Get Precision Performance from a Digitally Controlled Potentiometer (DCP) ® Application Note June 5, 2009 Typical resistance accuracy of the polysilicon DCP is in ±20% 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% 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 application note we will discuss how the DCP accuracy affects the design and some techniques to improve final system accuracy. VIN AN1425.1 VCC U1 RH RTOTAL ±20% V+ INRW RL OUT VOUT ±1% IN+ V- There are two major uses of DCP in application design – as a voltage divider and as a variable resistor. Voltage Divider Mode GND GND FIGURE 1. HIGH ACCURACY VOLTAGE DIVIDER 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 and will be the same from part to part, regardless of their total resistance accuracy. This is simply because of 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 the configuration shown in Figure 1, the output voltage VOUT for the wiper position m, can be calculated as Equations 1 and 2. R TOTAL V IN V OUT = ----------------------- × ----------------------- × m R TOTAL n–1 or: V IN V OUT = ------------ × m n–1 where, m is a current wiper position and n is a total number of taps. As can be seen from Equation 2, the resistance accuracy is canceled out and has no effect on VOUT. However, if a DCP has another resistor(s) on its RH and/or RL terminal, the accuracy of the output signal becomes a function of the initial accuracy of the DCP. This is because the scaled factor is not equal among the divider string, Figure 2. (EQ. 1) VCC VIN VIN (EQ. 2) VCC R1 ±1% U1 RTOTAL ±20% RH RL V+ INRW IN+ V- OUT U2 ACCURACY VOUT ±1% = V+ IN- RH OUT RW RTOTAL ±20% IN+ V- VOUT up to ±20% RL R2 ±1% GND GND GND GND FIGURE 2. EXAMPLE OF ACCURACY INEQUALITY 1 CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2008. All Rights Reserved All other trademarks mentioned are the property of their respective owners. Application Note 1425 The output function for the circuitry with R1 and R2 in Figure 2 is as shown in Equation 3: V IN R TOTAL V OUT = -------------------------------------------------- × ⎛ R 2 + ----------------------- × m⎞ ⎝ ⎠ R 1 + R TOTAL + R 2 (n – 1) (EQ. 3) improved by using certain techniques. For example, the design in Figure 2 can be slightly modified in order to get higher accuracy, as shown in Figure 4. 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, assuming that we have an ideal Op Amp. R1 ±1% R2 ±1% U2 RTOTAL ±20% V+ IN- RH RW VOUT ±2% OUT IN+ V- RL Rheostat Mode When a DCP is used as a variable resistor, its output accuracy becomes a combination of initial accuracy (± 20%) plus an additional error from wiper resistance, since the wiper switch is not ideal (it has a small resistance, typically about 70Ω) 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 (see Figure 3A). RH RW RTOTAL ±20% RH RL RW RTOTAL ±20% VCC VIN R3 ±1% GND GND FIGURE 4. EXAMPLE OF INCREASED ACCURACY IN VOLTAGE DIVIDER MODE In Figure 4, the input signal VIN is divided by the string of fixed resistors R1, R2 and R3, and a DCP is placed in parallel with 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 5. RL GND R1 ±1% GND FIGURE 3A. FIGURE 3B. FIGURE 3. RHEOSTAT/VARIABLE RESISTOR CONFIGURATION In rheostat configuration (Figure 3A), 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 3B. In this case, the wiper resistance is well known and usually provided as a graph in a data sheet that makes calculation of total resistance at each tap much easier. Equation 3 can be used to calculate resistance at tap m R TOTAL R m = ----------------------- × m + R WIPER + R OFFSET n–1 RTOTAL ±20% RH RW R2 ±1% RL FIGURE 5. DCP IN BOTH SERIAL AND PARALLEL CONFIGURATION WITH FIXED RESISTORS For example, using ±20% 10k 256 taps DCP and circuitry as in Figure 5, we can get a variable resistor from 5.5k to 10.695k with accuracy distributed from ±1.1% to ±8.5%, (Table 1). (EQ. 4) Design Examples Allow Increases in Circuitry Accuracy Even though the initial accuracy of the regular DCP is in ±20% range, the accuracy of the application can be 2 AN1425.1 June 5, 2009 Application Note 1425 TABLE 1. DCP IN BOTH SERIAL AND PARALLEL CONFIGURATION WITH FIXED RESISTORS DCP Rtotal (kΩ) R1 (kΩ) R2 (kΩ) MINIMUM TOTAL RESISTANCE, DCP AT TAP 1 (kΩ) Min 8 6.742 29.106 5.495 9.786 0.021 Nom 10 6.81 29.4 5.555 10.695 0.026 Max 12 6.878 29.694 5.615 11.541 0.031 There is also the ISL22317, a 1% precision, non-volatile, 128 tap DCP available with the resistance options of 10k, 50k and 100k that can be configured either as a two-terminal variable resistor or as a three-terminal potentiometer. The ISL22317 provides high accuracy, 0Ω of wiper resistance and low temperature coefficient, eliminating the need for complex precision algorithms and other additions. MAXIMUM TOTAL RESISTANCE, DCP AT TAP 255 (kΩ) STEP RESOLUTION (kΩ) TOTAL ACCURACY FOR TAP 1 TOTAL ACCURACY FOR TAP 255 -1.08% -8.50% 1.08% 7.90% within limited range, an 8-bit DCP can achieve even better resolution than a 10-bit DAC. The DCP resolution table as a function of terminal voltages and number of taps is shown in Table 2. Another practical usage of the DCPs is an alternative to the DACs. In most cases when the design needs fine tuning TABLE 2. DCP RESOLUTION PER TAP DIFFERENTIAL TERMINAL VOLTAGES: VH-VL 16 Taps 32 Taps 64 Taps 100 Taps 128 Taps 256 Taps 1024 Taps 4 Bits 5 Bits 6 Bits 7 Bits 7 Bits 8 Bits 10 Bits 10V 667 323 159 101 79 39 10 9V 600 290 143 91 71 35 9 8V 533 258 127 81 63 31 8 7V 467 226 111 71 55 27 7 6V 400 194 95 61 47 24 6 5.5V 367 177 87 56 43 22 5 5V 333 161 79 51 39 20 5 4V 267 129 63 40 31 16 4 3V 200 97 48 30 24 12 2.9 2V 133 65 32 20 16 8 2.0 1V 67 32 16 10 8 3.9 1.0 0.5V 33 16 8 5 3.9 2.0 0.5 NOTE: Resolution (chart specifies LSB in mV/tap) Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that the Application Note or Technical Brief is current before proceeding. For information regarding Intersil Corporation and its products, see www.intersil.com 3 AN1425.1 June 5, 2009

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