STM32W108C8

STM32W108C8
Analog-to-digital converter
Table 108.
Typical ADC input configurations (continued)
ADC P input ADC N input ADC_MUXP ADC_MUXN Purpose
ADC5 ADC4 5 4 Differential
GND VREF/2 8 9 Calibration
VREF VREF/2 10 9 Calibration
VDD_PADSA/2 VREF/2 11 9 Calibration
Input range
ADC inputs can be routed through input buffers to expand the input voltage range. The input
buffers have a fixed 0.25 gain and the converted data is scaled by that factor.
With the input buffers disabled the single-ended input range is 0 to VREF and the differential
input range is -VREF to +VREF. With the input buffers enabled the single-ended range is 0
to VDD_PADS and the differential range is -VDD_PADS to +VDD_PADS.
The input buffers are enabled for the ADC P and N inputs by setting the ADC_HVSELP and
ADC_HVSELN bits respectively, in the ADC_CFG register. The ADC accuracy is reduced
when the input buffer is selected.
Sample time
ADC sample time is programmed by selecting the sampling clock and the clocks per
sample.
● The sampling clock may be either 1 MHz or 6 MHz. If the ADC_1MHZCLK bit in the
ADC_CFG register is clear, the 6 MHz clock is used; if it is set, the 1 MHz clock is
selected. The 6 MHz sample clock offers faster conversion times but the ADC
resolution is lower than that achieved with the 1 MHz clock.
● The number of clocks per sample is determined by the ADC_PERIOD bits in the
ADC_CFG register. ADC_PERIOD values select from 32 to 4096 sampling clocks in
powers of two. Longer sample times produce more significant bits. Regardless of the
sample time, converted samples are always 16-bits in size with the significant bits leftaligned
within the value.
Table 109 shows the options for ADC sample times and the significant bits in the conversion
results.
Table 109.
ADC sample times
ADC_PERIOD Sample
clocks
Sample time (µs)
Sample frequency (kHz)
1 MHz clock 6 MHz clock 1 MHz clock 6 MHz clock
Significant
bits
0 32 32 5.33 31.3 188 5
1 64 64 10.7 15.6 93.8 6
2 128 128 21.3 7.81 46.9 7
3 256 256 42.7 3.91 23.4 8
4 512 512 85.3 1.95 11.7 9
5 1024 1024 170 0.977 5.86 10
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