Datasheet

LTC2480
20
2480fd
value with a temperature coefficient 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 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.
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
R V
SLOPE
R
R
T
C
SDO REF
SDO
SDO
=
= +
( )
•
–
• –
273
0
0 273 273
Reference Voltage Range
The LTC2480 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
/217), 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 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 LTC2480 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 LTC2480 converts the bipolar differential input
signal, V
IN
= IN
+
– IN
–
, from –FS to +FS where FS = 0.5 •
V
REF
/GAIN. Outside this range, the converter indicates the
overrange or the underrange condition using distinct output
codes. Since the differential input current cancellation does
not rely on an on-chip buffer, current cancellation as well
as DC performance is maintained rail-to-rail.
applicaTions inForMaTion
TEMPERATURE (°C)
–60
V
PTAT
(mV)
500
600
120
2480 F04
400
200
300 90–30 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
R V
SLOPE
T
R V
SLOPE
K
SDO REF
C
SDO REF
=
= °
•
•
–
in Kelvin
and
in C273
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
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
R V
T
SDO REF
=
+
0
0 273
•
This calibrated SLOPE can be used to calculate the tem-
perature.