LTC2984 datasheet(29/78 Pages) AD

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LTC2984

2984fb

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(4) Diode Ideality Factor

The last field in the channel assignment word (B21 to B0)

sets the diode ideality factor within the range 0 to 4 with

1/1048576(2–20)resolution.Thetoptwobits(B21toB20)

are the integer part and bits B19 to B0 are the fractional

part of the ideality factor (see Table 24).

Diode channel assignments follow the general convention

shown in Figure 8. The anode ties to CHD (where D is

the selected channel number) for both the single-ended

and differential modes of operation, and the cathode is

grounded.Fordifferentialdiodemeasurements,thecathode

is also tied to CHD-1.

APPLICATIONS INFORMATION

Table 24. Programming Diode Ideality Factor

(4) DIODE IDEALITY FACTOR VALUE

B21 B20 B19 B18 B17 B16 B15 B14 B13 B12 B11 B10 B9 B8

B1 B0

Example h

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

1.25

1.003 (Default)

1.006

Table 25. Diode Fault Reporting

BIT

FAULT

ERROR TYPE

DESCRIPTION

OUTPUT RESULT

D31

Sensor Hard Fault

Hard

Open, Short, Reversed, or Hard ADC

–999°C or °F

D30

Hard ADC-Out-of-Range

Hard

Bad ADC Reading (Could Be Large External Noise Event)

–999°C or °F

D29

Not Used for Diodes

N/A

Always 0

D28

Not Used for Diodes

N/A

Always 0

D27

Sensor Over Range

Soft

T > 130°C

Suspect Reading

D26

Sensor Under Range

Soft

T < –60°C

Suspect Reading

D25

ADC Out-of-Range

Soft

ADC Absolute Input Voltage Is Beyond ±1.125 • VREF/2

Suspect Reading

D24

Valid

Result Valid (Should Be 1) Discard Results if 0

Valid Reading

Fault Reporting – Diode

Each sensor type has unique fault reporting mechanism

indicated in the upper byte of the data output word.

Table 25 shows faults reported in the measurement of

diodes.

Bit D31 indicates the diode is open, shorted, not plugged

in, wired backwards, or the ADC reading is bad. Any of

these are hard faults and –999°C or °F is reported. Bit

D30 indicates a bad ADC reading. This can be a result of

either a broken (open) sensor or an excessive noise event

(ESD or static discharge into the sensor path). This is a

hard error and –999°C or °F is reported. In the case of

Figure 8. Diode Channel Assignment Convention

2984 F08

SINGLE-ENDED

= CHD (1 ≤ D ≤ 20)

COM

CHD

CHANNEL

ASSIGNMENT

DIFFERENTIAL

= CHD (2 ≤ D ≤ 20)

CHD

CHD-1

CHANNEL

ASSIGNMENT

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