DA6512.003
2 December, 2016
TRIMMING FOR SENSOR CAPACITANCE
MAS6512
has
two
different
capacitance
range minimum and maximum limits the average of
the ΔΣ-modulator output is 10% and 90%
respectively. In the following trimming equations we
denote these by
measurement modes. The output can be
proportional either to capacitance difference (CS-CR)
or to capacitance ratio (CS-CR)/CS.
For trimming it is necessary to know the sensor
capacitance CS range CS MIN…CS MAX. For optimal
utilization of the MAS6512 input range the trimming
is based on selecting the minimum linear range
capacitance same as CS MIN and maximum linear
range capacitance same as CS MAX. At the linear
DMIN = 0.1
DMAX = 0.9
In the capacitance measurement the internal
regulator should be disabled (TEMPREGEN=0) and
the external capacitive sensor selected (SEL=00).
MAS6512 in capacitance difference mode
The reference capacitor value CR is calculated from
CR = [CS MIN*(DMAX-0.5)-CS MAX*(DMIN-0.5)] / (DMAX-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 = [144mV*(DMAX-DMIN)*2*CREF] / (CS MAX-CS MIN
)
where CREF is nominally 6 pF, but also has ±10 % variation.
The gain register trim value is calculated from
REGE5HEX = [(VS-33 mV) / 734mV]*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*(0.9-0.5)-12pF*(0.1-0.5)]/(0.9-0.1) = 10pF
REGE4HEX = (10pF/22pF)*255 = 115.9 ~ 116
VS = [144mV*(0.9-0.1)*2*6pF] / (12pF-8pF) = 345.6mV
REGE5HEX = [(345.6-33 mV) / 734mV]*255 = 108.6 ~109
REGE3HEX = 0 (no internal CS capacitor matrix used)
22 (32)
MAS [ MICRO ANALOG SYSTEMS ]
