MAS1641 datasheet(5/14 Pages) MAS

5 / 14 page

DA1641.004

1 February, 2019

5 (14)

APPLICATION INFORMATION

System clock resistor (RCLK) value

The external RCLK resistor adjusts internal system clock frequency (fCLK) which defines rate at which measurements

are looped and intermittent alarm sound is repeated. Nominal frequency for the internal system clock frequency is

fCLK=10Hz which corresponds to operation flow chart timing presented in the figure 2.

Suitable RCLK resistor value is calculated using equation 1.

�� =

8192∙6.1��∙��

Equation 1.

With nominal system clock frequency fCLK=10Hz we get RCLK=1/(8192*6.1pF*10Hz)

2.0MΩ.

Measurement oscillator resistor (RCOM, RINC, RDEC) values

The external RCOM and RINC or RDEC resistor values adjust measurement oscillator frequencies used in reference

and monitoring capacitance measurements. Nominal oscillator frequency in reference measurement should be as

close to fREF = 100kHz when using nominal internal system clock 10Hz. Thus the frequency ratio fREF/fCLK should be

nominally close to 10000.

See figure 1 for resistor circuit configuration. Note that in capacitance decrease detection (DSEL=GND) the RINC

resistor is not needed and it is replaced by a short circuit (0

). Analogously in capacitance increase detection

(DSEL=VIN) the RDEC resistor is not needed and it is replaced by a short circuit (0

).

The resistance RREF seen from RREF pin to ground defines measurement oscillator frequency (fREF) during

reference measurement (the first measurement after power up). See equation 2.

�� = �� + �� =

2∙��∙��

Equation 2.

Similarly, resistance RMON seen from RMON pin to ground defines measurement oscillator frequency (fMON) during

monitoring measurements (repeated measurements after the reference measurement). See equation 3.

�� = �� + �� =

2∙��∙��

Equation 3.

Resistor values RCOM, RINC and RDEC are chosen to set fREF = 100kHz and capacitance threshold for alarm (CALARM)

to optimal point. Optimal value for the alarm threshold is mid value between reference capacitance (CREF) and

changed monitoring capacitance (CMON = CREF +

C) since this maximizes margin to both false and missing

alarms. See equation 4.

�� =

��+��

= �� +

∆��

Equation 4.

Mid valued capacitance threshold for alarm is illustrated in figure 3 for increasing and decreasing capacitance

cases.

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