LTC2420CS8 datasheet(14/36 Pages) LINER

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LTC2420

APPLICATIO S I FOR ATIO

INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%)

–12

–8

–4

048

2420 F05

–60

–70

–80

–90

–100

–110

–120

–130

–140

Figure 5. LTC2420 Normal Mode Rejection When

Using an External Oscillator of Frequency fEOSC

Whenever an external clock is not present at the FO pin, the

converter automatically activates its internal oscillator and

enters the Internal Conversion Clock mode. The LTC2420

operation will not be disturbed if the change of conversion

clock source occurs during the sleep state or during the

data output state while the converter uses an external

serial clock. If the change occurs during the conversion

state, the result of the conversion in progress may be

outside specifications but the following conversions will

not be affected. If the change occurs during the data output

state and the converter is in the Internal SCK mode, the

serial clock duty cycle may be affected but the serial data

stream will remain valid.

Table 3 summarizes the duration of each state as a

function of FO.

SERIAL INTERFACE

The LTC2420 transmits the conversion results and re-

ceives the start of conversion command through a syn-

chronous 3-wire interface. During the conversion and

sleep states, this interface can be used to assess the

converter status and during the data output state it is used

to read the conversion result.

Serial Clock Input/Output (SCK)

The serial clock signal present on SCK (Pin 7) is used to

synchronize the data transfer. Each bit of data is shifted out

the SDO pin on the falling edge of the serial clock.

In the Internal SCK mode of operation, the SCK pin is an

output and the LTC2420 creates its own serial clock by

dividing the internal conversion clock by 8. In the External

SCK mode of operation, the SCK pin is used as input. The

internal or external SCK mode is selected on power-up and

then reselected every time a HIGH-to-LOW transition is

detected at the CS pin. If SCK is HIGH or floating at power-

up or during this transition, the converter enters the inter-

nal SCK mode. If SCK is LOW at power-up or during this

transition, the converter enters the external SCK mode.

Table 3. LTC2420 State Duration

State

Operating Mode

Duration

CONVERT

Internal Oscillator

FO = LOW

133ms

(60Hz Rejection)

FO = HIGH

160ms

(50Hz Rejection)

External Oscillator

FO = External Oscillator

20510/fEOSCs

with Frequency fEOSC kHz

(fEOSC/2560 Rejection)

SLEEP

As Long As CS = HIGH Until CS = 0 and SCK

DATA OUTPUT

Internal Serial Clock

FO = LOW/HIGH

As Long As CS = LOW But Not Longer Than 1.26ms

(Internal Oscillator)

(24 SCK cycles)

FO = External Oscillator with

As Long As CS = LOW But Not Longer Than 256/fEOSCms

Frequency fEOSC kHz

(24 SCK cycles)

External Serial Clock with

As Long As CS = LOW But Not Longer Than 24/fSCKms

Frequency fSCK kHz

(24 SCK cycles)

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