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This article details how to combine digital potentiometers with other components, highlighting design considerations and specifications crucial for all use cases (ensuring designers achieve optimal system performance). It also discusses important design considerations and specifications to consider when combining digital potentiometers with other components (such as operational amplifiers) to create flexible, multi-purpose systems. Furthermore, it explores the design advantages and disadvantages of digital potentiometers compared to traditional potentiometers. Numerous examples demonstrate that the improvements offered by digital potentiometers are more significant than those of more traditional alternatives. For instance, in operational amplifiers, using a digital potentiometer as a feedback resistor allows the amplifier's gain to alternate according to the amplitude of the input signal.
Digital potentiometers are digitally controlled variable resistors that can replace functionally equivalent mechanical resistors. While functionally similar to mechanical potentiometers, digital potentiometers excel in specifications, reliability, and repeatability, making them suitable for many designs. A potentiometer adjusts voltage or current by changing the resistance of a device. This adjustment can then be used to set different levels or gains when used with other components, such as operational amplifiers. Designers can use variable components like digital potentiometers to design flexible, multi-functional systems. For example, in an operational amplifier, using a digital potentiometer as a feedback resistor allows the amplifier's gain to alternate according to the amplitude of the input signal. This allows designers to reduce the number of components (e.g., multiple operational amplifiers), maximize the types of input signals the system can support, and reduce PCB size. Digital potentiometers are characterized by their small size and versatility.
Digital potentiometers and mechanical potentiometers
Digital and mechanical potentiometers share some similarities and are interchangeable in many applications. Both are adjustable, offering a variety of end-to-end resistance options to meet user needs for adjustable resistance. Some advantages of mechanical potentiometers over digital potentiometers include: the ability to withstand higher voltages, greater current carrying capacity, and higher power consumption. However, due to design limitations, the performance of mechanical potentiometers may change over time, leading to reliability issues. They are more sensitive to shocks and vibrations, and the resistance of the mechanical vernier contacts may change due to oxidation, aging, and wear. This can shorten the usable lifespan of a mechanical potentiometer.
Digital potentiometers consist of multiple CMOS transmission gates (see Figure 1). Because there are no mechanical components, digital potentiometers are highly resistant to shock, wear, aging, and contact damage.
Figure 1. Digital potentiometer - internal structure
Factors to consider when using a digital potentiometer
As with all components, there are certain factors that must be considered when selecting the right component for a specific application. The order of importance of various specifications depends on the end use and other system considerations.
Table 1. Important considerations when selecting a digital potentiometer
The best way to understand these considerations is to see how they influence the choice of digital potentiometers in a specific application. Therefore, we will now look at two important use cases for digital potentiometers in more detail.
Common applications of digital potentiometers are as follows:
• DC and AC signal attenuators
• Change the gain of the operational amplifier
How to use a digital potentiometer as an attenuator
A digital potentiometer can be used to simulate a simple low-resolution digital-to-analog converter (DAC). Figure 2 shows this setup and some common terminology. The end-to-end resistance is defined as RAB, which is the resistance between terminals A and B. RAW and RWB refer to the resistance between the vernier and the terminals. Figure 2 also lists the transfer function.
Figure 2. Digital potentiometer as a low-resolution DAC
In this setup, there are three key parameters to consider when selecting a digital potentiometer: power supply voltage range, digital potentiometer resolution, and linearity.
Power supply voltage 1 and resolution 2 are crucial considerations, as these specifications affect the input range the digital potentiometer can handle and the number of different resistance levels it can achieve. The linearity of a digital potentiometer is expressed similarly to that of a DAC, using INL (Integral Nonlinearity) and DNL (Digital Nonlinearity) as measures. INL refers to the maximum deviation between the actual digital potentiometer and the ideal straight line drawn from zero level to full scale. DNL refers to the difference between the output of a continuous code and the ideal transfer function.
For AC applications, the same parameters as for DC power supplies apply (power supply voltage range, resolution, and linearity). Total harmonic distortion (THD) and bandwidth are also important factors to consider.
How to use a digital potentiometer when creating a variable gain operational amplifier
Digital potentiometers are extremely useful for changing the gain of operational amplifiers. Using a digital potentiometer, the Rb/Ra gain ratio can be precisely set and changed. Applications utilizing gain control include volume control, sensor calibration, and contrast/brightness in LCD displays. However, several characteristics of digital potentiometers must be considered during configuration.
