To control the temperature at several points using platinum or copper resistance thermometers, a simple and inexpensive eight-channel resistance-to-frequency converter will help, the advantages of which include noise immunity, unipolar power supply and low power consumption.
The sensing element of a resistance thermometer, also called resistance temperature detector (RTD), is a resistor made of a metal wire or film, which has a known dependence of electrical resistance on temperature. This dependence is established by the standard for the entire temperature measurement range for each type of resistance thermometer. The task of temperature measurement is reduced to measuring the electrical resistance of the sensitive element with subsequent conversion to the corresponding temperature value.
The schematic diagram of the eight-channel resistance-to-frequency converter is shown in Fig.1, and the connection of resistance thermometers to it is shown in Fig.2. The principle of operation of the converter is that the frequency of a rectangular digital signal at the FOUT output is proportional to the voltage on the sensing element through which a direct electric current passes. This current is known and constant. In this case, according to Ohm’s law, the voltage on the sensing element and its electrical resistance are interconnected by a well-defined linear relationship. Thus, the change in the frequency of the output signal with a change in the electrical resistance of the sensing element is linear.
The output signal, the frequency of which is a function of the voltage on the sensing element, is formed by the voltage-to-frequency converter D6. To work in this capacity, an inexpensive AD7742 chip manufactured by Analog Devices was chosen. Detailed information about this microcircuit can be found on the manufacturer’s website.
The connection of the control inputs of the D6 microcircuit shown in Fig.1 sets such a mode of its operation, when the conversion coefficient corresponds to the maximum sensitivity of the output signal frequency to a change in the input voltage, and only a pair of outputs Vin1 and Vin2 is used as a differential input, and the input signal is unipolar, then there is a voltage measured relative to the common wire at the input Vin2 should not exceed the voltage at the input Vin1. The source of clock pulses, as shown in the diagram, is the built-in clock generator with a quartz resonator ZQ1 connected to it and capacitors C10 and C11. The reference voltage necessary for the operation of the converter is supplied to the REFIN input intended for its supply also from the built-in source. The same voltage from the RFOUT output also serves as a reference for a stable current source based on the operational amplifier D5.
The eight-channel resistance-to-frequency converter works as follows. The analog multiplexers D2 and D3 connect to the differential input of the voltage-to-frequency converter D6 the resistance thermometer that is connected to a stable current source by the analog multiplexer D4. Which resistance thermometer is connected to the converter determines the state of the control digital signals A0..A2. The voltage from one or another resistance thermometer is supplied to the input of the D6 chip via two wires so that the own resistance of these wires and the resistance of the electronic keys of the multiplexers practically does not affect the operation of the circuit. This is due to the fact that the input currents of the voltage-to-frequency converter D6 and the leakage currents of analog multiplexers are negligible. In addition, pairwise controlled electronic analog switches of the dual four-channel multiplexers D2 or D3 almost do not differ from each other in their resistance, the influence of which can therefore only be co-phase and is effectively and finally suppressed by the differential circuit of the converter D6.
A stable current source is made according to the scheme with a “floating” load and includes of elements D5, VT1..VT3 and R1..R4. An external circuit with one or another resistance thermometer is connected to it between the collector of the transistor VT3 and the resistor R1 connected to the inverting input of the operational amplifier D5. A reference voltage is applied to the non-inverting input of this operational amplifier. The function of the current stabilizer is to maintain a constant and equal to the reference voltage on the reference resistor R1, through which the same current flows as through the resistance thermometer currently connected by the analog multiplexer D4. The current flowing through the resistance thermometer is defined as follows:
It is recommended to choose this current so that self-heating of the resistance thermometer does not lead to temperature measurement errors. With the resistance of the resistor R1 marked in the schematic diagram, a current of 0.926 mA will flow through the resistance thermometer. The stability of the current source is determined by the stability of the reference voltage, the resistance of the resistor R1 and the bias voltage of the operational amplifier D5.
The current mirror on the elements VT1..VT3, R3 and R4, drived by the operational amplifier D5, serves to decouple the output of the operational amplifier and external circuits, the parasitic self-capacitance of which can lead to self-oscillation of the operational amplifier D5.
The filter on elements L1, C8 and C9 serves to additionally suppress possible high-frequency common mode noise at the input of the converter. The coil L1 can be made with a double-twisted winding wire, wound with 4..8 turns either on a small ferrite ring or on a ferrite “binocular”. The wire diameter is chosen based on ferrite core dimentions.
PCB design must be done in accordance to common requirements for the A/D converters.
Copyright © Sergii Zadorozhnyi, 2009
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