Datasheet

LTC2498
23
2498ff
applications inForMation
Typically, CS remains LOW during the data output/input
state. However, the data output state may be aborted by
pulling CS HIGH any time between the 1st falling edge
and the 32nd falling edge of SCK, see Figure 7. On the
rising edge of CS, the device aborts the data output state
and immediately initiates a new conversion. In order to
program a new input channel, 8 SCK clock pulses are
required. If the data output sequence is aborted prior to
the 8th falling edge of SCK, the new input data is ignored
and the previously selected input channel remains valid.
If the rising edge of CS occurs after the 8th falling edge
of SCK, the new input channel is loaded and valid for the
next conversion cycle. If CS goes HIGH between the 8th
falling edge and the 16th falling edge of SCK, the new
channel is still loaded, but the converter configuration
remains unchanged. In order to program both the input
channel and converter configuration, CS must go HIGH
after the 16th falling edge of SCK (at this point all data
has been shifted into the device).
External Serial Clock, 3-Wire I/O
This timing mode uses a 3-wire
serial I/O interface. The
c
onversion result is shifted out of the device by an externally
generated serial clock (SCK) signal, see Figure 8. CS is
permanently tied to ground, simplifying the user interface
or isolation barrier.
The external serial clock mode is selected at the end of the
power-on reset (POR) cycle. The POR cycle is typically
concluded 4ms after V
CC
exceeds 2V. The level applied
to SCK at this time determines if SCK is internally gener-
ated or externally applied. In order to enter the external
SCK mode, SCK must be driven LOW prior to the end of
the POR cycle.
Since CS is tied LOW, the end-of-conversion (EOC) can be
continuously monitored at the SDO pin during the convert
and sleep states. EOC may be used as an interrupt to an
external controller. EOC = 1 while the conversion is in
Figure 7. External Serial Clock, Reduced Output Data Length and Valid Channel Selection
Hi-Z
2498 F07
CS
SCK
(EXTERNAL)
SDI
SDO
CONVERSION SLEEP DATA INPUT/OUTPUT SLEEPCONVERSION
V
CC
f
O
REF
+
REF
–
CH0
CH7
CH8
CH15
COM
SCK
SDI
SDO
CS
GND
28 35
29
30
8
15
16
23
7
38
37
1,3,4,5,6,31,32,33,39
36
34
REFERENCE
VOLTAGE
0.1V TO V
CC
ANALOG
INPUTS
= EXTERNAL OSCILLATOR
= INTERNAL OSCILLATOR
LTC2498
4-WIRE
SPI INTERFACE
•
•
•
•
•
•
•
•
•
•
•
•
EOC
BIT 28 BIT 27 BIT 26 BIT 25 BIT 24 BIT 23BIT 29BIT 30BIT 31
1 2 3 4 5 6 7 8
1 0 EN SGL A2 A1 A0ODD
DON'T CAREDON'T CARE
MSBSIG“0”
10µF
0.1µF
2.7V TO 5.5V