A digital potentiometer (informally a digipot) 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 microcontrollers. It is either built using an R-2R integrated circuit or a Digital-to-analog converter. The resistor ladder construction is the most common. Every step on the resistor ladder has its own switch which can connect this step to the output terminal of the digipot. The selected step on the ladder determines the resistance ratio of the digital potentiometer. The amount of steps of a digital potentiometer is normally indicated with a bit value e.g. 8 bits equals 256 steps. A digital potentiometer 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 amplifiers), small-signal audio-balancing, and offset adjustment.
Sometimes this device is also referred to as an RDAC, Resistive Digital-to-Analog Converter.[1]
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 midpoint.
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.
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 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.
See also
- Analog Potentiometer
- Console automation
References
- ^ [ Analog Devices: Digital Potentiometers: Digital to Analog Converters: Digital Potentiometers: Frequently Asked Questions]
External links
