
LTC2498
21
2498ff
applications inForMation
Figure 4. Internal PTAT Digital Output vs Temperature
Slope calibration is not required if the reference volt-
age (VREF) is known. A 5V reference has a slope of 314
LSBs24/°C. The temperature is calculated from the output
code (DATAOUT24) for a 5V reference using the following
formula:
TK = DATAOUT24/314 in Kelvin
IfadifferentvalueofVREFisused,thetemperatureoutputis:
TK =
DATAOUT24 VREF
1570
in Kelvin
IfthevalueofVREFisnotknown,theslopeisdeterminedby
measuringthetemperaturesensorataknowntemperature
TN (in °K) and using the following formula:
SLOPE = DATAOUT24/TN
This value of slope can be used to calculate further tem-
perature readings using:
TK = DATAOUT24/SLOPE
All Kelvin temperature readings can be converted to TC
(°C) using the fundamental equation:
TC = TK – 273
SERIAL INTERFACE TIMING MODES
The LTC2498’s 4-wire interface is SPI and MICROWIRE
compatible. This interface offers several flexible modes
of operation. These include internal/external serial clock,
3-or4-wireI/O,singlecycleorcontinuousconversion.The
following sections describe each of these timing modes
in detail. In all cases, the converter can use the internal
oscillator (fO = LOW or fO = HIGH) or an external oscillator
connectedtothefOpin.Foreachmode,theoperatingcycle,
data input format, data output format, and performance
remain the same. Refer to Table 5 for a summary.
Figure 5. Absolute Temperature Error
Table 5. LTC2498 Interface Timing Modes
CONFIGURATION
SCK
SOURCE
CONVERSION
CYCLE CONTROL
DATA OUTPUT
CONTROL
CONNECTION AND
WAVEFORMS
External SCK, Single Cycle
Conversion
External
CS and SCK
Figures 6, 7
External SCK, 3-Wire I/O
External
SCK
Figure 8
Internal SCK, Single Cycle
Conversion
Internal
CS↓
Figures 9, 10
Internal SCK, 3-Wire I/O,
Continuous Conversion
Internal
Continuous
Internal
Figure 11
TEMPERATURE (°K)
0
DATAOUT
24
60000
80000
100000
120000
140000
400
2498 F04
40000
0
300
200
100
20000
VCC = 5V
VREF = 5V
SLOPE = 314 LSB24/K
TEMPERATURE (°C)
–55 –30
–5
ABSOLUTE
ERROR
(°C)
5
4
3
2
1
–4
–3
–2
–1
0
120
95
70
45
20
2498 F05
–5