L9966 datasheet(36/122 Pages) STMICROELECTRONICS

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8.2

ADC2: resistance measurement

The device implements a dedicated function to perform high precision resistance measurements connected

between analog input pin IO[12:1] and ground. In Figure 17 a simplified circuit is reported..

Figure 17. Simplified circuit for resistance measurement

GADG0801180852PS

A wide measurement range is allowed by selecting the pull-up external reference resistors.

Digital out is RPD/RRx represented over 15 bits fixed point number (4 bits integer, 11 bits fractional). The result

from the SPI register needs then to be divided by 2048.

RPD=ADC2_RESULT

2048 *RRx

In order to reach the best accuracy, calibration of ADC, both ADC1 and ADC2, has to be activated by setting

CALIB_SEL to ‘1’. Calibration feature is guaranteed in case calibration fault CALIB_FLT flag in GEN_STATUS

By default the calibration is disabled (CALIB_SEL = ‘0’): the status of the CALIB_FLT flag should be verified, then

the calibration eventually enabled or kept disabled accordingly.

Depending on the selected external pull-up resistance RRx, a specific settling time has to be considered (see

Table 23. ADC2 parameters). Bit CT_PUx[3:0] in ADC_TIMING register can be used to set the proper settling

time for each pull up resistance selected (RRx).

Best accuracy is achieved when RPD/RRx is close to 1.

Table 23. ADC2 parameters

Symbol

Parameter

Condition

Min Typ Max Unit

UBSW = 7.5 V:36 V, VDD5V = 4.85 V:5.15 V, Tj-max = 150 °C unless otherwise specified.

RMEASrange

Resistance measurement range -

400K

RMEASacc_L

Total resistance measurement

accuracy

0.1<RPD/RRx<8

50 to 2K, RRx = 470 Ω Excl. external pull-up RRx

precision No ratiometric comparator running

CALIB_FLT=0 and CALIB_SEL=1

1.5

RMEASacc_M

Total resistance measurement

accuracy

0.1<RPD/RRx<8

2K to 30K, RRx = 9 kΩ Excl. external pull-up RRx

precision No ratiometric comparator running

CALIB_FLT=0 and CALIB_SEL=1

1.5

L9966

ADC2: resistance measurement

DS12459 - Rev 10

page 36/122

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