Application of MAX5481 in Franck-Hertz Experimental Apparatus

1. Frank-Hertz Experiment

The Franck-Hertz experiment is an important experiment in modern physics. It verifies the existence of atomic energy levels by directly measuring the energy transferred in electron collisions. The Franck-Hertz experiment instrument is to generate a variable voltage UG2K and measure the current IP and the size of UG2K. However, in early instruments, mechanical potentiometers were used to adjust UG2K. Here, a digital potentiometer is used to adjust UG2K.

2 Overall design of the instrument

Figure 1 shows the basic principle of the instrument and the IP-UG2K curve. In the design, the single-chip microcomputer STC89C54 is used. It has 16K Flash ROM, 1280B RAM, 16K E2PROM, standard MCS-51 single-chip microcomputer, and supports ISP download. The A/D converter uses TLC2543 with 12-bit 11 input channels and serial port interface. The 10-9~10-7A current In after three-stage amplification and the UG2K after resistor voltage division can be measured. The measurement results are displayed by 4-bit 7-segment LEDs and saved in the E2PROM of the single-chip microcomputer. Under the control of the P1.0~P1.3 pins of the single-chip microcomputer, the digital potentiometer MAX5481 generates a UG2K with a voltage of about 0~100 V and a resolution of about 0.1 V through the circuit. The RS232 serial port is used for ISP download. The overall block diagram of the instrument is shown in Figure 2.

3 Digital potentiometer MAX5481

MAX548l is a 10-bit (i.e. 1 024 taps) non-volatile, linearly variable, programmable voltage divider and variable resistor. Its two fixed-end resistors are 10 kΩ, which realizes the function of a mechanical potentiometer and can be configured as a 3-wire serial SPI compatible interface or an up/down digital interface. It has a non-volatile, electrically erasable programmable read-only memory (E2PROM) inside to store the initial position of the sliding end when power is on. It can be powered by a single power supply of +2.7 to +5.25 V or a dual power supply of ±2.5 V.

3.1 Internal structure

Figure 3 shows the internal functional block diagram of MAX5481. MAX5481 mainly includes power supply, power-on reset, non-volatile memory, latch, decoding circuit, SPI interface circuit, Up/Down interface circuit, interface selection circuit, variable resistor, etc.

3.2 Pin Function

MAX548 has two packages: 16-pin TQFN and 14-pin TSSOP. Its pin functions: H is the high-level end of the variable resistor; W is the sliding end of the variable resistor; L is the low-level end of the variable resistor; VDD is the positive pole of the power supply; GND is the power ground. VSS is the negative pole of the power supply. When the unipolar power supply is used, it is short-circuited with GND; CS is the chip select signal, which is valid at a low level; SPI/UD is used to select the interface mode. When the high level is selected, the SPI interface mode is selected, and when the low level is selected, the Up/Down mode is selected; SCLK (INC) is used to switch between the two modes. In the SPI mode, it is the clock signal input terminal; in the Up/Down mode, each falling edge increases or decreases the W terminal by J LSB; DIN (U/D) is used to switch between the two modes. In the SPI mode, DIN is the data signal input terminal; in the Up/Down mode, it determines the change direction of the W terminal.

3.3 MCU Control of MAX5481

After power-on, MAX5481 resets and reads the data in the non-volatile memory first, and moves the W terminal to a predetermined position through the latch and decoding circuit. The interface selection circuit selects the interface mode according to the SPI/UD pin level, rewrites the latch data through the SPI interface or the Up/Down interface, and changes the position of the W terminal after decoding, thereby changing the voltage divider ratio and the upper and lower resistance values.

3.3.1 SPI interface mode

When SPI/UD=1, MAX5481 enters SPl interface mode; when CS=0, at the rising edge of the clock pin SCLK(INC), the data of the data input pin DIN(U/D) is written into MAX5481. When writing data into the latch, 24 clocks are required to write the command and data into MAX5481; when copying data between the latch and the NV memory, 8 clocks can be used to write the command, or 24 clocks can be used to write the command and data into MAX5481, in which the latter 16 bits of data will be ignored. Table 1 shows its data format. The position of the W terminal is determined by the data in the 10-bit latch, and its voltage divider ratio can be calculated as follows:

Where: D(D9~DO) is the data in the latch. For example, when D(D9~DO)=000000 0000, the W terminal is at the L terminal.

3.3.2 Up/Down interface mode

When SPI/UD=0, MAX5481 enters Up/Down interface mode, which is simple to operate. When CS=0 device is selected, if pin DIN(U/D)=1, each falling edge of pin SCLK(INC) increases the W end by 1 LSB (moves to the H end); if pin DIN(U/D)=0, each falling edge of pin SCLK(INC) decreases the W end by 1 LSB (moves to the L end). In the state of SCLK(INC)=1, when the pin CS level is rising, MAX5481 will copy the latch data to the NV memory for storage.

3.3.3 UG2K voltage generation circuit

In terms of hardware, the microcontroller is connected to the pins CS, SCLK (INC), DIN (U/D), and SPI/UD through the P1.0~P1.3 ports. In terms of software, the SPI method is used to operate the MAX548l. Since the MAX548l is a 10-bit (1 024 taps) digital potentiometer, when there is no key, there are also decimal and hundred-bit acceleration keys to facilitate adjustment.

Since the load capacity of MAX5481 is limited, it is generally necessary to use an amplifier circuit to expand its load capacity. In the design, VT1 and VT3 are connected as a common-collector amplifier circuit, and VT2 is connected as a common-base amplifier circuit, so that the load capacity can be improved, the input voltage is about 0~100V, and the resolution is about 0.1 V change UG2K. The voltage is divided by R7 and R8 resistors for measurement.

4 Conclusion

Digital potentiometers can provide convenient digital control and regulation of resistance, voltage and current for analog circuits in various applications. In particular, many analog circuits have been developed for decades and the technology is mature, so only minor improvements are needed, and even digital potentiometers can be directly used to replace mechanical potentiometers to make the operation digital, which is convenient for control, improves system performance and simplifies design.