Data Sheet

ISM14585-L35 Specification

DOC-DS-14585-201807-3.0

Confidential Inventek Systems

Page 78

With bit GP_ADC_CTRL_REG[GP_ADC_MUTE], the ADC input is switched to the

center scale input level, so the ADC result ideally is 511.5. If instead a value of 515 is

observed, the output offset is +3.5 (adc_off_p = 3.5).

With bit GP_ADC_CTRL_REG[GP_ADC_SIGN] the sign of the ADC input and output is

changed. Two sign changes have no effect on the signal path, though the sign of the

ADC offset will change.

If adc_off_p = 3.5 the ADC_result with opposite GP_ADC_SIGN will be 508. The sum of

these equals 515 + 508 = 1023. This is the mid-scale value of an 11-bit ADC, so one

extra bit due to the over-sampling by a factor of two.

The LSB of this 11-bit word should be ignored if a 10-bit word is preferred. In that case

the result is 511.5, so the actual output value will be 511 or 512.

9.14.5 OFFSET CALIBRATION

A relative high offset caused by a very small dynamic comparator (up to 20 mV, so

approximately 20 LSB). This offset can be cancelled with the chopping function, but it

still causes unwanted saturation effects at zero scale or full scale. With the

GP_ADC_OFFP and GP_ADC_OFFN registers the offset can be compensated in the

ADC network itself.

To calibrate the ADC follow the steps in table below:

GPADC Calibration Procedure for Single-Ended and Differential Modes

Step

Single-Ended Mode (GP_ADC_SE = 1)

Differential Mode (GP_ADC_SE = 0)

Set GP_ADC_OFFP = GP_ADC_OFFN=0x200;

GP_ADC_MUTE = 0x1; GP_ADC_SIGN = 0x0

Set GP_ADC_OFFP=GP_ADC_OFFN = 0x200;

GP_ADC_MUTE = 0x1; GP_ADC_SIGN = 0x0

Start conversion

adc_off_p = GP_ADC_RESULT - 0x200

Set GP_ADC_SIGN = 0x1

Start conversion

adc_off_n = GP_ADC_RESULT - 0x200

GP_ADC_OFFP = 0x200 - 2*adc_off_p

GP_ADC_OFFN = 0x200 - 2*adc_off_n

(Note 4)

GP_ADC_OFFP = 0x200 - adc_off_p

GP_ADC_OFFN = 0x200 - adc_off_n

(Note 4)

Note 4 The average of GP_ADC_OFFP and GP_ADC_OFFN should be 0x200 (with a margin of 20

LSB). It is recommended to implement the above calibration routine during the initialization

phase of the ISM14585-L35. To verify the calibration results, check whether the

GP_ADC_RESULT value is close to 0x200 while bit GP_ADC_MUTE = 1.