MCP4441 datasheet(76/100 Pages) MICROCHIP

76 / 100 page

MCP444X/446X

DS22265A-page 76

8.5

Implementing Log Steps with a

Linear Digital Potentiometer

In audio volume control applications, the use of

logarithmic steps is desirable since the human ear

hears in a logarithmic manner. The use of a linear

potentiometer can approximate a log potentiometer,

but with fewer steps. An 8-bit potentiometer can

achieve fourteen 3 dB log steps plus a 100% (0 dB)

and a mute setting.

Figure 8-7 shows a block diagram of one of the

MCP44x1 resistor networks being used to attenuate an

input signal. In this case, the attenuation will be ground

referenced. Terminal B can be connected to a common

mode voltage, but the voltages on the A, B and Wiper

terminals must not exceed the MCP44x1’s VDD/VSS

voltage limits.

FIGURE 8-7:

Signal Attenuation Block

Diagram - Ground Referenced.

Equation 8-1 shows the equation to calculate voltage

dB gain ratios for the digital potentiometer, while

Equation 8-2 shows the equation to calculate

resistance dB gain ratios. These two equations assume

that the B terminal is connected to ground.

If terminal B is not directly resistively connected to

ground, then this terminal B to ground resistance

(RB2GND) must be included into the calculation.

Equation 8-3 shows this equation.

EQUATION 8-1:

dB CALCULATIONS

(VOLTAGE)

EQUATION 8-2:

dB CALCULATIONS

(RESISTANCE) - CASE 1

EQUATION 8-3:

dB CALCULATIONS

(RESISTANCE) - CASE 2

Table 8-1 shows the codes that can be used for 8-bit

digital potentiometers to implement the log attenuation.

The table shows the wiper codes for -3 dB, -2 dB, and

-1 dB attenuation steps. This table also shows the

calculated attenuation based on the wiper code’s linear

step. Calculated attenuation values less than the

desired attenuation are shown with red text. At lower

wiper code values, the attenuation may skip a step, if

this occurs the next attenuation value is colored

magenta to highlight that a skip occurred. For example,

in the -3 dB column the -48 dB value is highlighted

since the -45 dB step could not be implemented (there

are no wiper codes between 2 and 1).

P0A

MCP44X1

P0W

P0B

L = 20 * log10 (VOUT / VIN)

VOUT / VIN Ratio

-3

0.70795

-2

-1

0.79433

0.89125

L = 20 * log10 (RBW / RAB)

Terminal B connected to Ground (see Figure 8-7)

L = 20 * log10 ( (RBW + RB2GND) / (RAB + RB2GND) )

Terminal B through RB2GND to Ground

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