DA6512.003
2 December, 2016
TRIMMING FOR SENSOR CAPACITANCE
MAS6512 in capacitance ratio mode
The reference capacitor CR is calculated from
CR = [CS MIN*CS MAX*(DMAX-DMIN)] / (CS MAX*DMAX-CS MIN*DMIN
)
If an external CR is used, it is connected between pins CR and CC. If an internal CR is used, the trim code for CR
is calculated from
REGE4HEX = (CR/CR MAX)*255
where CR MAX is nominally 22pF, but subject to ±10 % (±3 sigma) process variation.
The reference voltage, VS, can be calculated using the following equation:
VS = 100.8mV * (CS MAX-CS MIN) / (CS MAX*DMAX-CS MIN*DMIN
The gain register trim value is calculated from
REGE5HEX = (VS / 133.3mV)*255
)
REGE4HEX and REGE5HEX are 8-bit values, so they range from 0 to 255. When their values are found, the same
values can be written to corresponding EEPROM addresses C4HEX and C5HEX. However, with SPI bus, the
address MSB in write operation is 0, so the addresses are actually 44HEX and 45HEX
.
Example: Single capacitance sensor
CS MIN=8pF
CS MAX=12pF
CR = [8pF*12pF*(0.9-0.1)] / (12pF*0.9-8pF*0.1) = 7.68pF
REGE4HEX = (7.68pF/22pF)*255 = 89.0 ~ 89
VS = 100.8mV*)/(12pF-8pF)/(12pF*0.9-8pF*0.1)=40.32mV
REGE5HEX = (40.32mV/133.3mV)*255 = 77.1 ~ 77
REGE3HEX = 0 (no internal CS capacitor matrix used)
23 (32)
MAS [ MICRO ANALOG SYSTEMS ]
