Discussion on Several Application Problems of Digital Potentiometer

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Discussion on Several Application Problems of Digital Potentiometer

Author: Love Follows Source: Unknown Added Date: 2005-5-27 1202

Abstract: This paper introduces a solution to the anti-jitter and repetitive action problems of push-button digital potentiometers, as well as common solutions to the voltage, current and series expansion problems of digital potentiometers.

1 Introduction

Digital potentiometers have been recognized by a large number of electronic engineering technicians for their accurate and convenient adjustment, long service life, little impact from the physical environment, and stable performance. In the family of digital potentiometers, X9511/14 is particularly popular for its direct button control feature. This article aims to briefly discuss the anti-shake and repeated action problems of X9511/14 in the button control process and the common problems encountered by digital potentiometers for application developers.

2 Introduction to Digital Potentiometer

Digital potentiometer is a new type of device that can control the position of the potentiometer sliding end with digital signal. It is generally divided into two types: button control and serial signal control. The following briefly introduces its principle by taking the non-volatile button control digital potentiometer X9511 of XICOR Company of the United States as an example.
X9511 is a product in the digital potentiometer family with button control and linear output characteristics. It contains 31 resistor units and 32-level output sliding ends. The sliding end is controlled by the negative pulse input to the pin to slide to the VH or VL end. The sliding end position can be stored in the non-volatile memory EEPROM, so that it can automatically return to the original position after power-on. The pins of X9511 are shown in Table 1, and the basic application is shown in Figure 1 (the figure shows the connection method of X9511/14 to automatically store the sliding end position when power is off).

Figure 1 X9511 basic applications

　
3 Common problems encountered by digital potentiometers in applications

Digital potentiometers are a new type of device that has appeared in my country in recent years. Many people do not understand them well enough in practical applications, which leads to many questions. The following is a brief discussion of three frequently asked questions.

The button-controlled digital potentiometer often has the problem that the number of button presses and the output value do not match the predicted value. The digital potentiometer itself can only withstand a limited current and voltage, and needs to be expanded.

In practical applications, the resistance range and resolution of digital potentiometers are not enough and need to be expanded.

3.1 Anti-jitter and repetitive triggering issues of button-controlled digital potentiometers

The first question mentioned above said that the number of key presses and the output value of the button-controlled potentiometer are inconsistent with the prediction. Usually, there is a repeated trigger action in one of the gears, so naturally the number of key presses and the output potential will not be consistent with the predicted value. The reason for this phenomenon is often that a breadboard is used for testing, or a low-quality button is used, resulting in poor contact, increased line noise, or non-standard human button action.
The application circuit of the button-type digital potentiometer provided by XICOR Company in the United States is directly controlled by a button, which may cause these problems. X9511/14 integrates a 40ms delay de-jitter circuit in its interior,

requiring the input control signal to have a short jitter time, the signal valid time between 40ms and 250ms, and no interference level during this period. However, due to the unpredictable actual application situation, it is impossible to avoid the repeated output action caused by the jitter of the input signal (the button time exceeding 250ms will also cause repeated output action), which is what many people do not want to see.
In order to control the jitter and noise of the input signal, a trigger is added to the control end of the digital potentiometer, as shown in Figure 2. The test results have significantly improved the output stability, but the button action is still required to be crisp and the line is free of interference, which is ultimately reflected in the input signal being clean and without fluctuations, otherwise repeated triggering cannot be avoided.
After many improvements, the circuit in Figure 3 solves the above problems better. Between the button and the control input end, a monostable circuit composed of a NAND gate circuit is added as shown in Figure 3, which has the characteristics of low cost, simple circuit, anti-jitter, and no repeated output action.

Figure 2 Adding anti-shake trigger

When button K in Figure 3 is not in action, the control end must be a stable high level. Once the button is pressed, the potential at point A is discharged through capacitor C1 through resistor R1 to the input low level threshold of 74HC00, and point B is logically high. At the same time, the level at point E (point D was originally a high level) is controlled to flip to low through point F, starting the X9511 action. At this time, since the potential of capacitor C2 will not change immediately, point D maintains its original high level. Capacitor C2 discharges through R2 and reaches the low level threshold of the gate circuit after a transient time, so that point E returns to a high level. After that, regardless of whether the button is kept pressed (keeping point D low) or released (point F is low), point E will remain in a high level state. During the transient period, the low level at point E is locked. Even if the circuit produces strong level jitter at point A, it will not have any effect on the output. Because the circuit has a shielding effect on noise during the transient time, and the control end is low

