DA1641.004
1 February, 2019
APPLICATION INFORMATION (continued)
Figure 3. Illustration of optimal alarm threshold in increasing or decreasing capacitance sensing
At alarm threshold following applies.
ꢊ푀푂푁 = ꢊ퐴퐿퐴ꢃ푀
Equation 5.
Equation 6.
ꢌ푀푂푁 = ꢌ
ꢃ퐸퐹
For external resistor value (RCLK, RCOM, RDEC and RINC) calculations one can derive following equations shown in
the table 1. Resistor values are calculated differently whether capacitance decrease or increase is sensed.
Table 1. Equations for calculating resistor values
Resistor Capacitance decrease sensing (DSEL=GND)
Capacitance increase sensing (DSEL=VIN)
퐂퐑퐄퐅 + 퐂퐌퐎퐍
∆퐂
ퟐ
퐂퐑퐄퐅 + 퐂퐌퐎퐍
∆퐂
ퟐ
CALARM
퐂퐀퐋퐀퐑퐌
=
= 퐂퐑퐄퐅
+
=
퐂퐀퐋퐀퐑퐌
=
= 퐂퐑퐄퐅
+
=
ퟐ
ퟐ
푹퐈퐍퐂
퐂퐑퐄퐅
=
= 퐂퐑퐄퐅 ∙ ꢍ
+ ퟏꢎ
푹퐃퐄퐂
푹퐂퐎퐌
푹퐂퐎퐌
(
+ ퟏ)
ퟏ
ퟏ
RCLK
RCOM
RDEC
RINC
퐑퐂퐋퐊
=
≈ ퟐ퐌훀
푹푪푳푲
퐑퐂퐎퐌
=
=
≈ ퟐ퐌훀
ퟖퟏퟗퟐ ∙ ퟔ. ퟏ퐩퐅 ∙ ퟏퟎ퐇퐳
ퟏ
ퟖퟏퟗퟐ ∙ ퟔ. ퟏ풑푭 ∙ ퟏퟎ푯풛
ퟏ
퐑퐂퐎퐌
=
ퟐ ∙ 퐟퐑퐄퐅 ∙ 퐂퐑퐄퐅
퐂퐑퐄퐅
ퟐ ∙ 퐟퐑퐄퐅 ∙ 퐂퐀퐋퐀퐑퐌
0 (shorted)
퐑퐃퐄퐂 = 퐑퐂퐎퐌 ∙ ꢍ
− ퟏꢎ
퐂퐀퐋퐀퐑퐌
ퟏ
0 (shorted)
퐑퐈퐍퐂
=
− 퐑퐂퐎퐌
ퟐ ∙ 퐟퐑퐄퐅 ∙ 퐂퐑퐄퐅
EXAMPLE 1: Capacitance decrease sensing
fCLK = 10Hz, fREF = 100kHz, CREF = 3.2pF, CMON = 2.7pF i.e. C=CMON - CREF = 2.7pF - 3.2pF = -0.5pF
CALARM = (3.2pF+2.7pF)/2=2.95pF
RCLK = 1/(8192*6.1pF*10Hz)/2=2M
RCOM = 1/(2*100kHz*3.2pF)=1.5625M
RDEC = 1.5625M*(3.2pF/2.95pF-1)=132.4k
RINC = 0 (shorted)
Important note: In above RCOM and RDEC calculations there must be used non-rounded calculated resistor
values. The resistor value rounding to standard series values can be done only in the next step.
RCOM (E24 5% series) = 1.5625M 1.6M (E24 5% series)
RDEC (E24 5% series) = 132.4k 130k (E24 5% series)
Alarm threshold check: CALARM=3.2pF/(130k/1.6M+1)=2.96pF OK! (very close to target 2.95pF)
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MAS [ MICRO ANALOG SYSTEMS ]
