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LDTC0520 / LDTC1020
PAGE 12
LDTC1020-00400-A Rev B
WTC OPERATION, continued
Confi guration of the LDTC for Alternate Temperature Sensors
The standard LDTC confi guration is for Thermistor temperature sensors with 100uA bias current. The
LDTC can be easily modifi ed to operate with other sensors and bias currents using the following instruc-
tions. Resistive temperature sensors are covered on this page while LM335 and AD590 sensors are
covered on the following page.
Resistive Temperature Sensors
(Thermistors and RTDs)
Figure 4 shows the location of resistor R
Bias
,
which sets the sensor bias current. R
Bias
can be
replaced with a 1206 sized resistor to modify
the current to the desired value. Table 3 lists
the suggested resistor values for R
Bias
and RS
(shorting jumper mentioned below) versus the
range of resistances of the resistive temperature
sensor.
The following equation demonstrates how to
calculate a value of R
Bias
given a desired sensor
bias current, IBIAS.
Sensor Gain
If a temperature sensor voltage becomes too
low (below 250 mV), the temperature controller
may have a diffi cult time achieving high levels
of precision and stability. The sensor voltage
may be amplifi ed by a factor of 10 by placing
a jumper between Pin 8 () and Pin 13
(GND) on the WTC3243 Temperature Controller
(please refer to Figure 5). Note that the set point
voltage must also refl ect this amplifi ed voltage
level for proper control. For example, in a system
that utilizes a 100
Ω
RTD sensor, with a desired
operating point of about 25° C, the expected
sensor voltage would be approximately 110
mV, with a 1 mA bias current. By adding the
jumper, the temperature controller will “see” an
approximate sensor voltage of 1.10 V at 25° C.
Therefore the set point voltage will have to be
set to 1.10 V to account for the sensor voltage
amplifi cation and to properly control the system
at the desired set point of 25°C.
RBIAS
Figure 4
Location of Sensor Bias Resistor
Figure 2
Location of Gain Resistor
RBIAS
Sensor
Resistance
Range
Sensor Bias
Resistor,
R
BIAS
Sensor
Bias Current
Sensor Gain
Resistor, R
S
Sensor
Gain
35
Ω
to 350
Ω
35k
Ω
to 350k
Ω
2 k
Ω
2 k
Ω
20 k
Ω
200 k
Ω
1mA
1mA
SHORT
10
350
Ω
to 3.5k
Ω
OPEN
OPEN
OPEN
1
1
1
3.5k
Ω
to 35k
Ω
100μA
10μA
Table 3
Sensor Gain Resistor R
S
vs Resistance
Temperature Sensor
R
BIAS
=
2
I
BIAS
[
Ω
]
