DA1641.004
1 February, 2019
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).
1
ꢌ
=
Equation 6.
ꢃ퐸퐹
ꢏ
ꢐ
푆ꢅꢉ푆ꢈꢄ
2∙ ꢃ퐶푂푀ꢋꢃ퐼푁퐶 ∙퐶
1
ꢌ푀푂푁
=
Equation 7.
ꢏ
ꢐ
푆ꢅꢉ푆ꢈꢄ
2∙ ꢃ퐶푂푀ꢋꢃ퐷퐸퐶 ∙퐶
Either from equation 6 or 7 we can solve the sensor capacitance value CSENSOR as follows.
1
ꢊꢑ퐸푁ꢑ푂ꢃ
ꢊꢑ퐸푁ꢑ푂ꢃ
=
=
Equation 8.
ꢏ
ꢐ
ꢄꢅꢆ
2∙ ꢃ퐶푂푀ꢋꢃ퐼푁퐶 ∙푓
1
Equation 9.
ꢏ
ꢐ
ꢇꢈꢉ
2∙ ꢃ퐶푂푀ꢋꢃ퐷퐸퐶 ∙푓
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.
푁
ꢌ
푂ꢑ퐶
=
Equation 10.
푇
ꢉꢈ푆ꢀ
9 (14)
MAS [ MICRO ANALOG SYSTEMS ]
