15
LTC2404/LTC2408
APPLICATIONS INFORMATION
WUU
U
Table 4. LTC2404/LTC2408 State Duration
State Operating Mode Duration
CONVERT Internal Oscillator F
O
= LOW (60Hz Rejection) 133ms
F
O
= HIGH (50Hz Rejection) 160ms
External Oscillator F
O
= External Oscillator 20480/f
EOSC
(In Seconds)
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 1.67ms
(Internal Oscillator) (32 SCK cycles)
F
O
= External Oscillator with As Long As CS = LOW But Not Longer Than 256/f
EOSC
ms
Frequency f
EOSC
kHz (32 SCK cycles)
External Serial Clock with As Long As CS = LOW But Not Longer Than 32/f
SCK
ms
Frequency f
SCK
kHz (32 SCK cycles)
MAXIMUM OUTPUT
WORD RATE
Figure 4. LTC2404/LTC2408 Normal Mode Rejection When
Using an External Oscillator of Frequency f
EOSC
INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%)
–12–8–404812
REJECTION (dB)
24048 F04
–60
–70
–80
–90
–100
–110
–120
–130
–140
The selection of 50Hz or 60Hz rejection can also be made
by driving F
O
to an appropriate logic level. A selection
change during the sleep or data output states will not
disturb the converter operation. If the selection is made
during the conversion state, the result of the conversion in
progress may be outside specifications but the following
conversions will not be affected.
When a fundamental rejection frequency different from
50Hz or 60Hz is required or when the converter must be
synchronized with an outside source, the LTC2404/
LTC2408 can operate with an external conversion clock.
The converter automatically detects the presence of an
external clock signal at the F
O
pin and turns off the internal
oscillator. The frequency f
EOSC
of the external signal must
be at least 2560Hz (1Hz notch frequency) to be detected.
The external clock signal duty cycle is not significant as
long as the minimum and maximum specifications for the
high and low periods t
HEO
and t
LEO
are observed.
While operating with an external conversion clock of a
frequency f
EOSC
, the LTC2404/LTC2408 provide better
than 110dB normal mode rejection in a frequency range
f
EOSC
/2560 ±4% and its harmonics. The normal mode
rejection as a function of the input frequency deviation
from f
EOSC
/2560 is shown in Figure 4.
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
LTC2404/LTC2408 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 4 summarizes the duration of each state as a
function of F
O
.
OWR
tt
inHz
CONVERT DATAOUTPUT
=
+
1
