DAC vs. Digital Potentiometer: Which is Right for My Application?

Abstract: This application note compares digital-to-analog converters (DACs) and digital potentiometers, which are traditionally used to replace mechanical potentiometers. As resolution increases and features increase, digital potentiometers can also replace traditional DACs in some applications. In addition, traditional DACs are larger and more expensive than digital potentiometers. However, as DACs become less expensive and packaged in smaller sizes, DACs can also replace digital potentiometers.

Overview

There are two options for fine-tuning analog outputs using digital inputs : digital potentiometers (pots) and digital-to-analog converters (DACs), both of which use digital inputs to control analog outputs. Digital potentiometers adjust analog voltages; DACs adjust currents as well as voltages. Potentiometers have three analog connections: a high side, a wiper (or analog output), and a low side (see Figure 1a). DACs have corresponding three terminals: a high side that corresponds to a positive reference voltage, a wiper that corresponds to the DAC output, and a low side that may correspond to ground or a negative reference voltage (see Figure 1b).


Figure 1. DACs typically include an output buffer, but digital potentiometers do not.

Traditionally, digital potentiometers have been used to replace simple mechanical potentiometers (for more information, see application note 3417: Digital Potentiometers Replace Mechanical Pots). As digital potentiometers have improved in resolution and added functionality, some traditional DAC applications have begun to be replaced by digital potentiometers. There are some significant differences between DACs and digital potentiometers, the most obvious being that DACs typically include an output amplifier/buffer, while digital potentiometers do not. Most digital potentiometers require an external buffer to drive a low-impedance load. In some applications, the user can easily choose between a DAC and a digital potentiometer; in other applications, both can meet the needs.

This article compares DACs and digital potentiometers to help users make the most appropriate choice.

Basic features and advantages of DAC

DACs are usually implemented in either a resistor string or R-2R ladder architecture. When using a resistor string, the DAC input controls a set of switches that divide the reference voltage through a series of matched resistors . For the DAC R-2R ladder architecture, the positive reference voltage is divided by switching each resistor, resulting in a controlled current. This current is fed into the output amplifier, where a voltage output DAC converts this current into a voltage output, while a current output DAC outputs the R-2R ladder current after buffering it through an amplifier.

If you are selecting a DAC, you should also consider specific specifications such as serial/parallel, resolution, number of input channels, current/voltage output, cost, and relative accuracy. The communication interface

of a DAC can be either serial or parallel . A serial interface sends data sequentially, one bit after another, over a single input or output line. A parallel interface typically sends all data bits, with each bit requiring a separate pin/connection point. Serial interfaces are generally classified into two types: 3-wire (SPI™, QSPI™, or MICROWIRE™ compatible) or 2-wire (I²C). Some 3-wire interfaces include digital output lines, which are called 4-wire interfaces. For simplicity, this article will refer to them as 3-wire interfaces. For systems that focus on speed, a parallel interface can be used. If cost and size are important, a 3-wire or 2-wire serial interface can be used. This device has a small number of pins and can significantly reduce costs. In addition, some 3-wire interfaces can reach a communication rate of 26MHz, and 2-wire interfaces can reach a rate of 3.4MHz. For applications that require multiple DACs to be cascaded, a 3-wire serial interface can be used. Both 3-wire and 2-wire interfaces can read back the data written to the DAC. Reading back data is another advantage of DACs over digital potentiometers. Another indicator of DACs is resolution. 16-bit or 18-bit DACs can provide microvolt control. For example, an 18-bit, 2.5V reference DAC has each least significant bit (LSB) corresponding to 9.54µV. High resolution is extremely important for industrial control (such as robots, engines, and other products). Currently, the highest resolution that digital potentiometers can provide is 10 bits or 1024 taps. Another advantage of DACs is the ability to integrate multiple converters on a single chip. For example, the MAX5733 has 32 DACs built in, each of which can provide 16 bits of resolution. Current digital potentiometers can only provide up to 6 channels. For example, the DS3930 is one of the few single-chip 6-channel potentiometers. DACs provide current or voltage output drive through R-2R ladders or resistor strings, output amplifiers, and MOSFETs. The most obvious difference between DACs and digital potentiometers is the DAC's output amplifier, which allows the DAC to drive low-resistance loads, but so far, few potentiometers provide output amplifiers. DACs can source or sink current, providing designers with greater flexibility. For example, the MAX5550 10-bit DAC can provide up to 30mA of output drive through an internal amplifier, p-channel MOSFET, and pull-up resistors. The MAX5547 10-bit DAC can provide 3.6mA of sink current with an amplifier, n-channel MOSFET, and pull-down resistor. In addition to current output, some DACs can also be connected to external amplifiers to provide additional output control. The latter DAC is also called a force/sense DAC. Because DACs usually have built-in amplifiers, they cost more than digital potentiometers, but as new DACs get smaller, the cost difference is getting smaller.

Basic features and advantages of digital potentiometers

As mentioned earlier, digital potentiometers can control resistance via digital inputs. The 3-terminal digital potentiometer in Figure 1a is actually an adjustable resistor divider with fixed end-to-end resistance. Digital potentiometers can be configured as 2-terminal variable resistors by connecting the center tap of the potentiometer to the high or low end, or by leaving the high or low end floating. Unlike DACs, digital potentiometers can connect the H end to the highest voltage, the L end to the lowest voltage, or inversely.

When selecting a digital potentiometer, users also need to consider specific specifications: linear or logarithmic adjustment, number of taps, number of tap levels, nonvolatile memory, cost, etc. Control interfaces include increase/decrease control, push buttons, SPI, and I²C.

Linear potentiometers are more versatile than logarithmic potentiometers. Each tap in a linear potentiometer has the same resistance, and the change from low end to high end is a linear transfer function. Logarithmic tap potentiometers are generally used to adjust audio signals. Because the decibel number corresponding to each step of change needs to be consistent with the response characteristics of the human ear.

Digital potentiometers communicate through various types of interfaces, including I²C and SPI. In addition, the digital potentiometer also provides 2-wire increment and decrement interface control; 3-wire interface slightly different from SPI; key increase/decrease control. The MAX5456 32-tap digital potentiometer combines a 2-wire key control interface, and the center taps of its two digital potentiometers can be adjusted up and down, or the audio signals of the left and right channels can be balanced.

DAC/Potentiometer Application Selection

In many applications, users can easily choose between DAC and potentiometer. Motor control, sensor or robotic systems that require high resolution need to use DAC. In addition, high-speed applications such as base stations and instruments have high requirements for speed and resolution, and even need DAC with parallel interface.

The linear characteristics of the potentiometer make it easy to build the feedback network of the amplifier. Compared with DAC, logarithmic potentiometer is more suitable for volume adjustment.

However, in many current applications, the boundary between DAC and digital potentiometer is blurred. The DAC and digital potentiometer in Figure 2 can both be used to control the brightness adjustment of the MAX1553 LED driver. The DC voltage at the MAX1553 BRT input and the current-sense resistor between FB and GND determine the LED current.


Figure 2. Use a digital potentiometer or DAC to control the BRT pin of the MAX1553 to adjust the LED current