MAS1641 datasheet(6/14 Pages) MAS

6 / 14 page

DA1641.004

1 February, 2019

6 (14)

APPLICATION INFORMATION (continued)

Figure 3. Illustration of optimal alarm threshold in increasing or decreasing capacitance sensing

At alarm threshold following applies.

�� = ��

Equation 5.

�� = ��

Equation 6.

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)

CALARM

�� =

�� + ��

��

= �� +

∆��

��

(

��

+ ��)

�� =

�� + ��

��

= �� +

∆��

��

= �� ∙ (

��

+ ��)

RCLK

�� =

��

�� ∙ ��. �� ∙ ��

≈ ��

�� =

��

�� ∙ ��. �� ∙ ��

≈ ��

RCOM

�� =

��

�� ∙ �� ∙ ��

�� =

��

�� ∙ �� ∙ ��

RDEC

�� = �� ∙ (

��

− ��)

 (shorted)

RINC

 (shorted)

�� =

��

�� ∙ �� ∙ ��

− ��

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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