LTC2485 LINER [Linear Technology], LTC2485 Datasheet - Page 20

• Manufacturer Part Number: LTC2485
• Description: 24-Bit ?? ADC with Easy Drive Input Current Cancellation and I2C Interface
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC2485
Ease of Use
The LTC2485 data output has no latency, filter 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
mately 2V. This feature guarantees the integrity of the
conversion result.
When the V
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
first conversion result following POR is accurate within the
specifications 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 coefficient 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 auto-
matic offset calibration is automatically selected for the in-
ternal PTAT signal measurement as well.
When using the internal temperature sensor, if the output
code is normalized to R
20
CC
voltage rises above this critical threshold,
U
SDA
U
= V
PTAT
CC
/V
W
drops below approxi-
REF
, the temperature
U
is calculated using the following formula:
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
ture T0 (in °C) and the SLOPE is calculated as:
This calibrated SLOPE can be used to calculate the
temperature.
If the same V
temperature measurement, the actual value of the V
not needed to measure the temperature as shown in the
calculation below:
and
SLOPE
T
C
T
T
=
=
K
C
Figure 11. Internal PTAT Signal vs Temperature
R
R
=
=
R
0
SDA
SDA
=
R
R
SLOPE
SDA
600
500
400
200
300
SDA
SDA
R
–60
SLOPE
SLOPE
REF
•
0
T
V
IM = 1
SLOPE = 1.40mV/°C
•
SDA
V
CC
0 273
•
•
REF
(
source is used during calibration and
–30
= 5V
T
+
V
V
SDA
0 273
REF
REF
•
–
+
V
, is measured at a known tempera-
TEMPERATURE (°C)
REF
273
0
–
in Kelvin
273
)
30
–
in C
273
60
°
90
2485 F11
120
REF
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