Manualshelf

Data Sheet

ManualsBrandsDFRobot ManualsRobot PartsDFRobot Capacitive Touch Kit For Arduino
12345678910
MPR121 Sensor
5 Freescale Semiconductor
The amount of charge (Q) applied is programmable by setting the charge current (I), and the charge time (T). Once the
electrode is charged, the peak voltage (V) at the end of charge is measured by internal 10bit ADC. This voltage V (that is the
ADC counts) is reverse proportional to the capacitance (C) on the sensing channel.
π‘ͺ =
𝑸
𝑽
=
𝑰 Γ— 𝑻
𝑽
, 𝑽 =
𝑸
π‘ͺ
=
𝑰 Γ— 𝑻
π‘ͺ
The programmable range for the current I is 1uA~63uA, see description on AFE Configuration 1 Register (0x5C). The
programmable range for time T is 0.5uS~32uS, see description on AFE Configuration 2 Register (0x5D). To maintain the
linearity of internal current source, the voltage on the sensing input suggested range is 0.7V ~ VDD-0.7V. Using these setting
limits for the above equation, the C measurable range can be calculated.
Taking VDD=1.8V as an example:
π‘ͺπ’π’π’˜ =
π‘°π’Žπ’Šπ’ Γ— π‘»π’Žπ’Šπ’
π‘½π’Žπ’‚π’™
=
𝟎. πŸ“π’–π‘Ί Γ— πŸπ’–π‘¨
𝟏. πŸ–π‘½ βˆ’ 𝟎. πŸ•π‘½
= 𝟎. πŸ’πŸ“π’‘π‘­
π‘ͺπ’‰π’Šπ’ˆπ’‰ =
π‘°π’Žπ’‚π’™ Γ— π‘»π’Žπ’‚π’™
π‘½π’Žπ’Šπ’
=
πŸ‘πŸπ’–π‘Ί Γ— πŸ”πŸ‘π’–π‘¨
𝟎. πŸ•π‘½
= πŸπŸ–πŸ–πŸŽπ’‘π‘­
Figure 5. MPR121 linear voltage range by the supply voltage
Note: If the exact C value is not cared and MPR121 is only used for touch detection, then the current source linearity is not an
issue and sensitivity is critical, the electrode can be charged up to the supply limit so maximum charge can be generated for
higher touch detection sensitivity. However, charge should not be so high that the voltage is clipped by supply rail which can
cause a reverse impact on sensitivity.
Since the ADC is 10bit, the equation for ADC reading result is as below:
𝑨𝑫π‘ͺ 𝒄𝒐𝒖𝒏𝒕𝒔 =
𝑽
𝑽𝒅𝒅
Γ— πŸπŸŽπŸπŸ’ =
𝑰 Γ— 𝑻
π‘ͺ
Γ—
𝟏
𝑽𝒅𝒅
Γ— πŸπŸŽπŸπŸ’
Re-arrange above equation, the capacitance C can be calculated as below:
π‘ͺ =
𝑰 Γ— 𝑻
𝑨𝑫π‘ͺ 𝒄𝒐𝒖𝒏𝒕𝒔
Γ—
𝟏
𝑽𝒅𝒅
Γ— πŸπŸŽπŸπŸ’
To calculate the capacitance resolution per ADC counts,
𝒅π‘ͺ
𝒅𝑨𝑫π‘ͺ 𝒄𝒐𝒖𝒏𝒕𝒔
=
𝑰 Γ— 𝑻
(𝑨𝑫π‘ͺ 𝒄𝒐𝒖𝒏𝒕𝒔)
𝟐
Γ—
𝟏
𝑽𝒅𝒅
Γ— πŸπŸŽπŸπŸ’ =
π‘ͺ
𝑨𝑫π‘ͺ 𝒄𝒐𝒖𝒏𝒕𝒔
From above equation, the resolution is dependent on the capacitance under measurement and the ADC reading counts. It’s
easy to understand smaller C or higher ADC counts result higher resolution.
To get the maximum resolution for a given C, the highest ADC counts is used, which gives out: