Circuit based on digital potentiometer to convert voltage into resistance value

Industrial control and variable bias circuits often require a converter that converts voltage to resistance. But it is difficult to implement. A simple method shown in the attached figure is to use two identical digital potentiometers to form such a converter.

Digital potentiometer U1 and comparator U3 form a digital track and hold circuit, where U1 adjusts its internal voltage divider so that VWIPER tracks VIN.

Therefore, the resistance value of the wiper point is proportional to VIN. Because the digital input terminals of U1 and U2 are connected together, the wiper point positions of U1 and U2 are the same, and there is the same resistance value between the corresponding terminals. When a voltage/resistance conversion is required, the output resistance value is proportional to VIN.

The digital signal track and hold works as follows: To track VIN, the wiper (center tip) of digital potentiometer U1 moves up or down with each clock pulse. Comparator U3 compares the analog input voltage (VIN) with the wiper voltage (VWIPER).

If VIN>VWIPER the comparator remains at logic high, causing the wiper position to move upward, increasing the VWIPER value, which keeps increasing until it is greater than VIN, then the comparator triggers downward and drives the wiper position. At each clock cycle, the wiper moves up or down as needed to track VIN.

The reference input of the voltage divider (VH and VL) sets the input voltage range: if the VH range is 0-5Vdo set VL=GND, VH=5 Vdc.

Because U1 and U2 are identical, their digital inputs are connected together and their wiper positions are the same. A logic low is applied to the LOCK input and the output resistance changes with VIN, while a logic high is applied and the resistance value is undefined.

LOCK can be permanently connected to ground potential. In this way, even if VIN is fixed, the output resistance is continuously switched between two adjacent states. For example, if the potentiometer is 10kΩ and the sliding point is set to 5kΩ, the output resistance switches between 5kΩ and 5.3125kΩ in each clock cycle.

If necessary, this effect can be filtered out by connecting a capacitor to the output wiper. The clock frequency can be set between 100Hz and 10kHz.

The output resistance value cannot change instantaneously with VIN, but it takes a number of clock cycles to reach the final value. The number of cycles (maximum 32) depends on the starting wiper position and the input voltage.

If higher resolution is required, use a 6-bit or 8-bit digital potentiometer instead of the 5-bit type shown.

Select digital potentiometers whose starting conditions are known.

Note that the MAX5160 has an active reset state that sets the wiper position to mid-scale, which synchronizes the two digital potentiometers to the same resistance value.