Differentiating the performance of digital potentiometers

Digital potentiometers (digipots) can provide convenient digital control and adjustment of resistance , voltage, and current for analog circuits in a variety of applications . This application note introduces the basic functions of digital potentiometers and explains how to modify digital potentiometers to improve system performance, simplify design, and meet the needs of special applications.

Overview

Digital potentiometers, or digipots, facilitate digital control and adjustment of resistance, voltage, and current in analog circuits. Digital potentiometers are commonly used for power supply calibration, volume control, brightness control, gain adjustment, and bias/modulation current adjustment of optical modules. In addition to the basic functions, digital potentiometers provide many other features to enhance system performance and simplify design. These features include: different types of non-volatile memory, zero-crossing detection, debounce button interface , temperature compensation, and write protection. These features are designed for different applications.

Basic Digital Potentiometer Design

The potentiometer is actually a three-terminal component (see Figure 1a). The low-end VL is internally connected to the device ground or as a pin output for easy design. The structure of the three-terminal digital potentiometer is essentially an adjustable voltage divider resistor with a fixed end-to-end resistance.

A variable resistor is a two-terminal potentiometer with a variable resistance at the wiper and one end of a resistor string (see Figure 1b). By adjusting the wiper position of a variable resistor digital potentiometer, the end-to-end resistance of the digital potentiometer can be changed.

Figure 1. (a) The structure of a three-terminal digital potentiometer is essentially an adjustable voltage divider with a fixed end-to-end resistance. (b) The variable resistor is a two-terminal digital potentiometer with the wiper internally connected to one end of the potentiometer.

Simply put, a digital potentiometer is an analog output controlled by a digital input, similar to the definition of a digital-to-analog converter (DAC). Unlike a DAC, which provides a buffered output, most digital potentiometers cannot drive low-impedance loads without an external buffer .

For digital potentiometers, the maximum wiper current ranges from a few hundred microamps to milliamps. When the wiper of a digital potentiometer is connected to a low-resistance load, whether it is a variable resistor or a true digital potentiometer, it is important to ensure that the wiper current is within the acceptable IWIPER range under the worst-case operating conditions. The worst-case load for a variable resistor occurs when VW is close to VH. At this point, there may be no resistance in the circuit other than the wiper resistor to limit the current. However, some applications may require a very large wiper current. In this case, the voltage drop across the potentiometer wiper needs to be considered, which limits the output dynamic range of the digital potentiometer.

Improve the design according to application requirements

Digital potentiometers have a wide range of applications, and some designs may require external devices to meet the "precision adjustment" requirements of digital potentiometers. For example, the end-to-end resistance range of digital potentiometers is 10kΩ and 200KΩ, while small resistors are often required to control LED brightness. The solution to this problem is the DS3906, which is used in parallel with a fixed resistor of 105Ω to provide an equivalent resistance of 70Ω to 102Ω. This configuration can obtain a step adjustment of 0.5Ω to accurately adjust the brightness of the LED. Another solution is a multi-channel digital potentiometer, such as the MAX5477 or MAX5487, which can combine multiple channels to obtain different adjustment resistance steps to meet the resolution requirements of the digital potentiometer.

Some situations may require more specialized digital potentiometer functionality. For voltage or current regulation that requires temperature compensation, such as the biasing of an optical driver in an optical module, a variable resistor based on a lookup table can be selected. Some digital potentiometers integrate EEPROM (for storing calibration data when the temperature changes) and an internal temperature sensor (for measuring the ambient temperature). The digital potentiometer retrieves the corresponding value in the lookup table according to the measured temperature and adjusts the variable resistor. Digital potentiometers based on temperature lookup tables are often used to correct for nonlinear temperature responses of circuit components, such as laser diodes or LED /' target='_blank'>photodiodes; it is also possible to intentionally establish a nonlinear temperature response for a resistor, depending on the application.