Figure 3 Anti-shake monostable circuit

If the transient time exceeds 250ms, the output of X9511/14 will have continuous jumps. Therefore, the value of R2 can be adjusted to make the transient time within the range of X9511 non-repeated action time as long as possible (for example, the transient time can be between 150ms and 220ms) to shield the possible noise interference during this period. The values of R2 and C2 can be obtained according to the formula of transient time T.
T=(R2+R0)·C2·Ln[(Vol－Voh) / (Vol－Vth)]
In the formula, R0 is the output resistance of 74HC00;
Vol is the low-level output voltage of 74HC00;
Voh is the high-level output voltage of 74HC00;
Vth is the high-level flip threshold voltage of 74HC00.
This circuit has been repeatedly verified to have good results. The control line length before X9511 can reach 200 meters.
In fact, the digital potentiometer at this time can be other models controlled by the interface, not limited to the button-controlled X9511/14. (The resistance value of R2 can be adjusted to the input pulse width allowed by the device model)
Another reliable solution is to use a cheap microprocessor, such as GMS97C1051, to control the digital potentiometer. The button signal is sent to the MCU, and software is used to debounce. At the same time, the LED can be used to display the control action, and more complex multi-channel mixed control can be completed. The disadvantage is that it will prolong the development cycle.

3.2 Expansion of digital potentiometer terminal current and voltage

At present, the current that the terminals of all digital potentiometers can withstand is not very large, only 1 to 3 mA. The voltage that can be tolerated is also not high, between -5V and +5V, or between 0 and 15V. Figures 4 and 5 are two expansion solutions provided by XICOR, which are applicable to various types of digital potentiometers.

Figure 4 An example of output current expansion

Figure 5 An example of output voltage increase

3.3 Using cascaded digital potentiometers to expand resolution and resistance range

(1) The series cascade of digital potentiometers
is shown in Figure 6 (a). The potentiometers W1 and W2 are connected in series, the sliding end of W1 is short-circuited with one end, and the sliding end of W2 is used as the output. The sliding end of W1 divides it into two parts, set as R1 and R2, and the sliding end of W2 divides W2 into two parts, R3 and R4. Assume that the input voltage signal is Ui and the output is Uo, then: When used as a variable resistor, as shown in Figure 6 (b), the resistance value is: R0=R1+R3.
If the original number of taps of W1 and W2 are P1 and P2 respectively, then the number of taps after series connection is P1+P2-1. At this time, the number of control buttons also increases accordingly, and the resistance range also increases accordingly.

(2) Parallel connection of digital potentiometers
Parallel connection can improve the resolution to a greater extent. If two digital potentiometers are connected in parallel as shown in Figure 7 (a), the output is:

When used as a variable resistor, as shown in Figure 7 (b), the resistance is: R0 = (R2·R3)/(R2 + R3) In practical applications, W1 can be used as a coarse adjustment and W2 as a

Figure 6 Series cascade

Figure 7 Parallel cascade

For fine-tuning. Assume that the number of taps of W1 is P1, and the number of taps of W2 is P2. As shown in Figure 7, after cascading, the number of adjustment levels is (P1-1)·P2. When three X9511 potentiometers are connected in series and parallel as shown in Figure 8, there will be 31744 different outputs. For other digital potentiometers, there are (P1-1)·P2·P3 different outputs, where P1, P2, and P3 are the number of taps of W1, W2, and W3 respectively. The output of Figure 8 (a) is: The variable resistor is connected as shown in Figure 8 (b), and its resistance value is:
R0 = R1 + (R2 + R5) // R3 + R6 This situation is more suitable for cooperating with a microprocessor to perform calculations and control the output

Figure 8 Series-parallel cascade

Output. Note that the current and voltage of the potentiometer must be controlled within the allowable range during the parallel cascade process, and it should be noted that the output is no longer linear.
Supplementary explanation to the above formula: In microprocessor interface control applications, digital potentiometers are not limited to X9511/14. In the control operation process, if the number of W1 taps is P1, the adjustment step can be set to N1, (N1∈[0, P1-1]), for example, the P1 of X9511 is 32, N1∈[0, 31], then:
R1=W1·[N1/(P1-1)]
R2=W1·[(P1-N1-1)/(P1-1)]
Calculate the adjustment variable N1 to control the output variable and resistance value, which will not be described in detail here.
After understanding the use characteristics of digital potentiometers, you will find that digital potentiometers have many novel applications in some circuits, and the basis for flexible use is the understanding of the basic use skills of digital potentiometers.