LTC2431 datasheet(20/40 Pages) LINER

Part #	LTC2431

Description	20-Bit No Latency Delta-Sigma ADCs with Differential Input and Differential Reference

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Manufacturer	LINER [Linear Technology]

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LTC2430/LTC2431

20

24301f

internal pull-up is not available to restore SCK to a logic

HIGH state. This will cause the device to exit the internal

serial clock mode on the next falling edge of CS. This can

be avoided by adding an external 10k pull-up resistor to

the SCK pin or by never pulling CS HIGH when SCK is LOW.

Whenever SCK is LOW, the LTC2430/LTC2431’s internal

pull-up at pin SCK is disabled. Normally, SCK is not exter-

nally driven if the device is in the internal SCK timing mode.

However, certain applications may require an external

driver on SCK. If this driver goes Hi-Z after outputting a LOW

signal, the LTC2430/LTC2431’s internal pull-up remains

disabled. Hence, SCK remains LOW. On the next falling

edge of CS, the device is switched to the external SCK timing

mode. By adding an external 10k pull-up resistor to SCK,

this pin goes HIGH once the external driver goes Hi-Z. On

the next CS falling edge, the device will remain in the in-

ternal SCK timing mode.

A similar situation may occur during the sleep state when

CS is pulsed HIGH-LOW-HIGH in order to test the conver-

sion status. If the device is in the sleep state (EOC = 0),

SCK will go LOW. Once CS goes HIGH (within the time

period defined above as tEOCtest), the internal pull-up is

activated. For a heavy capacitive load on the SCK pin, the

internal pull-up may not be adequate to return SCK to a

HIGH level before CS goes low again. This is not a concern

under normal conditions where CS remains LOW after

detecting EOC = 0. This situation is easily overcome by

adding an external 10k pull-up resistor to the SCK pin.

Internal Serial Clock, 2-Wire I/O,

Continuous Conversion

This timing mode uses a 2-wire, all output (SCK and SDO)

interface. The conversion result is shifted out of the device

by an internally generated serial clock (SCK) signal, see

Figure 10. CS may be permanently tied to ground, simpli-

fying the user interface or isolation barrier.

The internal serial clock mode is selected at the end of the

power-on reset (POR) cycle. The POR cycle is concluded

approximately 1ms after VCC exceeds 2V. An internal weak

pull-up is active during the POR cycle; therefore, the

internal serial clock timing mode is automatically selected

if SCK is not externally driven LOW (if SCK is loaded such

that the internal pull-up cannot pull the pin HIGH, the

external SCK mode will be selected).

During the conversion, the SCK and the serial data output

pin (SDO) are HIGH (EOC = 1). Once the conversion is

complete, SCK and SDO go LOW (EOC = 0) indicating the

conversion has finished and the device has entered the

Figure 10. Internal Serial Clock, CS = 0 Continuous Operation

APPLICATIO S I FOR ATIO

SDO

SCK

(INTERNAL)

CS

MSB

SIG

BIT 0

LSB

BIT 19

BIT 18

BIT 20

BIT 21

BIT 22

EOC

BIT 23

DATA OUTPUT

CONVERSION

2431 F10

VCC

FO

REF+

REF–

SCK

IN+

IN–

SDO

GND

CS

REFERENCE

VOLTAGE

0.1V TO VCC

ANALOG INPUT RANGE

–0.5VREF TO 0.5VREF

1

µF

2.7V TO 5.5V

2-WIRE I/O

LTC2430/

LTC2431

= 50Hz REJECTION

= EXTERNAL OSCILLATOR

= 60Hz REJECTION

VCC

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