Datasheet

19
LTC2421/LTC2422
24212f
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 t
EOCtest
), 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 11. CS may be permanently tied to ground (Pin 6),
simplifying 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 0.5ms after V
CC
exceeds 2.2V. 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 ex-
ternal 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
sleep state. The part remains in the sleep state a minimum
amount of time (1/2 the internal SCK period) then imme-
diately begins outputting data. The data output cycle begins
on the first rising edge of SCK and ends after the 24th
rising edge. Data is shifted out the SDO pin on each falling
edge of SCK. The internally generated serial clock is out-
put to the SCK pin. This signal may be used to shift the
conversion result into external circuitry. EOC can be latched
on the first rising edge of SCK and the last bit of the
conversion result can be latched on the 24th rising edge
of SCK. After the 24th rising edge, SDO goes HIGH
(EOC = 1) indicating a new conversion is in progress. SCK
remains HIGH during the conversion.
Figure 10. Internal Serial Clock, Reduced Data Output Length
APPLICATIO S I FOR ATIO
WUUU
V
CC
10k
V
CC
F
O
FS
SET
ZS
SET
SCK
CH1 SDO
GND
CS
REFERENCE VOLTAGE
ZS
SET
+ 0.1V TO V
CC
0V TO FS
SET
– 100mV
CH0
= INTERNAL OSC/50Hz REJECTION
= EXTERNAL CLOCK SOURCE
= INTERNAL OSC/60Hz REJECTION
1µF
110
9
8
7
6
2
3
4
5
2.7V TO 5.5V
LTC2422
V
CC
ANALOG INPUT RANGE
ZS
SET
– 0.12V
REF
TO
FS
SET
+ 0.12V
REF
(V
REF
= FS
SET
– ZS
SET
)
SDO
SCK
(INTERNAL)
CS
>t
EOCtest
MSBEXRSIG
BIT 8
TEST EOCTEST EOC
BIT 19 BIT 18BIT 20BIT 21BIT 22
EOC CH0/CH1
BIT 23
EOC
BIT 0
SLEEP DATA OUTPUT
Hi-Z Hi-Z Hi-Z Hi-Z Hi-Z
DATA OUTPUT
CONVERSIONCONVERSIONSLEEP
24212 F10
<t
EOCtest
TEST EOC