Digital Potentiometers

Potentiometers are three terminal device, two fixed end terminals and one wiper terminal which is used to vary the resistance. It has multitude of applications, for eg. - calibrating a system, adjusting offset voltage or gain in amplifiers, tuning filters, controlling screen brightness, radio adjustment, volume control and many more. However mechanical potentiometers suffer from some serious disadvantages which make it unsuitable for applications where precision is inevitable. Size, wiper contamination, mechanical wear, resistance drift, sensitivity to vibration, humidity, etc. are some of the main disadvantages of mechanical potentiometer. Hence to overcome these drawbacks digital potentiometers are more common in applications since it provides higher accuracy.

Construction of Digital Potentiometers

The circuit of a digital potentiometer consists of two parts, first the resistive element along with electronic switches and second the control circuit of the wiper. Figure below shows both the part respectively.
The first part is an array of resistors, and each node is connected to a common point W, except the end points A and B, via a two way electronic switch. The terminal W is the wiper terminal. Each of the switches is designed using CMOS technology and only one of the switches out of all is in ON state at any given time of the potentiometer operation. The switch which is ON determines the potentiometer resistance and the number of switches determines the resolution of the device. Now which switch is to be made ON is controlled by the control circuit. The control circuit consists of a RDAC register which can be written digitally using interface such as SPI, I²C, up/down or can be manually controlled by push buttons or a digital encoder. The diagram above shows that of a push button controlled digital potentiometer. One button is for “UP” or increasing the resistance and the other for “DOWN” i.e. decreasing the resistance.

Generally the wiper position is at the middle switch when switched off. After power is switched on, depending upon our requirement we can increase or decrease the resistance by suitable push button operation. Besides, advanced digital potentiometers also have an inbuilt on board memory which can store the last position of wiper. Now this memory can be of volatile type or permanent type both, depending upon the application. For example, in case of volume control of a device, we expect the device to remember the volume setting we used last even after we switch it on again. Hence a permanent type memory such as EEPROM is suitable here. On the other hand for systems which recalibrates the output continuously and it is not necessary to restore previous value, a volatile memory is used.

Advantages and Disadvantages of Digital Potentiometers

Now we will discuss on advantages and disadvantages of digital potentiometers.

Advantages of Digital Potentiometers

• Higher reliability
• Increased accuracy
• Small size, multiple potentiometers can be packed on a single chip
• Negligible resistance drift
• Unaffected by environmental conditions like vibrations, humidity, shocks and wiper contamination
• No moving part
• Tolerance up to ±1%
• Very low power dissipation, up to tens of millwatts

Disadvantages of Digital Potentiometers

• Not suitable for high temperature environment and high power application
• Due to parasitic capacitance of the electronic switches, there is a bandwidth consideration which comes into picture in digital potentiometer. It is the maximum signal frequency that can cross the resistance terminals with less than 3 dB attenuation in the wiper. The transfer equation is similar to that of a low-pass filter.
• The nonlinearity in the wiper resistance adds a harmonic distortion to the output signal. The total harmonic distortion, or THD, quantifies the degree to which the signal is degraded after crossing through the resistance.