Temperature sensing with NTC thermistor circuit
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Hello. Welcome to our short video on how to design a circuit that senses temperature using a Negative Temperature Coefficient, or NTC, thermistor. This schematic represents a temperature sensing circuit for an NTC thermistor. An NTC thermistor has a negative temperature coefficient, which means its resistance decreases with temperature.
A resistor in series with an NTC thermistor forms a resistor divider. A change in temperature changes the NTC resistance, which changes the input voltage Vin. Vin is amplified by non-inverting amplifier. A reference voltage, Vref, offsets the gain which helps utilize the full output swing of the op amp.
The equation for VO is the transfer function of the circuit. The first term, VDD times R1 divided by R1 plus RNTC, is the input voltage Vin. The second term, R3 plus R2 divided by R2 represents a non-inverting gain of the amplifier circuit.
Finally, the third term, Vref times negative R3 divided by R2, represents the reference voltage times the inverting gain of the amplifier. The first goal for this circuit is to output a voltage of 3.25 volts at a maximum temperature of 50 degrees Celsius and to output a voltage of 50 millivolts at the minimum temperature of 25 degrees Celsius. This output voltage range from 50 millivolts to 3.25 volts will maximize the linear output voltage swing of the amplifier using a 3.3 volt single supply.
The first design step is to calculate the resistance of R1. This is done by taking the square root of the sum of the NTC resistance at 25 degrees Celsius and 50 degrees Celsius. This equates to a resistance of 1.359 kiloohms. The next closest standard resistor value is 1.37 kiloohms, so we will use this value for the design, as shown by R1 in the schematic.
Now that we know the input voltage divider resistances, the next step is to calculate the maximum and minimum input voltage range. The minimum input voltage is calculated using the NTC resistance at 25 degrees Celsius. The maximum input voltage is calculated using the NTC resistance at 50 degrees Celsius.
The minimum input voltage is calculated to be 1.248 volts and the maximum input voltage is calculated to be 2.065 volts. The next step is to calculate the ideal non-inverting gain required to produce an output voltage from 50 millivolts to 3.25 volts, given the input voltage range calculated on the previous slide. The non-inverting gain is calculated by dividing the output voltage swing by the input voltage swing. The value is calculated as 3.917 volts per volt.
The non-inverting gain is set by resistors R2 and R3, and is calculated with the equation gain equals R2 plus R3 divided by R2. Choosing a standard resistor value of 1 kiloohms for R2 allows us to calculate a resistor value of 2.917 kiloohms for R3. Since 2.917 kiloohms is not a standard resistor value, we will use the next closest value of 2.87 kiloohms, as shown in the schematic.
However, since there are not standard resistors available to give us our ideal gain, we will need to calculate the actual gain we will get using the standard resistor values. The actual gain of our circuit is calculated to be 3.87 volts per volt. Next, we need to calculate the output voltage swing based on the actual gain of the circuit. The output voltage swing is calculated as the maximum input voltage minus the minimum input voltage multiplied by the actual gain.
The output voltage swing is calculated to be 3.162 volts. Now that we know the output voltage swing, we need to calculate the maximum output voltage when the output voltage is symmetrical around mid-supply, so that we can calculate the reference voltage, or Vref, for the circuit. The maximum output voltage is calculated to be 3.231 volts. This calculation can also be done using the minimum output voltage.
Using the transfer function of the circuit, we can now calculate the reference voltage required for the design. For the calculation, we will use the maximum output voltage, maximum input voltage, and actual gain. Solving the transfer function for Vref, we calculate the reference voltage to be 1.659 volts.
This simulation shows a sweep of the temperature from 25 degrees Celsius to 50 degrees Celsius. The output voltage ranges from 71 millivolts to 3.231 volts. This verifies the functionality of the circuit. Running an AC sweep analysis, we find that the bandwidth of the circuit is 235.3 kilohertz.
When designing a temperature sensing circuit with an NTC thermistor, there are a few design notes to be aware of. First, the resistor R1 is chosen based on the temperature range and the NTC's value. Next, be sure to always check the linear output swing of the amplifier, which is usually given in the conditions section of the AOL data sheet specification. Finally, the reference voltage Vref can be created using a DAC or voltage divider. However, if a voltage divider is used, the equivalent resistance of the voltage divider will influence the gain of the circuit, so it should be buffered with an op amp.
Texas Instruments has many online resources to help you design circuits with op amps. This includes reference designs and guides, educational videos, simulation and prototyping tools, support resources, and search tools. Thank you for taking the time to watch this short presentation and how to design a temperature sensing with an NTC thermistor circuit. Please visit www.ti.com for additional information and resources.