DA6505.005
11 June 2018
AFE AND ADC INPUT SIGNAL RANGE DEFINITIONS
The MAS6505 has PI and NI input pins which allow measuring differential sensor bridge signal with both positive
and negative polarities. The differential input signal VIN is defined as voltage difference between positive (PI) and
negative (NI) input pin voltages VPI and VNI respectively.
VIN = VPI −VNI
Equation 2
Overall minimum and maximum differential input signals are defined by AFE GAIN and ADC OFS% settings as
follows.
1
2
OFS%
Equation 3
VINMIN = ISRADC −
+
+
/GAIN
/GAIN
100%
1
2
OFS%
Equation 4
VINMAX = ISRADC +
100%
where
ISRADC = 1400mV Overall input signal range of ADC
OFS% = -49% .. +49% ADC offset setting in percentage
GAIN = ±(1x .. 55.8x) AFE gain setting
Overall input signal range (ISR) of the MAS6505 is difference between maximum and minimum allowed
differential input signals.
ISRADC
Equation 5
ISR = VINMAX −VINMIN
=
GAIN
Signal clipping occurs for signals beyond VINMIN and VINMAX limits and exceeding the ISR value.
The ISRADC corresponds to overall input signal range of the ADC but from which 80% is considered linear
(ISRLINADC) and suitable for accurate measurements.
Equation 6
ISRLINADC = 80% ISRADC
The AFE gain and ADC offset settings should be chosen so that minimum and maximum sensor signal voltages
are within the linear input signal range of the MAS6505. The minimum (VNLINMIN) and maximum (VINLINMAX
differential input signals in the linear input signal range are as follows.
)
80% OFS%
Equation 7
VINLINMIN == ISRADC −
+
/ GAIN
2
100%
80% OFS%
Equation 8
VINLINMAX == ISRADC +
+
/GAIN
2
100%
Linear input signal range (ISRLIN) is difference between maximum and minimum allowed differential input signals
which are in the linear signal range of the MAS6505. Signal distortion occurs beyond this signal range due to
increased non-linearity.
ISRLINADC 80% ISRADC
Equation 9
ISRLIN =VINLIN MAX −VINLIN MIN
=
=
GAIN
GAIN
Signal distortion occurs for signals beyond VINLINMIN and VINLINMAX limits and exceeding the ISRLIN value due
to increased non-linearity.
Example
Temperature signal settings: OFST%=-7%, GAINT=+18.9
VINMIN =1400mV*(-0.5+(-7%)/100%)/18.9 -42.2mV
VINMAX=1400mV*(0.5+(-7%)/100%)/18.9 +31.9 mV
VINLINMIN =1400mV*(-0.5*80%+(-7%)/100%)/18.9 -34.8mV
VINLINMAX=1400mV*(0.5*80%+(-7%)/100%)/18.9 +24.4mV
34 (44)
MAS [ MICRO ANALOG SYSTEMS ]
