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Design Notes for a Generic Platinum RTD Processor
1
Kenneth A. Kuhn
Sept. 8, 2005
Introduction
This paper discusses the design of a generic platinum RTD (Resistance
Thermometer Device) processor to work with RTDs from 13 to over 200 Ohms
and produce an output voltage of 10 mV per degree C. The standard resistance
of an RTD is its resistance at zero C. At low to medium temperatures, platinum
has a fairly linear coefficient of resistance of 3850 parts per million per degree C.
At higher temperatures this factor decreases (about 3700 at 200 degrees C,
about 3600 at 300 degrees C, about 3500 at 400 degrees C). The circuit
described here does not include linearization so will indicate low as shown in the
following table.
Actual
Temperature
0
100
200
300
400
Normalized
Resistance
1.0000
1.3850
1.7584
2.1202
2.4704
Output
Voltage
0.0000
1.0000
1.9699
2.9096
3.8192
Indicated
Temperature
0
100
197
291
382
Operational Overview
A constant current is passed through a 3-wire RTD and the voltage drop across
the RTD element is sensed and amplified to produce a scale factor of 10 mV per
degree C. A 3-wire connection is used so that the resistance of the connecting
wires can be removed from the measurement. Calibration is accomplished by
adjusting the magnitude of the constant current so that the correct output voltage
is achieved at a known temperature of the RTD. The voltage produced by the
resistance of the RTD at zero degrees C is subtracted from the output thus
making the output voltage directly scaled to the RTD temperature in degrees C.
Detailed Operation (Figure 1)
The constant current source works by the control action of operational amplifier,
U2, on Q1 to cause the voltage drop across R1 to match the voltage on the wiper
of the calibration potentiometer, R4. The voltage across the calibration
potentiometer is regulated by the temperature compensated shunt regulator, U1.
Thus, the constant current is independent of ambient temperature. The purpose
of R3 is to limit the gate current to Q1 under any fault condition. The purpose of
R2 and C1 is to form a dominant time constant for control loop stability. Zener
diode, D2, limits the gate source voltage of Q1to a maximum of 5 volts to prevent
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Design Notes for a Generic Platinum RTD Processor
damage under any fault condition. The purpose of the zener diode, D1, is to
reduce the operating voltages of the circuit to be below the maximum commonmode voltage of the operational amplifier, U2. Resistor, R5, sets the operating
current of the shunt regulator. The range of constant current that can be
developed by this circuit is from less than 1 milliampere to 25 milliamperes.
When sensing the resistance of an RTD it is important to exclude the resistance
of the connecting wires and connector contact resistance as these are error
terms. This is achieved by a three-wire connection to the RTD. Two of the wires
are for conducting the constant current through the RTD and the third wire (which
conducts no current) is used to instrument the voltage drop in the current return
wire (the voltage drop in the current source wire is identical since the wires are
the same). Thus, the total voltage across the RTD and connecting resistance is
known (i.e. the input voltage to amplifier, U3) and the voltage across one of the
connecting resistances is known (i.e. the input voltage to amplifier, U4). A simple
algebra implemented by amplifiers, U3 and U4, combines these two
measurements to produce a voltage that is proportional only to the voltage
across the RTD element excluding the connecting resistances. U3 is connected
to have a gain of two and U4 is connected to have a gain of 4. The output
voltage of U3 is 2 * VRTD + 4 * VRw – 4 * VRw = 2 * VRTD. Components, R6, C2,
and R8, C3 are low-pass filters to reduce interference picked up by the RTD
cable. Resistors, R7 and R9, provide a DC path for U3 and U4 if the RTD is not
connected.
The equation for the voltage across the RTD is:
VRTD = (I * RTD0) *(1 + 0.00385 * T)
Eq. 1
where:
VRTD is the voltage across the RTD
I is the constant current in amperes
RTD0 is the resistance of the RTD at 0 degrees C
0.00385 is the resistance temperature coefficient of platinum at low temperatures
T is temperature in degrees C
The current, I, is adjusted so that the quantity (I * RTD0) is a constant for any
value of RTD0 over the range of resistances this circuit is designed to work over.
This is what makes the circuit independent of the value of RTD0. For standard
100 ohm miniature RTDs the constant current is generally selected to be small
enough so that the power dissipated in the RTD is no more than about 1 mW.
For this example, the current would be 3.16 mA and the constant, (I * RTD 0), is
0.316.
The output voltage / temperature scale factor is chosen to be 10 mV / degree C
as this is a very common and easy to work with value. The required gain, G, that
U5 must have to achieve this scale factor is:
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Design Notes for a Generic Platinum RTD Processor
G = 0.01 / (2 * 0.00385 * (I * RTD0))
Eq. 2
Note that the factor of two in Equation 2 is because of the gain of two by U3.
Continuing with the example, G computes to be 4.107. Although this gain can be
achieved with the right combination of R21 and R22, it is better to choose R21
and R22 to be convenient values and adjust the arbitrary constant (I * RTD 0) as
needed (remember that this value is set by potentiometer, R4). Since a gain of
four already exists in the circuit (U4) then R21 is chosen to be equal to R12 and
R22 is chosen to be equal to R13. Thus, G is four. Equation 2 is then solved to
determine the required constant, (I * RTD0). (I * RTD0) = 0.01 / (2 * 0.00385 * 4)
= 0.3247. This is the design constant for the circuit. Potentiometer, R4, is used
to calibrate a given RTD. The following table shows the nominal current for a
variety of RTD0 values.
RTD0 value
13.00 ohms
100.00 ohms
200.00 ohms
400.00 ohms
Constant current
24.977 mA
3.247 mA
1.623 mA
0.812 mA
Dissipation at 0 degrees C
8.11 mW
1.054 mW
0.527 mW
0.263 mW
To complete the design, the output voltage of U5 must be offset to be zero volts
when the RTD temperature is zero degrees C. The required offset voltage is:
Vos = 2 * 0.3247 * 4 = 2.5974 volts
This value is divided by the inverting gain of three for R21/R22 to be 0.8658 volts
at the output of U6. This voltage is derived by a combination of a temperature
stable 2.5 volt reference and a resistor and potentiometer network, R16 to R19.
This network derives the dominate portion of the voltage from stable resistors
rather than a potentiometer that could drift. The network is designed so that the
junction of R17 and R18 is at nearly 1.25 volts (half the reference voltage). With
this arrangement the potentiometer, R15, is nominally at the midway point and
thus there is little or no current in the wiper. The value of R16 is chosen to
provide about a +-2 percent adjustment range over the extreme range of the
potentiometer. Normally the pot would be adjusted so that the output voltage of
U6 is 0.8658 volts. However, the exact adjustment is for the output of U5 to be
0.000 volts when the RTD is at 0.0 degrees C – this is not generally convenient
to achieve.
The purpose of R20 in series with the output of U5 is to provide protection from
output faults such as a short circuit. The net output resistance of this circuit
remains at near zero ohms because R20 is within the feedback of U5. Capacitor,
C4, provides high frequency stability for this network.