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MCP4022 Datasheet(PDF) 44 Page - Microchip Technology |
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MCP4022 Datasheet(HTML) 44 Page - Microchip Technology |
44 / 64 page MCP4021/2/3/4 DS21945E-page 44 © 2006 Microchip Technology Inc. 6.2 Wiper Resistance Wiper resistance is the series resistance of the wiper. This resistance is typically measured when the wiper is positioned at either zero-scale (00h) or full-scale (3Fh). The wiper resistance in potentiometer-generated voltage divider applications is not a significant source of error. The wiper resistance in rheostat applications can create significant non-linearity as the wiper is moved toward zero-scale (00h). The lower the nominal resistance, the greater the possible error. Wiper resistance is significant depending on the devices operating voltage. As the device voltage decreases, the wiper resistance increases (see Figure 6-4 and Table 6-2). In a rheostat configuration, this change in voltage needs to be taken into account, particularly for the lower resistance devices. For the 2.1 k Ω device, the maximum wiper resistance at 5.5V is approximately 6% of the total resistance, while at 2.7V, it is approximately 15.5% of the total resistance. In a potentiometer configuration, the wiper resistance variation does not effect the output voltage seen on the terminal W pin. The slope of the resistance has a linear area (at the higher voltages) and a non-linear area (at the lower voltages), where resistance increases faster than the voltage drop (at low voltages). FIGURE 6-4: Relationship of Wiper Resistance (RW) to Voltage Since there is minimal variation of the total device resistance over voltage, at a constant temperature (see Figure 2-6, Figure 2-17, Figure 2-27 or Figure 2-37), the change in wiper resistance over voltage can have a significant impact on the INL and DNL error. TABLE 6-2: TYPICAL STEP RESISTANCES AND RELATIONSHIP TO WIPER RESISTANCE RW VDD Note: The slope of the resistance has a linear area (at the higher voltages) and a non- linear area (at the lower voltages). Resistance ( Ω)RW / RS (%) (1) RW / RAB (%) (2) Typical Wiper (RW) RW = Typical RW = Max @ 5.5V RW = Max @ 2.7V RW = Typical RW = Max @ 5.5V RW = Max @ 2.7V Total (RAB) Step (RS) Typical Max @ 5.5V Max @ 2.7V 2100 33.33 75 125 325 225.0% 375.0% 975.0% 3.57% 5.95% 15.48% 5000 79.37 75 125 325 94.5% 157.5% 409.5% 1.5% 2.50% 6.50% 10000 158.73 75 125 325 47.25% 78.75% 204.75% 0.75% 1.25% 3.25% 50000 793.65 75 125 325 9.45% 15.75% 40.95% 0.15% 0.25% 0.65% Note 1: RS is the typical value. The variation of this resistance is minimal over voltage. 2: RAB is the typical value. The variation of this resistance is minimal over voltage. |
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