Temperature detection circuit design summary (four analog circuit design schematics)

SMD aluminum electrolytic capacitor

Parallel thermistor / NTC thermistor / product complete / Sunlord first-class agent

Memory chip

Temperature detection circuit design (1)

The temperature detection circuit usually consists of temperature detection, digital-to-analog conversion, and result processing. The temperature detecting circuit converts the ambient temperature into a corresponding electrical signal, and the analog-to-digital conversion circuit converts the electrical signal into a digital quantity, and then sends the processor to perform necessary processing to obtain corresponding ambient temperature parameters, as shown in FIG. 1 .

Among them, the temperature detecting part generally uses a thermistor, but the thermistor has lower precision and a slower response speed. At present, some special temperature detection chips have appeared, the precision is greatly improved, and the sensitivity to temperature changes has reached a very high standard, such as NaTIonal's IM35 series.

The temperature detecting circuit generally converts the change of temperature into a change of the voltage signal, so it needs to be converted into a digital signal by the analog-to-digital conversion circuit to be accepted by the processor, thereby obtaining real temperature information by calculation for further processing by the processor.

Taking the IM35 series temperature sensor of NaTIonal Company and the AD7812 analog-to-digital converter of AD Company as an example, the design idea based on multi-channel and high-precision temperature detection scheme supporting serial bus is discussed. The circuit block diagram is shown in Figure 2.

The voltage is proportional to the current ambient temperature, and its detection accuracy can reach ±1/4 at room temperature. C, in the range of 55 ~ +150 ° C, its accuracy can reach ± ​​34 ~ C, its typical trend is that the temperature changes every l ° C, the voltage changes 10mV, its temperature / voltage conversion formula is:

V-10mV/°C·C(1)

Where: to convert the output voltage, C is the actual temperature of the system.

The AD7812 is a serial AD conversion chip that supports up to 8 channels of input (4 channels for the AD78ll), so we can easily design circuits that support multiple temperature sensing.

The working mode of the AD7812 is determined by an internal control register. It can be converted by the pulse input of the Convst pin, or it can be converted by software control. In the actual design, because the software control is more flexible, the software control method is generally used for conversion. Its control register is defined as follows:

software programming

The software control mainly controls the AD conversion chip AD78l2. Need to complete the functions of analog-to-digital conversion, serial data reading, etc. The AD7812 has two operating states. Mode 1 does not turn off the power after the conversion is complete; Mode 2 turns off the power after the conversion is complete. In general, the mode l working mode is selected. The following mainly discusses the mode 1 working mode.

Figure 3 is a typical control timing diagram, first set PD0, PD1 bit to l, turn on the on-chip power supply, so that the chip starts working; after the next startup conversion is completed, the data on the data bus is valid, the conversion data can be stringed The line is output. As can be seen from the figure, the data of the first conversion is invalid, because the on-chip power supply has not been turned on, which is where the programmer needs to pay attention.

Figure 4 is the actual communication timing diagram. The RFS is a receive data sync signal, and the TFS is a transmit data sync signal. Normally, the output of Dout is in a high-impedance state. At the first SCIK rising edge after the rising edge of RFS, the Dour output data is valid. After the 11th SCIK rising edge, Dour returns to the high-impedance state; after the falling edge of TFS The first SCIK falling edge. The data on the Din line is serially shifted into the chip. At the l3th SCIKF falling edge, the on-chip control register contents are updated. It should be noted here that the number of SCIK clocks must be guaranteed, otherwise it is easy to cause problems.