Datasheet

14
LTC2420
APPLICATIO S I FOR ATIO
WUU
U
INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%)
–12–8–404812
REJECTION (dB)
2420 F05
–60
–70
–80
–90
–100
–110
–120
–130
–140
Figure 5. LTC2420 Normal Mode Rejection When
Using an External Oscillator of Frequency f
EOSC
Whenever an external clock is not present at the F
O
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 F
O
.
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 F
O
= LOW 133ms
(60Hz Rejection)
F
O
= HIGH 160ms
(50Hz Rejection)
External Oscillator F
O
= External Oscillator 20510/f
EOSC
s
with Frequency f
EOSC
kHz
(f
EOSC
/2560 Rejection)
SLEEP As Long As CS = HIGH Until CS = 0 and SCK
DATA OUTPUT Internal Serial Clock F
O
= LOW/HIGH As Long As CS = LOW But Not Longer Than 1.26ms
(Internal Oscillator) (24 SCK cycles)
F
O
= External Oscillator with As Long As CS = LOW But Not Longer Than 256/f
EOSC
ms
Frequency f
EOSC
kHz (24 SCK cycles)
External Serial Clock with As Long As CS = LOW But Not Longer Than 24/f
SCK
ms
Frequency f
SCK
kHz (24 SCK cycles)