Nonvolatile memory is a relatively common low-cost feature incorporated into digital potentiometers. Standard EEPROM-based nonvolatile (NV) digital potentiometers are placed into a known state during power-on reset (POR). EEPROMs can guarantee 50,000 write cycles, greatly improving system reliability compared to mechanical potentiometers. One-time programmable (OTP) digital potentiometers, such as the MAX5427/MAX5428/MAX5429, use fuse settings to permanently store the default wiper position. Like EEPROM-based digital potentiometers, OTP digital potentiometers are initialized to a known state after POR. However, the POR state of an OTP digital potentiometer cannot be rewritten once programmed. Therefore, OTP is well suited for factory programming or production calibration. The fuse permanently sets the POR wiper position of the OTP digital potentiometer, eliminating the need to lock the wiper position. Some OTP digital potentiometers have a wiper that can be adjusted after fuse programming; others have a wiper position that is permanently set, resulting in an accurate, calibrated resistor divider. Some digital potentiometers offer a lock register, or digital control input, that makes the digital potentiometer interface high impedance, preventing inappropriate wiper adjustment. The write protection feature of EEPROM digital potentiometers also reduces power consumption.

Digital potentiometers can be used to calibrate voltage and current in power supplies or other systems that require factory calibration. Compared with time-consuming and inaccurate manual calibration of mechanical potentiometers or discrete resistors, digital potentiometers help manufacturers improve production capacity and calibration accuracy and repeatability. In addition, digitally controlled potentiometers facilitate remote debugging and recalibration. When multiple voltages and/or currents need to be calibrated, a triple NV digital potentiometer such as the DS3904/DS3905 is ideal (Figure 2). In this case, a small digital potentiometer can replace three mechanical potentiometers. Replacing mechanical potentiometers with digital potentiometers also helps increase circuit layout flexibility because digital potentiometers do not require mechanical adjustment during installation or maintenance. Calibration is a typical application of OTP or EEPROM write protection functions, where EEPROM write protection is more convenient for design.

Figure 2. The DS3904/DS3905 triple nonvolatile digital potentiometers are ideal for systems that need to calibrate multiple voltage/current paths. This small IC can replace three mechanical potentiometers.

Although not a digital potentiometer, a sample/hold voltage reference with a simple single-wire digital control interface such as the DS4303 can also be used for product calibration (Figure 3). The compact design is very suitable for calibration needs. The voltage reference output depends on the input voltage before being locked by the control signal. After the output is locked, the output will no longer change unless reprogrammed or powered off, regardless of the input voltage. The latest products store the locked output voltage in EEPROM and can be restored after power is turned on.

Figure 3. The DS4303 nonvolatile sample/hold voltage reference, although not a digital potentiometer, is ideal for product calibration. During calibration, the DS4303 output (VOUT) depends on the input voltage (VIN) before being locked by the control signal (ADJ).

Improved pushbutton interfaces complement traditional interfaces such as SPI®, I®C, up/down, and rotary controls. The MAX5486 digital potentiometer with buffered output uses this interface. This debounced pushbutton interface controls the tap movement at varying speeds based on how long the button is pressed. The pushbutton interface does not require a microcontroller, reducing the complexity of the system design. The debounced pushbutton interface is especially important for volume control.

Digital potentiometers designed for audio applications often provide zero-crossing detection circuits, which can suppress audible noise when the wiper jumps from one position to another. When this function is enabled, the zero-crossing detection circuit delays the wiper action until VL approaches VH. Many zero-crossing detection circuits also provide a delay of the maximum wiper change, which is convenient for DC regulation and other special circuits.

in conclusion

Simple volatile digital potentiometers are still practical in system design, while digital potentiometers and variable resistors designed for special applications provide more functions. Currently, many designers hope to replace mechanical potentiometers, improve system reliability and performance over the entire operating temperature range, eliminate system microprocessors, or suppress clicks/pops. For these needs, digital potentiometers fully demonstrate their advantages, and their applications are becoming more and more common.