MCP4021 datasheet(44/64 Pages) MICROCHIP

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MCP4021/2/3/4

DS21945E-page 44

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

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

125

325

225.0%

375.0%

975.0%

3.57%

5.95%

15.48%

5000

79.37

125

325

94.5%

157.5%

409.5%

1.5%

2.50%

6.50%

10000

158.73

125

325

47.25%

78.75%

204.75%

0.75%

1.25%

3.25%

50000

793.65

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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