MCP4021 datasheet(43/64 Pages) MICROCHIP

43 / 64 page

DS21945E-page 43

MCP4021/2/3/4

6.1

Resistor Configurations

6.1.1

RHEOSTAT CONFIGURATION

When used as a rheostat, two of the three digital

potentiometer’s terminals are used as a resistive

element in the circuit. With terminal W (wiper) and

either terminal A or terminal B, a variable resistor is

created. The resistance will depend on the tap setting

of the wiper and the wiper’s resistance. The resistance

is controlled by changing the wiper setting.

The unused terminal (B or A) should be left floating.

Figure 6-2 shows the two possible resistors that can be

used. Reversing the polarity of the A and B terminals

will not affect operation.

FIGURE 6-2:

Rheostat Configuration.

This allows the control of the total resistance between

the two nodes. The total resistance depends on the

“starting” terminal to the wiper terminal. At the code

00h, the RBW resistance is minimal (RW), but the RAW

resistance in maximized (RAB + RW). Conversely, at the

code 3Fh, the RAW resistance is minimal (RW), but the

RBW resistance in maximized (RAB + RW).

The resistance step size (RS) equates to one LSb of the

resistor.

The change in wiper-to-end terminal resistance over

temperature is shown in Figure 2-6, Figure 2-17,

Figure 2-27 and Figure 2-37. The most variation over

temperature will occur in the first few codes due to the

wiper resistance

coefficient affecting the

total

resistance. The remaining codes are dominated by the

total resistance tempco RAB.

6.1.2

POTENTIOMETER

CONFIGURATION

When used as a potentiometer, all three terminals are

tied to different nodes in the circuit. This allows the

potentiometer to output a voltage proportional to the

input voltage. This configuration is sometimes called

voltage divider mode. The potentiometer is used to

provide a variable voltage by adjusting the wiper

position between the two endpoints as shown in

Figure 6-3. Reversing the polarity of the A and B

terminals will not affect operation.

FIGURE 6-3:

Potentiometer Configuration.

The temperature coefficient of the RAB resistors is

minimal by design. In this configuration, the resistors all

change uniformally, so minimal variation should be

seen.

The wiper resistor temperature coefficient is different

from the RAB temperature coefficient. The voltage at

node V3 (Figure 6-3) is not dependent on this wiper

resistance, just the ratio of the RAB resistors, so this

temperature coefficient in most cases can be

ignored.

Note:

To avoid damage to the internal wiper

circuitry in this configuration, care should

be taken to insure the current flow never

exceeds 2.5 mA.

Resistor

RAW

RBW

Note:

To avoid damage to the internal wiper

circuitry in this configuration, care should

be taken to insure the current flow never

exceeds 2.5 mA.

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