[introduction] KWIK (technical know-how and integrated knowledge) circuit application note provides the guide dividing a pace that answers specific design challenge. To given requirement of circuit of a group of application, the article says how palpability uses current formula to answer these demands, make they expand easily other and similar application standards. This sensor model supports pair of resistor temperature detector (of RTD) electric undertake with physical character SPICE is emulated. SPICE model used descriptive RTD (its resistor of temperature translate into) the parameter of physical conduct character. It still supplied circuit of a typical drive and signature manage, use this circuit to be able to demonstrate the behavior of RTD model.
RTD is block sex component, its resistor changes along with temperature and change. The behavior of RTD already was person place hep, can use at undertaking accurate temperature measure, precision can amount to 0.1 ° C the following. RTD is twined by a paragraph normally form in the lead all round pottery and porcelain or vitreous core, but the thick film resistor that also can go up in underlay by plating is formed. Use resistor silk is platinic normally, but also can be nickel or copper. PT100 is a kind of common RTD, you Bo is made, when 0 ° C resistor is 100 Ω . Still have additionally when 0 ° C resistor is 200, 500, the 1000 RTD component with 2000 Ω . The relation of platinic RTD resistor and temperature is described by equation of Callendar-Van Dusen (CVD) . Equation 1 described PT100 RTD to be in the RTD resistor under 0 ° C, equation 2 described its to be in the RTD resistor of above of 0 ° C.
The coefficient in Callendar-Van Dusen equation is defined by IEC-60751 standard. R0 is RTD the resistor when 0 ° C. To PT100 RTD, r0 is 100 Ω . To PT100 RTD of IEC 60751 standard, coefficient is:
From – 200 ° C arrives 850 ° C, if the graph is shown 1 times,the resistor of PT100 RTD changes.
Graph 1. From – 200 ° C arrives the PT100 RTD resistor of 850 ° C
Design a description
This RTD model (graph 2) use LTSPICE undertakes emulation, but as compatible as PSPICE also. Use this model, the user is OK the sensor of electric current of imitate reference drive is laden, move signal manage circuit to join to RTD. Can be opposite so all in all model, poor cent and source impedance effect undertake emulation. Resistor of RTD of this model hypothesis follows temperature and change. Be opposite only nominal sensor norms undertakes building a model. T1 is the parameter that the model uses, express to describe the temperature in the equation of RTD behavior. This states with what use in SPICE the variable Temp of global temperature is different. This kind of method makes the model can demonstrate the behavior of RTD only, and in won’t affecting circuit other yuan of function of parts of an apparatus.
Design skill / note
1. uses model of sensor of drive of a current source, the action of current source is to make RTD resistor OK be measured as voltage.
2. joins RTD sensor output use at in all model, poor cent, full span and the random that precision emulates are tall input impedance signal recuperates circuit.
3. takes SPICE parameter step (.step Param) and dc analysis (.op) union is used, scan at the smallest temperature of sensor model from action the biggest temperature.
1. moving SPICE is emulated (use scanning parameter) , affirm RTD output voltage anticipates with what give temperature output is consistent. Ask an attention, – of Vrtd = (Vrtd+) (Vrtd- )
2. receives sensor model incentive electric current and signature manage circuit repeatedly with imitate complete application.
The design is emulated
Emulate use 1mA incentive electric current to undertake – the RTD temperature scanning of 200 ° C to 850 ° C. Express the 1 copy true value that showed RTD outputs voltage and computational value give typical examples (equation of use Callendar-Van Dusen) .
Express 1. Emulation result and ideal outcome
Graph 2. Indication RTD model and the principle of emulation parameter pursue
Graph 3. The relation of the emulation voltage of model of sensor of use PT100 SPICE RTD and 1mA incentive electric current and temperature pursues
The representative application circuit of sensor model is shown 4 times like the graph. Vc is undertaken dividing press and be generatinged by fiducial to 4.096V voltage, picks Vc cost should be in the dc of AD8538 operation amplifier in all inside modular limits, when its action at high accuracy (when resistor of 0.1%) 3.01k Ω , generation about the RTD incentive electric current of 1mA. The gain of road of tall be surrounded by mountains that AD8538 installs forces the incentive electricity that carries RTD model to be:
The input that resistor of two 499 Ω is AD8538 and output bring a foot to provide ESD protection, 1nF capacitance is used at EMI and RFI filter wave, 2.2nF capacitance is used at ensuring annulus road stability. RTD outputs voltage to be undertaken recuperating by AD8422 appearance amplifier, resistor of Ω of a 2.21k is placed between the RG terminal of this appearance amplifier, it is with installing its gain 9.959. Choosing this gain to be worth is to maintain the output voltage of AD8422 the input of the ADC that using 4.096V fiducial tension likewise inside limits. The action of the resistor that AD8422 input carries and capacitance is right in applying actually the noise of infuse wire cable undertakes differring dividing mixing in all modular filter wave. The resistor that is used at gain and filter wave and capacitance value undertake choosing according to the data manual of AD8422. Graph the concern plan that the 5 emulation that showed applied circuit output voltage and temperature. Although this applies circuit to use model of 2 lines RTD, but it can be adjusted easily for 3 lines or model of 4 lines RTD, if pursue,6 are shown. V1rtd and V4rtd are 0V voltage source, principle graph includes his, such node label won’t conflict (SPICE emulation tool does not support name of two different node to indicate identical node) . 0V voltage source was not affected to emulating a result (expression is short circuit) , and conduce to make RTD model better imitate RTD sensor is using medium physical wiring means actually. Same, these models can undertake adjustment in the light of PT200, PT500, PT1000 and PT2000 RTD, the corresponding value that the R0 in needing to pursue the principle only is worth a setting to need RTD for place (the resistor when 0 ° C) can. Watch 2 show, inside whole temperature limits, RTD voltage is located in inside the input range that AD8422 linear runs place to need, inside the input limits of the ADC that and use the total output tension of circuit is located in use 4.096V fiducial voltage. Ask an attention, LT1461 can be used at offerring this fiducial voltage, but stem from simplify the reason that the principle pursues, did not include its in the graph.
Express 2. Emulation result and ideal outcome
Graph 4. Indication drive and the RTD of PT100 2 line that signal recuperates circuit use circuit
Graph 5. The emulation of circuit of application of 2 lines RTD outputs the concern plan of voltage and temperature
Graph 6. Adjust model of 2 lines RTD to apply in order to get used to 3 lines and 4 lines RTD
Design parts of an apparatus
Express 3. Series connection is fiducial voltage source
Express 4. Appearance amplifier
Express 5. Operation amplifier (output bumper according to needing to be used at fiducial voltage source and DAC)
“Sensor signal recuperates practical design technology “
Edit by Walt Kester, ADI company, 1999, ISBN-0-916550-20-6.
Tool of plan of appearance amplifier diamond
Diamond plan tool is process of a network application, the output voltage range that can make specific configuration and input in all plan of modular voltage concern, also be called diamond plan, apply to ADI appearance amplifier
LTSpice? It is a high-powered SPICE III graph of emulation software, principle collects tool and weaveform to examine implement, compositive enhance function and model, simplified the emulation of switch manostat, linear manostat and signal catenary circuit.
Express one’s thanks to
ADI firm is main and advisory:
Tim Green, accurate technology and group of product of platform ministry linear are senior use an engineer
About ADI company
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