Application of 12-bit serial A/D converter MAX187

　　MAX187 is available in 8-pin DIP package and 16-pin SO package. Figure 1 shows the pin arrangement of the DIP package. Please refer to the literature [1] for the SO package. Table 1 is the pin function description.

　　2. Make timing

　　MAX187 uses a sampling/holding circuit and a bit-by-bit comparison register to convert the input analog signal into a 12-bit digital signal. Its sampling/holding circuit does not require external capacitors. MAX187 has 2 operating modes: normal mode and sleep mode. It will be set to low level to enter sleep mode, and the current consumption will drop below 10μA. Set high or left floating to enter normal operating mode.

　　The complete operating sequence is shown in Figure 2. When using the internal reference, after the power is turned on, the 4.7μF capacitor of the reference pin is charged after 20 ms, and normal conversion operations can be performed. The working process of A/D conversion is: when it is low level, the T/H circuit of MAX187 enters the holding state on the falling edge and starts conversion. After 8.5μs, DOUT outputs high level as a conversion completion mark. At this time, a series of pulses can be input at the SCLK terminal to move the result out of the DOUT terminal and read into the microcontroller for processing. Will be set high after data reading is completed. It should be noted that after setting to low level to start A/D conversion, SCLK shift pulses can be sent to read data only after DOUT is detected to be valid (or delayed by more than 8.5μs), and the number of SCLKs is at least 13. After sending the pulse, it should be set to high level.

　　3 Application examples

　　Figure 3 is an application example of MAX187. Use the P1 port of the microcontroller AT89C51 to control the conversion of MAX187. P1.1 is connected to clock SCLK, P1.2 is connected to chip select, and P1.3 is connected to data DOUT.

　　(1) Hardware wiring diagram

　　The MAX187 power supply requires a decoupling capacitor. A common method is to use a 4.7μF capacitor and a 0.1μF capacitor in parallel. In order to ensure the sampling accuracy, it is best to power the MAX187 and the microcontroller separately. Pin 4 is connected to a 4.7μF capacitor for the reference terminal, which uses the internal 4.096 V reference voltage. The voltage range of the input analog signal is 0~4.096 V. If the analog input voltage is not within this range, an external circuit must be added to convert the voltage range. MAX187 has only one analog input channel. If the input is multiple signals, multiple analog switches must be added.

　　(2) Programming

　　The program is compiled in C51 and passed debugging in the KEILC51 V6.20 environment. The basic idea of ​​the program is: define P1.1 pin as clock SCLK, P1.2 as data DOUT, and P1.3 as chip select. After the chip select is valid, the delay is more than 8.5μs to ensure that the conversion is completed. Under the action of the clock SCLK, the converted data is read from the data output terminal and stored in two unsigned character variables. These two character variables are combined into a 16 Bit unsigned integer variable function return value returns, the lower 12 bits of the return value are valid. 　　　　

　　4 Conclusion

　　MAX187 is an A/D converter with a serial interface, with only 8 pins and very few peripheral connections. Small size, fast speed and high precision. Suitable for instrumentation, sensors, engineering testing, etc.

　　references

[1] Ma Zhongmei. C language application program design for single-chip microcomputer［M］． Beijing: University of Aeronautics and Astronautics Press, 2001

[2] MAX187 2002 product set.