Datasheet
LTC2481
21
2481fc
APPLICATIONS INFORMATION
conversion result following POR is accurate within the
specifi cations of the device if the power supply voltage is
restored within the operating range (2.7V to 5.5V) before
the end of the POR time interval.
On-Chip Temperature Sensor
The LTC2481 contains an on-chip PTAT (proportional to
absolute temperature) signal that can be used as a tempera-
ture sensor. The internal PTAT has a typical value of 420mV
at 27°C and is proportional to the absolute temperature
value with a temperature coeffi cient of 420/(27 + 273) =
1.40mV/°C (SLOPE), as shown in Figure 11. The internal
PTAT signal is used in a single-ended mode referenced to
device ground internally. The GAIN is automatically set to
one (independent of the values of GS0, GS1, GS2) in order
to preserve the PTAT property at the ADC output code
and avoid an out of range error. The 1x speed mode with
automatic offset calibration is automatically selected for
the internal PTAT signal measurement as well.
When using the internal temperature sensor, if the output
code is normalized to R
SDA
= V
PTAT
/V
REF
, the temperature
is calculated using the following formula:
T
K
=
R
SDA
•V
REF
SLOPE
in Kelvin
and
T
C
=
R
SDA
•V
REF
SLOPE
– 273 in ° C
where SLOPE is nominally 1.4mV/°C.
Since the PTAT signal can have an initial value variation
which results in errors in SLOPE, to achieve absolute
temperature measurements, a one-time calibration is
needed to adjust the SLOPE value. The converter output of
the PTAT signal, R0
SDA
, is measured at a known temperature
T0 (in °C) and the SLOPE is calculated as:
SLOPE =
R0
SDA
•V
REF
T0 + 273
This calibrated SLOPE can be used to calculate the tem-
perature.
If the same V
REF
source is used during calibration and
temperature measurement, the actual value of the V
REF
is not needed to measure the temperature as shown in
the calculation below:
T
C
=
R
SDA
•V
REF
SLOPE
– 273
=
R
SDA
R0
SDA
•T0+ 273
(
)
– 273
Reference Voltage Range
The LTC2481 external reference voltage range is 0.1V
to V
CC
. The converter output noise is determined by
the thermal noise of the front-end circuits, and as such,
its value in nanovolts is nearly constant with reference
voltage. Since the transition noise (600nV) is much less
than the quantization noise (V
REF
/2
17
), a decrease in the
reference voltage will increase the converter resolution. A
reduced reference voltage will also improve the converter
performance when operated with an external conversion
clock (external f
O
signal) at substantially higher output
data rates (see the Output Data Rate section). V
REF
must
be ≥1.1V to use the internal temperature sensor.
The reference input is differential. The differential reference
input range (V
REF
= REF
+
– REF
–
) is 100mV to V
CC
and the
common mode reference input range is 0V to V
CC
.
Figure 11. Internal PTAT Signal vs Temperature
TEMPERATURE (°C)
–60
V
PTAT
(mV)
500
600
120
2481 F11
400
200
30090–30 60
300
V
CC
= 5V
IM = 1
SLOPE = 1.40mV/°C