LTC2983
57
2983fc
For more information www.linear.com/LTC2983
SUPPLEMENTAL INFORMATION
Table 64. 2- and 3-Cycles Conversion Modes
TYPE OF SENSOR CONFIGURATION NUMBER OF
CONVERSION
CYCLES
MAXIMUM OUTPUT
TIME
Thermocouple OC = 0 2 167.2ms
RTD All 2 167.2ms
Thermistor Non-Autorange
Current
2 167.2ms
Diode Two Readings 2 167.2ms
Thermocouple OC = 1 3 251ms
Thermocouple OC = 0, 3-Cycle
Cold Junction
3 251ms
Thermistor Autorange
Current
3 251ms
Diode Three Readings 3 251ms
Fault Protection and Anti-Aliasing
The LTC2983 analog input channels draw a maximum
of 1nA DC. As a result, it is possible to add anti-aliasing
and fault protection circuitry directly to the input of the
LTC2983. The most common input circuitry is a low pass
filter with 1k to 10k resistance (limited by excitation current
for RTDs and thermistors) and a capacitor with 100pF-0.1µf
capacitance. This circuit can be placed directly between
the thermocouples and 4-wire RTDs and the LTC2983.
In the case of 3-wire RTDs, mismatch errors between
the protection resistors can degrade the performance.
Thermistors requiring input projection should be tied to
the LTC2983 through a Kelvin type connection in order to
avoid errors due to the fault protection resistors.
2- and 3-Cycle Conversion Modes
The LTC2983 performs multiple internal conversions in
order to determine the sensor temperature. Normally, two
internal conversion cycles are required for each tempera-
ture result providing a maximum output time of 167.2ms.
The LTC2983 uses these two cycles to automatically
remove offset/offset drift errors, reduce 1/f noise, auto-
calibrate matched internal current sources, and provide
simultaneous 50/60Hz noise rejection.
In addition to performing two conversion cycles per result,
the LTC2983 also offers several unique features by utilizing
a 3rd conversion cycle. In this case, the maximum output
time is 251ms and all the benefits of the 2-cycle modes
are present (see Table 64).
One feature utilizing the three conversion cycle mode is the
internal open circuit detect mode. Typically, thermocouple
open circuit detection is performed by adding a high re-
sistance pull-up between the thermocouple and V
DD
. This
method can be used with the LTC2983 while operating
in the two conversion cycle mode (OC=0). This external
pull-up can interact with the input protection circuitry and
lead to temperature measurement errors and increased
noise. These problems are eliminated by selecting the
internal open circuit detection mode (OC=1). In this case,
a current is pulsed for 8ms and allowed to settle during
one conversion cycle. This is followed by the normal two
conversion cycle measurement of the thermocouple. If
the thermocouple is broken, the current pulse will result
in an open circuit fault.
A second feature taking advantage of the 3rd conversion
cycle is thermistor excitation current auto ranging. Since
a thermistor’s resistance varies many orders of magni-
tude, the performance in the low resistance regions are
compromised by the small currents required by the high
resistance regions of operation. The auto ranging mode
applies a test current during the first conversion cycle in
order to determine the optimum current for the resistance
state of the thermistor. It then uses that current to perform
the thermistor measurement using the normal 2-cycle
measurement. If a 3-cycle thermistor measurement is used
as the cold junction sensor for a 2-cycle thermocouple
measurement, the thermocouple conversion result is
ready after three cycles.
A third feature requiring a 3rd conversion cycle is the
three current diode measurement. In this mode, three
ratioed currents are applied to the external diode in order
to cancel parasitic lead resistance effects. This is useful
in applications where the diode is remotely located and
significant, unknown parasitic lead resistance requires
cancellation. If a 3-cycle diode or thermistor measure-
ment is used as the cold junction sensor for a 2-cycle
thermocouple measurement, the thermocouple conversion
result is ready after three cycles.
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