A Programmable potentiometer is a component-level controlling device in which the resistance between two externally accessible terminals can be varied as programed by the master/host device.
The Digital potentiometer actually consists of a number of resistive elements connected in a series combination between the two terminals. The taps to these resistive elements are controlled by CMOS switches. These resistive elements are composed of polycrystalline silicon and are insulated from other parts of the circuit through the oxide layer coating. Only one of the CMOS switches is closed at a time leading to a certain resistance between the terminals.
There are special control registers which store the switching instructions for every wiper taps. The number of bits in the control registers depends on the number of resistive elements and the greater the number of resistive elements inside the digital pot, the greater the number of resistance values can be obtained. The CAT5241 programmable digital potentiometer IC, for example, has four independent digital pots on the same chip. Each of these four potentiometers has 63 resistive elements and need a 6-bit control resister to control their tapping.
The control registers can be programmed by a computer through I2C transmission. The wiper settings stored in these control registers are non-volatile, which means that the instructions are not erased from its memory even if the power is turned OFF.
Modes of operation
A programmable potentiometer may be used as a three-terminal device or as a two-terminal device. These applications are briefly explained with the help of the following examples:
As a three-terminal device
The digital pot can be used as a potential divider to get a certain voltage output anywhere between the voltages of the two terminals as the wiper voltage. This can be used to make a simple digital to analog converter which can give continuous varying voltage output by connecting the wiper to the input of an op-amp. The digital signal sent by the host device will be converted to an analog signal at the output of the op-amp. The greater the number of resistive elements in the potentiometer, the greater the resolution can be attained for the output signal.
As a two-terminal device
The digital potentiometer can also be used as a programmable resister in circuits where the circuit parameters depend on a certain resistor. For example, we can use the programmable pot to vary the voltage gain in an op-amp by using it as the feedback element. Or we can vary the cutoff frequency of a high-pass or low-pass filter by using it as the biasing resistance.
