Datasheet

 2008-2013 Microchip Technology Inc. DS22107B-page 69
MCP454X/456X/464X/466X
8.4 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 8-5 shows a block diagram of one of the
MCP45x1 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 MCP45x1’s V
DD
/V
SS
voltage limits.
FIGURE 8-5: 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
(R
B2GND
) 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
MCP45X1
P0W
P0B
L = 20 * log
10
(V
OUT
/ V
IN
)
dB V
OUT
/ V
IN
Ratio
-3 0.70795
-2
-1
0.79433
0.89125
L = 20 * log
10
(R
BW
/ R
AB
)
Terminal B connected to Ground (see Figure 8-5)
L = 20 * log
10
( (R
BW
+ R
B2GND
) / (R
AB
+ R
B2GND
) )
Terminal B through R
B2GND
to Ground