Datasheet

LTC2494
21
2494fd
applications inForMation
GAIN (GS2, GS1, GS0)
The input referred gain of the LTC2494 is adjustable
from 1 to 256 (see Tables 5a and 5b). With a gain of 1,
the differential input range is ±V
REF
/2 and the common
mode input range is rail-to-rail. As the gain is increased,
the differential input range is reduced to ±0.5 • V
REF
/Gain
but the common mode input range remains rail-to-rail.
As the differential gain is increased, low level voltages
are digitized with greater resolution. At a gain of 256, the
LTC2494 digitizes an input signal range of ±9.76mV with
over 16,000 counts.
Temperature Sensor
The LTC2494 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
.
Slope calibration is not required if the reference voltage
(V
REF
) is known. A 5V reference has a slope of 2.45
LSBs
16
/°C. The temperature is calculated from the output
code (where DATAOUT
16
is the decimal representation of
the 16-bit result) for a 5V reference using the following
formula:
T
K
= DATAOUT
16
/2.45 in Kelvin
If a different value of V
REF
is used, the temperature output
is:
T
K
= DATAOUT
16
• V
REF
/12.25 in Kelvin
If the value of V
REF
is not known, the slope is determined by
measuring the temperature sensor at a known temperature
T
N
(in °K) and using the following formula:
SLOPE = DATAOUT
16
/T
N
This value of slope can be used to calculate further tem-
perature readings using:
T
K
= DATAOUT
16
/SLOPE
All Kelvin temperature readings can be converted to T
C
(°C) using the fundamental equation:
T
C
= T
K
– 273
Table 5a. Performance vs Gain in Normal Speed Mode (V
CC
= 5V, V
REF
= 5V)
GAIN 1 4 8 16 32 64 128 256 UNIT
Input Span ±2.5 ±0.625 ±0.312 ±0.156 ±78m ±39m ±19.5m ±9.76m V
LSB 38.1 9.54 4.77 2.38 1.19 0.596 0.298 0.149 µV
Noise Free Resolution* 65536 65536 65536 65536 65536 65536 32768 16384 Counts
Gain Error 5 5 5 5 5 5 5 8 ppm of FS
Offset Error 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 µV
Table 5b. Performance vs Gain in 2x Speed Mode (V
CC
= 5V, V
REF
= 5V)
GAIN 1 2 4 8 16 32 64 128 UNIT
Input Span ±2.5 ±1.25 ±0.625 ±0.312 ±0.156 ±78m ±39m ±19.5m V
LSB 38.1 19.1 9.54 4.77 2.38 1.19 0.596 0.298 µV
Noise Free Resolution* 65536 65536 65536 65536 65536 65536 45875 22937 Counts
Gain Error 5 5 5 5 5 5 5 5 ppm of FS
Offset Error 200 200 200 200 200 200 200 200 µV
*The resolution in counts is calculated as the FS divided by LSB or the RMS noise value, whichever is larger.