Data Sheet

MPR121 Sensor
6 Freescale Semiconductor
(
𝒅𝑪
𝒅𝑨𝑫𝑪 𝒄𝒐𝒖𝒏𝒕𝒔
)
𝒎𝒊𝒏
=
𝑪
(𝑨𝑫𝑪 𝒄𝒐𝒖𝒏𝒕𝒔)
𝒎𝒂𝒙
=
𝑪
𝟏𝟎𝟐𝟒
≈ 𝟎. 𝟎𝟎𝟏𝑪
For example, with C=10pF, when setting I and T so that the ADC reading counts is 1000, the sensing resolution for this
capacitance at this reading counts will be 0.01pF. The conclusion is that to get the maximum sensing resolution, the
capacitance shall be charged close to the upper counts limit. This scheme is used by the MPR121 in electrode auto-
configuration where the optimized I and T are chosen to set the reading counts close to upper counts limit for the capacitance
on the electrode.
For touch sensing application, to achieve the best sensing sensitivity, smaller C is desired, which means unnecessary stray
capacitance on the channel inputs shall be kept as little as possible. The stray capacitance (Cb) can cause the sensing charge
field return back to ground directly without following through Cx (See Figure 3) which is the capacitance change need to be
detected. So ground layer and shield shall be carefully decided and designed so that both the EMC design consideration and
the sensitivity target can be both achieved. Refer to MPR121 design guideline for more information on sensitivity issue.
2. Three levels of filtering
The ADC raw data outputs run through 3 levels of digital filtering to filter out the high frequency and low frequency noise
encountered.
The first level filter is a simple running average filter configured by FFI (see description on AFE Configuration 1 Register
(0x5C)) which is the number of samples used for the filter.
The 2
nd
level filter result is 10bit and stored in the output data registers 0x04~0x1D as the immediate capacitance of each
sensing input (see description on Filtered Data Registers). The filter control parameter is SFI and ESI (see description on
AFE Configuration 2 Register (0x5D)).
The 3
rd
level filter result is an even lower frequency content of signal change using the 2
nd
level filter output, and is mainly used
as the baseline value representing the capacitance variation over the long term and slow environment change such as
atmospheric moisture and dirt for touch detection. There are two sets of filter control parameters, all the 12 electrodes’ baseline
share the same set of parameters, while the 13
th
channel ELEPROX is controlled by another independent set of parameters (
refer to description on Baseline Filter Control Registers).
The 3
rd
level filtered result is internally 10bit but only high 8 bits are readable from registers 0x1E~0x2A as the baseline value
output for each channel ( see description on Baseline Value Registers).
Figure 4. MPR121 Capacitance Measurement