DA6505.005
11 June 2018
AFE AND ADC INPUT SIGNAL RANGE DEFINITIONS (continued)
Pressure signal settings: OFSP%=+35%, GAINP=+28.6
VINMIN=1400mV*(-0.5+35%/100%)/28.6 -7.3mV
VINMAX=1400mV*(0.5+35%/100%)/28.6 +41.6mV
VINLINMIN=1400mV*(-0.5*80%+35%/100%)/28.6 --2.4mV
VINLINMAX=1400mV*(0.5*80%+35%/100%)/28.6 +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.
1
2
VIN GAIN OFS%
Equation 10
CODE = CODEFS
+
−
ISRADC
100%
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.
ISRADC
GAIN
CODE
1
2
OFS%
Equation 11
VIN =
−
+
CODEFS
100%
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-
linearity or even signal clipping. The optimal offset (OFS%OPT) and gain (GAINOPT) values are values which
maximize resolution while keeping the amplified signal within the ISRLINADC. To be able to determine these
values the sensor signal minimum (VSMIN) and maximum (VSMAX) values need to be known.
The optimal GAIN and OFS% values can be calculated using equations 12 and 13.
VSMAX +VSMIN
2 VSMAX −VSMIN
1
Equation 12
OFS%OPT
=
100%
80% ISRADC
VSMAX −VSMIN
Equation 13
GAINOPT
=
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.
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MAS [ MICRO ANALOG SYSTEMS ]