If a digital potentiometer is used in potentiometer mode, the transfer function of the digital potentiometer must be known as the resistance increases from zero to full scale. As the resistance between RAW lines increases, the resistance between RBW lines decreases, resulting in a logarithmic transfer function. The logarithmic transfer function is more suitable for human auditory and visual responses (Figure 3(a)). If the application requires a linear response, the digital potentiometer can be linearized by: using the digital potentiometer in variable resistor mode (Figure 3(b)); using a vernier DAC configuration (Figure 3(c)); or through a linear gain setting mode, a feature unique to ADIdigiPOT+ series devices (such as the AD5144) (Figure 3(d)).
Figure 3. Potentiometer configuration
Using discrete resistors in combination in variable resistor mode
Using a digital potentiometer in series with a discrete resistor in variable resistor mode can linearize the output (Figure 3(b)). Although this design is simple, some design factors must be considered to maintain system accuracy.
For different reasons, both mechanical and digital potentiometers have a certain resistance tolerance. For mechanical potentiometers, the tolerance may vary depending on the difficulty of achieving repeatable values. For digital potentiometers, although the manufacturing process also introduces tolerances, their values are much more repeatable than those of mechanical potentiometers.
Offsets in discrete surface-mount resistors can be as low as 1%, while some digital potentiometers can have end-to-end resistance tolerances as high as 20%. This mismatch can lead to decreased resolution and potentially serious problems, especially in open-loop applications where monitoring to compensate for errors is not feasible. In applications where monitoring is possible, the inherent flexibility of digital potentiometers allows for easy calibration of the vernier position and adjustment for any offsets.
Analog Devices' digital potentiometer portfolio offers rated tolerances from 1% to 20% to meet the most demanding accuracy and precision requirements. Some digital potentiometers (such as the AD5258/AD5259) undergo factory tolerance testing, with the results stored in user-accessible memory for resistance matching during production.
Linear gain setting mode
The final approach utilizes the linear gain setting mode unique to the ADIdigiPOT+ portfolio. Figure 3(d) illustrates how the values of each RAW and RWB string can be independently programmed using the proprietary architecture. This mode allows for linear output by fixing the output of one string (RWB) and setting the output of the other string (RAW). This approach is similar to combining a digital potentiometer in rheostat mode with discrete resistors, but with an overall tolerance of less than 1% and without requiring any additional parallel or series resistors.
This is due to resistor error, which is common in both resistor string arrays and can be ignored. Figure 4 shows that the mismatch error between the two resistors is small at higher codes. When the code is less than ¼ of the range, the mismatch does exceed ±1%, but this is caused by the internal CMOS switching resistor effect increasing the error, which cannot be ignored.
Figure 4.10k resistor mismatch error
Why is memory so important in applications?
When using digital potentiometers to set circuit levels or calibrate sensor and gain settings, the power-on state of the digital potentiometer is crucial for ensuring accurate and rapid configuration. Digital potentiometers offer a variety of options to ensure that devices are powered on in the user-preferred state. There are two types of digital potentiometers:
• Non-volatile — The device integrates on-chip memory elements to store the cursor position selected by the user and configured upon power-up.
• Volatility—The device does not have a programmable memory; instead, the vernier position is powered at zero, intermediate, or full scale depending on the device configuration. Please refer to the datasheet for each product for details.
There are also some other options for non-volatile digital potentiometers:
•EEPROM
• One-time programmable (OTP)
• Multi-programmable Programmable (MTP)
A wide range of memory options allows for the customization of digital potentiometer selection for specific systems. For example, volatile digital potentiometers can be used for systems requiring constant adjustment. OTP potentiometers can be used for systems requiring only factory testing and calibration. EEPROM digital potentiometers can be used to hold the last cursor position, so that upon power-up, the digital potentiometer returns to its previous state and can continue to be adjusted as needed after power-up.
summary
As shown above, digital potentiometers can replace mechanical potentiometers to create easy-to-use adjustable signal chains, thereby improving specifications, reliability, and PCB area. These improvements and reduced system design considerations can be achieved by taking these factors into account during the design process.
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