MCP4017 datasheet(44/66 Pages) MICROCHIP

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MCP4017/18/19

DS22147A-page 44

6.3

Wiper Resistance

Wiper resistance is the series resistance of the analog

switch that connects the selected resistor ladder node

to the Wiper Terminal common signal (see Figure 6-1).

A value in the volatile wiper register selects which

analog switch to close, connecting the W terminal to

the selected node of the resistor ladder.

The resistance is dependent on the voltages on the

analog switch source, gate, and drain nodes, as well as

the device’s wiper code, temperature, and the current

through the switch. As the device voltage decreases,

the wiper resistance increases (see Figure 6-4 and

Table 6-4).

The wiper can connect directly to Terminal B or to

Terminal A. A zero scale connections, connects the

Terminal W (wiper) to Terminal B (wiper setting of

000h). A full scale connections, connects the Terminal

W (wiper) to Terminal A (wiper setting of 7Fh). In these

configurations the only resistance between the

Terminal W and the other Terminal (A or B) is that of the

analog switches.

The wiper 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 nonlinearity as the wiper is moved

toward zero scale (00h). The lower the nominal

resistance, the greater the possible error.

In a rheostat configuration, this change in voltage

needs to be taken into account. Particularly for the

lower resistance devices. For the 5.0 k

Ω device the

maximum wiper resistance at 5.5V is approximately

3.2% of the total resistance, while at 2.7V it is

approximately 6.5% of the total resistance.

In a potentiometer configuration, the wiper resistance

variation does not effect the output voltage seen on the

W pin.

The slope of the resistance has a linear area (at the

higher voltages) and a non-linear area (at the lower

voltages). In where resistance increases faster then 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 2-11, Figure 2-29, Figure 2-47, or Figure 2-65),

the change in wiper resistance over voltage can have a

significant impact on the INL and DNL error.

TABLE 6-4:

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

5000

39.37

100

170

325

254.00%

431.80%

825.5%

2.00%

3.40%

6.50%

10000

78.74

100

170

325

127.00%

215.90%

412.75%

1.00%

1.70%

3.25%

50000

393.70

100

170

325

25.40%

43.18%

82.55%

0.20%

0.34%

0.65%

100000

787.40

100

170

325

12.70%

21.59%

41.28%

0.10%

0.17%

0.325%

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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