MAS1641 datasheet(9/14 Pages) MAS

9 / 14 page

DA1641.004

1 February, 2019

9 (14)

Figure 4. TEST pin signal of reference and first monitoring measurement bursts after 4.9s(typ) from power up (VIN)

The frequencies of reference (fREF) and monitoring (fMON) measurement signal bursts have following dependency

on external RCOM and RMON resistors and sensed capacitance (CSENSOR).

�� =

2∙(��+��)∙��

Equation 6.

�� =

2∙(��+��)∙��

Equation 7.

Either from equation 6 or 7 we can solve the sensor capacitance value CSENSOR as follows.

�� =

2∙(��+��)∙��

Equation 8.

�� =

2∙(��+��)∙��

Equation 9.

For example if in capacitance decrease sensing (DSEL=GND, RINC=0

) we measure the reference measurement

signal frequency as fREF= 97.66kHz and have RCOM=1.60M

 then CSENSOR=1/[2*1.60M*97.66kHz]3.2pF.

If instead we measure the monitoring measurement signal frequency as fMON= 90.32kHz and have RCOM=1.60M



and RINC=130k

 then CSENSOR=1/[2*(1.60M+130k)*90.32kHz]3.2pF. Thus the sensed input capacitance

CSENSOR can be defined either from reference or any monitoring measurement burst frequency.

Hint: When using oscilloscope it is better to measure period of at minimum ten or more oscillator signal pulses (N

pcs) to get better time resolution i.e. better clock frequency (fOSC) measurement accuracy. The oscillator signal

frequency can be calculated from equation 10 when TNOSC period of N pulses is measured.

�� =

��

Equation 10.

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