Datasheet

LTC2486
19
2486fd
Temperature Sensor
The LTC2486 includes an integrated temperature sensor.
The temperature sensor is selected by setting IM = 1. The
digital output is proportional to the absolute temperature
of the device. This feature allows the converter to perform
cold junction compensation for external thermocouples or
continuously remove the temperature effects of external
sensors.
The internal temperature sensor output is 28mV at 27°C
(300°K), with a slope of 93.5μV/°C independent of V
REF
(see Figures 4 and 5). Slope calibration is not required if
the reference voltage (V
REF
) is known. A 5V reference has
a slope of 2.45 LSBs
16
/°C. The temperature is calculated
from the output code (where DATAOUT
16
is the decimal
representation of the 16-bit result) for a 5V reference using
the following formula:
T
K
= DATAOUT
16
/2.45 in Kelvin
If a different value of V
REF
is used, the temperature output
is:
T
K
= DATAOUT
16
• V
REF
/12.25 in Kelvin
If the value of V
REF
is not known, the slope is determined by
measuring the temperature sensor at a known temperature
T
N
(in °K) and using the following formula:
SLOPE = DATAOUT
16
/T
N
APPLICATIONS INFORMATION
Table 5a. Performance vs Gain in Normal Speed Mode (V
CC
= 5V, V
REF
= 5V)
GAIN 1 4 8 16 32 64 128 256 UNIT
Input Span ±2.5 ±0.625 ±0.312 ±0.156 ±78m ±39m ±19.5m ±9.76m V
LSB 38.1 9.54 4.77 2.38 1.19 0.596 0.298 0.149 μV
Noise Free Resolution* 65536 65536 65536 65536 65536 65536 32768 16384 Counts
Gain Error 5 5 5 5 5 5 5 8 ppm of FS
Offset Error 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 μV
Table 5b. Performance vs Gain in 2x Speed Mode (V
CC
= 5V, V
REF
= 5V)
GAIN 1 2 4 8 16 32 64 128 UNIT
Input Span ±2.5 ±1.25 ±0.625 ±0.312 ±0.156 ±78m ±39m ±19.5m V
LSB 38.1 19.1 9.54 4.77 2.38 1.19 0.596 0.298 μV
Noise Free Resolution* 65536 65536 65536 65536 65536 65536 45875 22937 Counts
Gain Error 5 5 5 5 5 5 5 5 ppm of FS
Offset Error 200 200 200 200 200 200 200 200 μV
*The resolution in counts is calculated as the FS divided by LSB or the RMS noise value, whichever is larger.
This value of slope can be used to calculate further tem-
perature readings using:
T
K
= DATAOUT
16
/SLOPE
All Kelvin temperature readings can be converted to T
C
(°C) using the fundamental equation:
T
C
= T
K
– 273
SERIAL INTERFACE TIMING MODES
The LTC2486’s 4-wire interface is SPI and MICROWIRE
compatible. This interface offers several fl exible modes
of operation. These include internal/external serial clock,
3- or 4-wire I/O, single cycle or continuous conversion. The
following sections describe each of these timing modes
in detail. In all cases, the converter can use the internal
oscillator (f
O
= LOW) or an external oscillator connected to
the f
O
pin. For each mode, the operating cycle, data input
format, data output format, and performance remain the
same. Refer to Table 6 for a summary.
External Serial Clock, Single Cycle Operation
This timing mode uses an external serial clock to shift out
the conversion result and CS to monitor and control the
state of the conversion cycle (see Figure 6).