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Achieving precision performance from digital pots

Posted: 23 Jul 2008 ?? ?Print Version ?Bookmark and Share

Keywords:precision performance? digitally controlled potentiometers?

Digitally controlled potentiometers (DCPs) have become very popular in a wide variety of applications including control, parameter adjustment and signal processing. The digital pots replace mechanical potentiometers and provide advantages such as remote operation and programmability, higher resolution, much smaller form factor, increased reliability, ability to store multiple wiper positions, and lower total system cost.

There are also some limitations that should be considered when designing with a DCP, such as limited terminal voltage and accuracy. These limitations come from the fabrication process of polysilicon resistors and their integration in a CMOS IC.

The DCP is implemented with a combination of resistor elements, R, connected in series and CMOS switches (Figure 1).

Figure 1: DCP resistor array.

The physical ends of the resistor array, the RH and RL terminals, are equivalent to the fixed terminals of a mechanical potentiometer. The wiper RW is connected to the intermediate nodes by the CMOS switches, one at a time, and is equivalent to the wiper terminal of the mechanical potentiometer.

Typical resistance accuracy of the polysilicon DCP is in ㊣20 percent range. However, the relative accuracy or matching of the resistive elements in the particular resistor array is excellent, and usually is in range of ㊣1 percent or better. Thus, this discrepancy between relative and total accuracy should be carefully calculated during the design stage, in order to avoid or minimize an additional adjustment of the application circuitry in production. In this article we will discuss how the DCP accuracy affects the design and some techniques to improve final system accuracy.

There are two major uses of DCP in application design: as a voltage divider and as a variable resistor.

Voltage-divider mode
When a DCP is used as a voltage divider and its RH and RL terminals are connected to the voltage rails, the final accuracy of the wiper RW depends only on the internal resistance matching. Thus, it will be the same from part to part, regardless of their total resistance accuracy.

This is simply because the voltage between RH and RL terminals is divided in-between particular number of taps, i.e. scaled down among n numbers of equal resistive elements in the divider string. For example, for configuration shown in Figure 2,

Figure 2: High-accuracy voltage divider.

the output voltage Vout for the wiper position m, can be calculated as (Equation 1a and 1b):

or

where m is a current wiper position and n is a total number of taps.

As can be seen from Equation 1b, the resistance accuracy is canceled out and has no effect on Vout.

However, if a DCP has another resistor(s) on its RH or/and RL terminal, the accuracy of the output signal become a function of the initial accuracy of the DCP. This is because the scaled factor is not equal among the divider string (Figure 3).

Figure 3: Example of accuracy inequality.

The output function for the circuitry with R1 and R2 in Figure 3 is (Equation 2):

where n is the total number of taps and m is the current wiper position.

Note that wiper resistance is not included, because it has no effect in this particular configuration, and assuming that we have an ideal op amp.

Rheostat mode
When a DCP is used as a variable resistor, its output accuracy becomes a combination of initial accuracy (㊣20 percent) plus an additional error from wiper resistance. Since the wiper switch is not ideal〞it has a small resistance, typically about 70次, and its value may vary among the taps. The wiper resistance can be lowered in rheostat configuration, e.g. when the wiper is connected to one of the end terminals, Figure 4a.

Figure 4: Rheostat/variable resistor configuration.

In rheostat configuration, Figure 4a, wiper resistance appears in parallel with the part of the resistor string and its effect depends on the selected wiper position.

Another possible configuration is to leave one of the end terminals floating as in Figure 4b. In this case, the wiper resistance is well known and usually provided as a graph in a datasheet that makes calculation of total resistance at each tap much easier. Equation 3 can be used to calculate resistance at tap m:

Design examples
Even though the initial accuracy of the regular DCP is in the ㊣20 percent range, the accuracy of the application can be improved by using certain techniques. For example, design in Figure 3 can be slightly modified in order to get higher accuracy as in Figure 5.

Figure 5: Example of increased accuracy in voltage-divider mode.

In the above example, the input signal Vin is divided by the string of fixed resistors R1, R2 and R3 and a DCP is placed in parallel with the R2. This configuration preserves the flexibility of the variable output with much higher accuracy. Note that in order to get desired accuracy, the value of Rtotal has to be about five to ten times the value of R2.

Better accuracy comes when the DCP is used as a variable resistor by combining the DCP with high-precision fixed resistors in parallel and serial configuration (Figure 6).

Figure 6: DCP in both serial and parallel configuration with fixed resistors.

For example, using ㊣20 percent, 10k次, 256 taps DCP and circuitry as in Figure 6, we can get a variable resistor from 5.5k次 to 10.695k次 with accuracy distributed from ㊣1.1 percent to ㊣8.5 percent (Table 1).

Table 1: DCP in both serial and parallel configuration with fixed resistors.

Another practical usage of DCPs is an alternative to the DACs. In most cases, when the design needs fine tuning within limited range, an 8-bit DCP can achieve even better resolution than a 10bit DAC. The DCP resolution table as function of terminal voltages and number of taps is in Table 2.

Table 2: DCP resolution per tap.

- Yuriy Kurtsevoy
Senior Applications Engineer
Intersil Corp.





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