small. Parasitic resistance effects can be removed

by setting bit B25 high, enabling three conversion cycles

(one at

1I, one at 4I and one at 8I).

BitB24enablesarunningaverageofthediodetemperature

reading. This reduces the noise when the diode is used

as a cold junction temperature element on an isothermal

block where temperatures change slowly.

Thealgorithmusedfordiodeaveragingisasimplerecursive

running average. The new value is equal to the average of

the current reading plus the previous value.

NEW VALUE =

CURRENT READING

PREVIOUS VALUE

If the current reading is 2°C above or below the previous

value, the new value is reset to the current reading.

(3) Excitation Current

The next field in the channel assignment word (B23

to B22) controls the magnitude of the excitation cur-

rent applied to the diode (see Table 23). In the two

conversion cycle mode, the device performs the

first conversion at a current equal to 8x the excita-

tion current

1I. The second conversion occurs at 1I.

Alternatively, in the three conversion cycle mode the first

conversion excitation current is

8I, the second is 4I and

the 3rd is

1I.

Table 21. Diode Channel Assignment Word

(1) SENSOR

TYPE

(2) SENSOR

CONFIGURATION

(3) EXCITATION

CURRENT

(4) DIODE IDEALITY FACTOR VALUE

TABLE 22

TABLE 23

TABLE 24

Measurement Class 31 30 29 28 27

22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Diode

Type = 28

SGL=1

DIFF=0

2 or 3

Readings

Avg

Current [1:0] Non-Ideality Factor (2, 20) Value from 0 to 4 with 1/1048576 Resolution

All Zeros Uses a Factory Set Default of 1.003

Table 22. Diode Sensor Selection

(1) SENSOR TYPE

B31

B30

B29

B28

B27

SENSOR TYPE

Diode

APPLICATIONS INFORMATION

Table 23. Diode Excitation Current Selection

(3) EXCITATION CURRENT

B23

B22

10µA

40µA

80µA

20µA

80µA

160µA

40µA

160µA

320µA

80µA

320µA

640µA

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