DA6505.005

11 June 2018

35 (44)

AFE AND ADC INPUT SIGNAL RANGE DEFINITIONS (continued)

Pressure signal settings: OFSP%=+35%, GAINP=+28.6

 -7.3mV

 +41.6mV

 --2.4mV

 +36.7mV

The digital A/D conversion result (CODE) is an unsigned 24-bit number between 0 and 11184810. The maximum

A/D conversion code value 11184810 is referred here as CODEFS. The linear input signal range codes range

from 10% to 90% of CODEFS i.e. from 1118481 to 10066329. Thus it is possible to see from the A/D conversion

result whether or not the input signal is within the linear input signal range of the ADC (ISRLINADC). The A/D

conversion result (CODE) depends on the input signal (VIN) as follows.









−



+



=

%

100

%

2

1

OFS

ISRADC

GAIN

VIN

CODEFS

CODE

Equation 10

where

CODEFS = 11184810 A/D-converter maximum code (minimum code is zero)

ISRADC = 1400mV Overall input signal range of ADC

GAIN = ±(1x .. 55.8x) AFE gain setting

OFS% = -49% .. +49% ADC offset setting in percentage

The corresponding input signal voltage (VIN) can be calculated from the A/D conversion result (CODE) as follows.













+

−



=

%

100

%

2

1

OFS

CODEFS

CODE

GAIN

ISRADC

VIN

Equation 11

Examples

VIN=+20mV, GAIN=+16, %OFS=+21%:

CODE=11184810*(0.5+20mV*16/1400mV-21%/100%)+)

5800123

CODE=5160991, GAIN=+16, %OFS=+21%:

VIN=(1400mV/16)*11184810*(5160991/11184810-0.5+21%/100%)

+15.0mV

AFE GAIN AND ADC OFFSET SELECTIONS

The AFE gain (GAIN) and ADC offset (OFS%) settings should be chosen such a way that the sensor signal (VS)

stays in the linear input signal range of the ADC (ISRLINADC) in all conditions to avoid inaccuracies due to non-

maximize resolution while keeping the amplified signal within the ISRLINADC. To be able to determine these

The optimal GAIN and OFS% values can be calculated using equations 12 and 13.

%

100

2

1

%



−

+



=

MIN

MAX

MIN

MAX

OPT

VS

VS

VS

VS

OFS

Equation 12

MIN

MAX

OPT

VS

VS

ISRADC

GAIN

−



=

%

80

Equation 13

With these OFS% and GAIN values the sensor signal is fit perfectly into the linear range of the ADC. In practice

the MAS6505 supports discrete OFS% and GAIN options so that rounding to closest values are necessary. Also

this ideal calculation may lead to out of range OFS% and/or GAIN values that are not supported by the MAS6505.

In this case the search for the best values that maximize the gain setting but also keep the signal within the linear

input signal range requires more steps.