MAS6505 datasheet(12/44 Pages) MAS

12 / 44 page

DA6505.005

11 June 2018

12 (44)

FUNCTIONAL DESCRIPTION (continued)

 Analog front-end and ADC trimming

The

pressure

and

temperature

sensors

are

interfaced

with

analog

front-end

(AFE)

which

comprises of a low noise chopper amplifier with 32

selectable gain options from 1x up to 55.8x and an

input signal polarity selection. The amplified sensor

signal is then fed to input of the delta-sigma (

ΔΣ)

analog-to-digital converter (ADC). The input signal

range of the ratiometric ADC is ISRADC=1400 mV

at the nominal sensor supply voltage VDDS=1.68 V.

This corresponds to differential signal ranging from -

700mV to +700mV. The ADC offers eight input offset

options with both polarities to adjust to different

sensor signals.

For the best resolution and to avoid signal clipping

the amplifier gain and the ADC offset settings need

to be trimmed optimally for both the temperature and

the pressure sensor signals. In optimal cases the

signal ranges of amplified sensor signals cover

maximally the linear input signal range (ISRLIN) of

the ADC. The ISRLIN is 10%...90% of the ISR i.e.

ISRLIN = 80%*ISR = 1120 mV at VDDS=1.68 V. See

further details in chapters AFE AND ADC INPUT

SIGNAL RANGE DEFINITIONS and AFE GAIN AND

ADC OFFSET SELECTIONS.

MAS6505 has individual AFE and ADC trimming

settings

for

both

pressure

and

temperature

measurement. The AFE gain and polarity for

pressure is trimmed using Pressure gain trim register

(FC/7CHEX).

See

table

The

corresponding

EEPROM storage address is (BC/3CHEX). The AFE

gain and polarity for temperature is trimmed using

Temperature gain trim register (FD/7DHEX). See table

9. The corresponding EEPROM storage address is

(BD/3DHEX). The ADC offsets and polarities for

pressure

and

temperature are

trimmed using

Temperature and pressure offset trim register

(FE/7EHEX). See table

10. The corresponding

EEPROM storage address is (BE/3EHEX).

 Internal clock oscillator

MAS6505 has an internal clock oscillator making

external clock unnecessary. In the forced mode it is

turned on only during the A/D conversions and

turned off when the sleep mode is entered. In the

normal mode the internal clock oscillator is turned on

continuously

run

selected

measurements

periodically.

The internal oscillator frequency is factory trimmed

to 250 kHz using a 7-bit register. See table 11 for

Clock oscillator frequency trim register (FF/7FHEX).

The factory trimming value has been stored into

EEPROM

address

(BF/3FHEX).

Note:

recommended to not touch the factory trimming

value of the internal clock oscillator. The converter

runs from a divided system clock which is 125 kHz.

 EEPROM memory

The 512-bit (64 byte) EEPROM memory is available

for storing trimming and calibration data on chip. Six

of the EEPROM bytes are reserved for trimming

purposes. One byte is for selecting temperature

sensor and trimming sensor bridge resistance that is

used in sensor bridge resistance based temperature

measurements. Three bytes are for trimming AFE

gain and ADC offset in the pressure and temperature

measurements.

Two

EEPROM

addresses

are

reserved for storing factory trimming values of

EEPROM oscillator and internal clock oscillator.

These EEPROM addresses must be left untouched.

The remaining 464 bits (58 bytes) in addresses

80/00HEX

… B9/39HEX are free for other use such as

storing sensor calibration coefficients. See table 1.

After power on reset the EEPROM is by default

inactive. To use EEPROM (read or write) it needs to

be first activated. To prevent accidental overwriting

or erasing of the trimming and calibration memory

the EEPROM write requires an additional enable

setting. These EEPROM functions are controlled

using EEPROM control register (ED/6DHEX). See

table 2 for EON and EWE bit settings. When

activated the EEPROM draws typically 60µA current.

To save current the EEPROM should be always

disabled after use.

See also chapters EEPROM WRITE PROCEDURE

and EEPROM READ ONLY PROCEDURE.

 Calibration

MAS6505

performs

pressure

and temperature

measurements resulting raw measurement values.

Thus calibrated temperature and pressure reading

calculations need to be done outside in the host

system by utilizing calibration coefficients that are

stored into the EEPROM during sensor calibration

procedure. See additional DAE6505 document

regarding sensor system calibration.

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