Datasheet

LTC2485
20
2485fc
APPLICATIONS INFORMATION
Ease of Use
The LTC2485 data output has no latency, fi lter settling
delay or redundant data associated with the conversion
cycle. There is a one-to-one correspondence between the
conversion and the output data. Therefore, multiplexing
multiple analog voltages is easy.
The LTC2485 performs offset and full-scale calibrations
every conversion cycle. This calibration is transparent to
the user and has no effect on the cyclic operation described
above. The advantage of continuous calibration is extreme
stability of offset and full-scale readings with respect to
time, supply voltage change and temperature drift.
Power-Up Sequence
The LTC2485 automatically enters an internal reset state
when the power supply voltage V
CC
drops below ap-
proximately 2V. This feature guarantees the integrity of the
conversion result.
When the V
CC
voltage rises above this critical threshold,
the converter creates an internal power-on-reset (POR)
signal with a duration of approximately 4ms. The POR
signal clears all internal registers. Following the POR signal,
the LTC2485 starts a normal conversion cycle and follows
the succession of states described in Figure 1. The fi rst
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 LTC2485 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 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 tem-
perature 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
TEMPERATURE (°C)
–60
V
PTAT
(µV)
500
600
120
2485 F11
400
200
30090–30 60
300
V
CC
= 5V
IM = 1
SLOPE = 1.40mV/°C
Figure 11. Internal PTAT Signal vs Temperature