LTC2984 datasheet(61/78 Pages) AD

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LTC2984

2984fb

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DIRECT ADC MEASUREMENTS

Inadditiontomeasuringtemperaturesensors,theLTC2984

can perform direct voltage measurements. Any channel

can be configured to perform direct single-ended or dif-

ferential measurements. Direct ADC channel assignments

follow the general convention shown in Figure 36. The

32-bit channel assignment word is programmed into a

memory location corresponding to the input channel.

The channel assignment word is 0xF000 0000 for differ-

ential readings and 0xF400 0000 for single-ended. The

SUPPLEMENTAL INFORMATION

positive input channel ties to CHADC for both single-ended

and differential modes. For single-ended measurements

the ADC negative input is COM while for differential mea-

surements it is CHADC-1. For single ended measurements,

COM can be driven with any voltage above GND – 50mV

and below VDD – 0.3V.

The direct ADC results are available in memory at a

location corresponding to the conversion channel.

The data is represented as a 32-bit word (see Table 67)

where the eight most significant bits are fault bits and

2984 F36

CHADC

SINGLE-ENDED

CHANNEL

ASSIGNMENT

= CHADC (1 ≤ ADC ≤ 20)

= CHADC (2 ≤ ADC ≤ 20)

24-BIT

∆∑ ADC

24-BIT

∆∑ ADC

CHANNEL

ASSIGNMENT

DIFFERENTIAL

COM

CHADC

CHADC-1

–

Figure 36. Direct ADC Channel Assignment Conventions

Table 67. Direct ADC Output Format

START ADDRESS

START ADDRESS + 1

START ADDRESS + 2

START ADDRESS + 3

(END ADDRESS)

D31

D30 D29 D28 D27

D26

D25

D24

D23 D22 D21 D20 D19 D18 D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0

Fault Data

SIGN MSB

LSB

Volts

Sensor

Hard

Fault

Range

Hard

Fault

NA NA Soft

Above

Soft

Below

Soft

Range

Valid

Always

± 2V 1V

0.5V 0.25V ...

Integer

Fraction

>VREF

CLAMPED to Factory Programmed Value

of VREF

1.75 • VREF/2

1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1.125 • VREF/2

1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

VREF/2

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

VREF/222

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

–VREF/222

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

–VREF/2

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

–1.125 • VREF

0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

–1.75 • VREF

0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

< –VREF

CLAMPED to Factory Programmed Value

of –VREF

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