Datasheet

LTC2484
18
2484fd
APPLICATIONS INFORMATION
fi rst conversion result following POR is accurate within
the specifi cations of the device if the power supply volt-
age is restored within the operating range (2.7V to 5.5V)
before the end of the POR time interval.
On-Chip Temperature Sensor
The LTC2484 contains an on-chip PTAT (proportional
to absolute temperature) signal that can be used as a
temperature sensor. The internal PTAT has a typical
value of 420mV at 27°C and is proportional to the abso-
lute temperature value with a temperature coeffi cient of
420/(27 + 273) = 1.40mV/°C (SLOPE), as shown in Figure 4.
The internal PTAT signal is used in a single-ended mode
referenced to device ground internally. The 1x speed mode
with automatic offset calibration is automatically selected
for the internal PTAT signal measurement as well.
to adjust the SLOPE value. The converter output of the
PTAT signal, R0
SDO
, is measured at a known temperature
T0 (in °C) and the SLOPE is calculated as:
SLOPE =
R0
SDO
•V
REF
T0 + 273
This calibrated SLOPE can be used to calculate the
temperature.
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
SDO
•V
REF
SLOPE
– 273
=
R
SDO
R0
SDO
•T0+ 273
()
– 273
Reference Voltage Range
The LTC2484 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.
A reduced reference voltage will 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 negative reference input to the converter is internally
tied to GND. GND (Pin 8) should be connected to a ground
plane through as short a trace as possible to minimize
voltage drop. The LTC2484 has an average operational
current of 160μA and for 0.1Ω parasitic resistance, the
voltage drop of 16μV causes a gain error of 3.2ppm for
V
REF
= 5V.
Input Voltage Range
The analog input is truly differential with an absolute/com-
mon mode range for the IN
+
and IN
–
input pins extending
from GND – 0.3V to V
CC
+ 0.3V. Outside these limits, the ESD
protection devices begin to turn on and the errors due to
input leakage current increase rapidly. Within these limits,
the LTC2484 converts the bipolar differential input signal,
TEMPERATURE (°C)
–60
V
PTAT
(mV)
500
600
120
2484 F04
400
200
30090–30 60
300
V
CC
= 5V
IM = 1
f
O
= GND
SLOPE = 1.40mV/°C
Figure 4. Internal PTAT Signal vs Temperature
When using the internal temperature sensor, if the output
code is normalized to R
SDO
= V
PTAT
/V
REF
, the temperature
is calculated using the following formula:
T
K
=
R
SDO
•V
REF
SLOPE
in Kelvin
and
T
C
=
R
SDO
•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 better tem-
perature measurements, a one-time calibration is needed