A digital potentiometer (dpot) is a common component that uses various packages, resistances and resolutions. However, it has no utility other than the conventional linear function between resistance and setting. For applications that require a wide dynamic gain adjustment range (for example, tens of octave bands), this can cause problems.
Taking an amplifier as an example, you use an 8-bit (1 of 256) resolution potentiometer and set its gain to 0 to 10,000 (80dB). When the potentiometer setting has a linear relationship with resistance (linear taper), the dpot setting has a linear relationship with gain. In a 256 potentiometer setting, each step represents a gain increase of approximately 40 (that is, gain steps of 0, 40, 80, 120, 160, etc.).
For dpot settings of 8 or above (gain> 300), this provides a good resolution for the gain setting and can achieve gain control of 1dB or less per step. However, when the setting value is lower than 8, the gain resolution is greatly reduced. For example, if you need to set the gain to 100 or below, you have no way to achieve the necessary value with any meaningful precision, you can only choose a value around 80 or 120.
If you have an accurate, stable, high-resolution, digital potentiometer with logarithmic taps (the logarithm of the resistance is proportional to the setting), it is easy to arrange the gain control circuit to provide a constant resolution (increased by Unit of measure: dB). Unfortunately, there is currently no logarithmic digital potentiometer (logarithmic dpot) with excellent resolution (eg, steps less than 6dB).
But it is not all useless. The Design Idea shown in Figure 1 uses a common linear tapped potentiometer (for example, the inexpensive bipolar AD5200 provided by ADI), and the AD5200 to achieve similar log gain control.
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