Controllable Gain Amplifier Based on X9241 Digital Potentiometer

X9241 is a digital potentiometer that integrates four E2POT digital potentiometers on a single CMOS integrated circuit. Its functional block diagram is shown in Figure 1. It contains four resistor arrays, each of which contains 63 resistor units. There are tap points between each unit and at both ends that can be accessed by the sliding unit. The position of the sliding unit in the array is controlled by the user through the I2C bus. Each resistor array is associated with a sliding end count register (WCR) and four 8-bit data registers. The four data registers can be read and written by the user. The content of the sliding end count register controls the position of the sliding end in the resistor array. The content of the data register can be transferred to the sliding end count register to set the sliding end position, and the current sliding end position can also be transferred to any data register associated with it. The sliding end count register is volatile. When the device is powered on, the sliding end count register is automatically loaded with the value of the data register R0. The four data registers are non-volatile. If the potentiometer does not need to be set in multiple ways in the application, they can be used as general storage units to save system parameters or user data. The X9241 model has suffixes of Y, W, U and M, indicating the different compositions of the internal potentiometers. The resistance values ​​of the four potentiometers in the X9241M are 2 kΩ, 10 kΩ, 10 kΩ and 50 kΩ respectively. The X9241 digital potentiometer can be used to easily realize the design of controllable gain amplifiers.

2 Design of Controllable Gain Amplifier

Solution 1: Use a digital signal processing system consisting of A/D/A+DSP to achieve this. This solution has a complex system structure and high cost.

Solution 2: Use a programmable amplifier. Since a dedicated chip is used, gain control is limited to the capabilities provided by the chip, resulting in poor flexibility and high cost.

Solution 3: As we all know, the gain of the amplifier is related to the resistance. Changing the resistance of the corresponding resistor can change the gain of the amplifier. Since the solution of changing the resistance to control the gain of the amplifier has a clear concept, simple circuit composition, easy implementation, and low cost, it can better meet the actual requirements. This solution is usually used. The specific implementation methods are as follows:

(1) The resistance is changed by switching fixed resistors of different resistance values ​​through a small relay. Each fixed resistor is determined after theoretical calculation and debugging.

(2) Use a resistive optocoupler (such as AD521L/H, etc.) to change the resistance value by controlling the current of its light-emitting tube.

(3) Use a digital potentiometer (such as X9312, X9241, etc.) to change the resistance value by writing the value into the potentiometer's related register through software control.

The method of using small relays is only applicable to the case where the resistance to be controlled is small. When the total number of gain types required to be controlled is large, a relay array is formed, which makes the circuit structure bulky and its control reliability is greatly reduced. In addition, the relay switching process will inevitably interfere with other circuits.

The resistive optocoupler has a good isolation effect, but it must be combined with a D/A conversion circuit and an amplifier circuit to work. Its circuit structure is complex and the cost is high. In addition, the current of the light-emitting diode of the resistive optocoupler and its resistance value are nonlinear, which makes the control more difficult.

The third method uses a digital potentiometer, which has the advantages of simple circuit structure and easy use. As long as the selected digital potentiometer has a high enough resolution, it can meet the requirements for gain control. The X9241M digital potentiometer can be used. It has 4 potentiometers with three resistance values. The resolution of each potentiometer is 1/63. The chip control uses the I2C bus. Potentiometers with different resistance values ​​can be selected for different voltage gains. The resistance value of the sliding end relative to the fixed end is changed by changing the value of the counting register of the sliding end of the potentiometer, thereby achieving gain adjustment.

3 Implementation of Controllable Gain Amplifier Based on X9241

Taking the display circuit part of the automatic range conversion of the current sensor as an example, the controllable gain amplifier implemented by X9241 is introduced. Assume that the sensor range is: 100 A, 300 A and 500 A. No matter what range the sensor is in, its full-scale output current is 100 mA. The resistance used for I/V conversion is 5 Ω, and its output VM voltage at full scale is 0.5 V. In order to correctly display the magnitude of the measured current with a digital voltmeter (three and a half digits, 2 V range), only the three digits of the digital voltmeter are used, and it is necessary to add the amplifier circuit shown in Figure 2. The corresponding magnifications are: 0.2, 0.6 and 1 times, respectively. The feedback resistors of the first-stage amplifier should be: 0.2 kΩ, 0.6 kΩ and 10 kΩ, respectively.