Data Sheet
ISM14585-L35 Specification
DOC-DS-14585-201807-3.0
Confidential Inventek Systems
Page 79
9.14.6 ZERO-SCALE ADJUSTMENT
The GP_ADC_OFFP and GP_ADC_OFFN registers can also be used to set the zero-
scale or full-scale input level at a certain target value. For instance, they can be used to
calibrate GP_ADC_RESULT to 0x000 at an input voltage of exactly 0.0 V, or to calibrate
the zero scale of a sensor.
9.14.7 COMMON MODE ADJUSTMENT
The common mode level of the differential signal must be 0.6 V (or 1.8 V with
GP_ADC_ATTN3X = 1). If the common mode input level of 0.6 V cannot be achieved,
the common mode level of the GP_ADC can be adjusted (the GP_ADC can tolerate a
common mode margin up to 50 mV) according to below table.
CM Voltage (Vcmm)
GP_ADC_OFFP = GP_ADC_OFFN
0.3 V
0x300
0.6 V
0x200
0.9 V
0x100
Common Mode Adjustment
Any other common mode level between 0.0 V and 1.2 V can be calculated from the
table above. Offset calibration can be combined with common mode adjustment by
replacing the 0x200 value in the offset calibration routine by the value required to get
the appropriate common mode level.
Note: The input voltage limits for the ADC in differential modes are: -1.3 V to +1.3 V (for
GP_ADC_ATTN3X = 0, see Table for GPADC Input Channels and Voltage Scale). The
differential input range of the ADC is: -1.2 V < V[P0_0,P0_1] < +1.2 V. Therefore, if
Vcmm < 0.5 V or Vcmm > 0.7 V, the input can no longer cover the whole ADC range.
9.14.8 INPUT IMPEDANCE, INDUCTANCE, AND INPUT SETTLING
The GPADC has no input buffer stage. During sampling phase a capacitor of 0.2 pF is
switched to the input line. The precharge of this capacitor is at mid-scale level so the
input impedance is infinite. At 100 ksample/s, zero or full-scale single-ended input
signal, this sampling capacitor will load the input with:
ILOAD = V * C * fS = ±0.6 V * 0.2 pF * 100 kHz = ±12 nA (differential: ±1.2 V * 0.2 pF *
100 kHz = ±24 nA at both pins).
During sampling phase a certain settling time is required. A 10-bit accuracy requires at
least 7 time constants of the output impedance of the input signal source and the 0.2 pF