MAS6512 datasheet(22/32 Pages) MAS

22 / 32 page

DA6512.003

2 December, 2016

22 (32)

TRIMMING FOR SENSOR CAPACITANCE

MAS6512

has

two

different

capacitance

measurement

modes.

The

output

can

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

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

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)

MAS6512 数据表(PDF) 22 Page - Micro Analog systems

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