Digital potentiometer
Encyclopedia
A digital potentiometer is a digitally controlled electronic component that mimics the analog functions of a potentiometer
. It is often used for trimming and scaling analog signals by microcontroller
s. It is either built using an R-2R
integrated circuit
or a Digital-to-analog converter
. A digital potentiometer is an electronic component that is often controlled by digital protocols like I²C
and SPI
, as well as more basic Up/Down protocols. Some typical uses of digital potentiometers are in circuits requiring gain control of amplifiers (frequently instrumentation amplifier
s), small-signal audio-balancing, and offset adjustment.
Sometimes this device is also referred to as an RDAC, Resistive Digital-to-Analog Converter.
Some Digipots come with non-volatile memory, so that they retain their last programmed position after they have been power cycled. Most, though, are volatile, i.e. after they are power cycled they will default to a standard value, which is usually the mid-point.
The former can be useful, but when they are controlled by a microprocessor, or even via a Field Programmable Gate Array
(FPGA), these devices can retain, in other non-volatile memory, the value to initialise the Digipot with. In these circumstances, the need for non-volatile Digipots is less obvious.
The non-volatile Digipots also differ from their electro-mechanical cousins in that on power up, the resistance will default to (possibly) a different value after a power cycle.
Similarly, the Digipot resistance is only valid when the correct DC supply voltage(s) are present. When voltages are removed, the resistance between the two end points and the (nominal) Wiper are undefined. In an operational amplifier circuit, the Off-State impedance of a real potentiometer can help stabilise the DC operating point of the circuit during the power-up stage. This may not be the case when a Digipot is used.
Like their electro-mechanical counterparts, Digipots suffer similar weaknesses. Real potentiometers and Digipots generally have poor tolerances (typically +/- 20%), poor temperature coefficients (many hundreds of ppm per degree C), and a stop resistance that is typically about 0.5-1% of the full scale resistance. Note that Stop Resistance is the residual resistance when the terminal to wiper resistance is set to the minimum value.
Potentiometer
A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...
. It is often used for trimming and scaling analog signals by microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
s. It is either built using an R-2R
Resistor Ladder
A resistor ladder is an electrical circuit made of repeating units of resistors. Two configurations are discussed below, a string resistor ladder and a R-2R ladder....
integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
or a Digital-to-analog converter
Digital-to-analog converter
In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
. A digital potentiometer is an electronic component that is often controlled by digital protocols like I²C
I²C
I²C is a multi-master serial single-ended computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, cellphone, or other electronic device. Since the mid 1990s, several competitors I²C ("i-squared cee" or "i-two cee"; Inter-Integrated Circuit;...
and SPI
Serial Peripheral Interface Bus
The Serial Peripheral Interface Bus or SPI bus is a synchronous serial data link standard named by Motorola that operates in full duplex mode. Devices communicate in master/slave mode where the master device initiates the data frame. Multiple slave devices are allowed with individual slave select ...
, as well as more basic Up/Down protocols. Some typical uses of digital potentiometers are in circuits requiring gain control of amplifiers (frequently instrumentation amplifier
Instrumentation amplifier
An instrumentation amplifier is a type of differential amplifier that has been outfitted with input buffers, which eliminate the need for input impedance matching and thus make the amplifier particularly suitable for use in measurement and test equipment...
s), small-signal audio-balancing, and offset adjustment.
Sometimes this device is also referred to as an RDAC, Resistive Digital-to-Analog Converter.
Some Digipots come with non-volatile memory, so that they retain their last programmed position after they have been power cycled. Most, though, are volatile, i.e. after they are power cycled they will default to a standard value, which is usually the mid-point.
The former can be useful, but when they are controlled by a microprocessor, or even via a Field Programmable Gate Array
Field-programmable gate array
A field-programmable gate array is an integrated circuit designed to be configured by the customer or designer after manufacturing—hence "field-programmable"...
(FPGA), these devices can retain, in other non-volatile memory, the value to initialise the Digipot with. In these circumstances, the need for non-volatile Digipots is less obvious.
Limitations
These devices are extremely useful in the modern, digitally controlled world, but have some limitations. While quite similar to a normal potentiometer, digital potentiometers are somewhat constrained by current limits in the tens of milliamperes. Also, most, if not all digital potentiometers limit the input voltage range to the digital supply range (often 0–5 VDC), so some ingenuity is often required when attempting to replace standard resistive potentiometers with digital potentiometers. Further, instead of the seemingly continuous control that can be obtained from a multiturn resistive potentiometer, digital potentiometers have discrete steps in resistance. Eight-bit pots (256-steps) are most common, but potentiometers between 5 and 10 bits (32 to 1024 steps) are available. A fourth constraint is that special logic is often required to check for zero crossing of an analog AC signal to allow the resistance value to be changed without causing an audible click in the output for audio amplifiers.The non-volatile Digipots also differ from their electro-mechanical cousins in that on power up, the resistance will default to (possibly) a different value after a power cycle.
Similarly, the Digipot resistance is only valid when the correct DC supply voltage(s) are present. When voltages are removed, the resistance between the two end points and the (nominal) Wiper are undefined. In an operational amplifier circuit, the Off-State impedance of a real potentiometer can help stabilise the DC operating point of the circuit during the power-up stage. This may not be the case when a Digipot is used.
Like their electro-mechanical counterparts, Digipots suffer similar weaknesses. Real potentiometers and Digipots generally have poor tolerances (typically +/- 20%), poor temperature coefficients (many hundreds of ppm per degree C), and a stop resistance that is typically about 0.5-1% of the full scale resistance. Note that Stop Resistance is the residual resistance when the terminal to wiper resistance is set to the minimum value.