Datasheet

LTC2499
20
2499fd
Rejection Mode (FA, FB)
The LTC2499 includes a high accuracy on-chip oscillator
with no required external components. Coupled with an
integrated fourth order digital lowpass filter, the LTC2499
rejects line frequency noise. In the default mode, the
LTC2499 simultaneously rejects 50Hz and 60Hz by at least
87dB. If more rejection is required, the LTC2499 can be
configured to reject 50Hz or 60Hz to better than 110dB.
Speed Mode (SPD)
Every conversion cycle, two conversions are combined
to remove the offset (default mode). This result is free
from offset and drift. In applications where the offset is
not critical, the auto-calibration feature can be disabled
with the benefit of twice the output rate.
While operating in the 2x mode (SPD = 1), the linearity
and full-scale errors are unchanged from the 1x mode
performance. In both the 1x and 2x mode there is no
latency. This enables input steps or multiplexer changes
to settle in a single conversion cycle, easing system over-
head and increasing the effective conversion rate. During
temperature measurements, the 1x mode is always used
independent of the value of SPD.
Temperature Sensor
The LTC2499 includes an integrated temperature sensor.
The temperature sensor is selected by setting IM = 1.
During temperature readings, MUXOUTN/MUXOUTP
remains connected to the selected input channel. The
ADC internally connects to the temperature sensor and
performs a conversion.
The digital output is proportional to the absolute tem-
perature of the device. This feature allows the converter
to perform cold junction compensation for external
thermocouples or continuously remove the temperature
effects of external sensors.
The internal temperature sensor output is 28mV at 27°C
(300°K), with a slope of 93.5µV/°C independent of V
REF
(see Figures 4 and 5). Slope calibration is not required if
the reference voltage (V
REF
) is known. A 5V reference has
a slope of 314 LSBs
24
/°C. The temperature is calculated
from the output code (where DATAOUT
24
is the decimal
representation of the 24-bit result) for a 5V reference using
the following formula:
T
K
=
DATAOUT
24
314
in Kelvin
If a different value of V
REF
is used, the temperature
output is:
T
K
=
DATAOUT
24
• V
REF
1570
in Kelvin
If the value of V
REF
is not known, the slope is determined
by measuring the temperature sensor at a known tempera-
ture T
N
(in K) and using the following formula:
SLOPE =
DATAOUT
24
T
N
Figure 4. Internal PTAT Digital Output vs Temperature Figure 5. Absolute Temperature Error
applications inForMation
TEMPERATURE (K)
0
DATAOUT
24
60000
80000
100000
120000
140000
400
2499 F04
40000
0
300200100
20000
V
CC
= 5V
V
REF
= 5V
SLOPE = 314 LSB
24
/K
TEMPERATURE (°C)
–55 –30 –5
ABSOLUTE ERROR (°C)
5
4
3
2
1
–4
–3
–2
–1
0
12095704520
2499 F05
–5