12.3.9 12-Bit Mode
When configured for 12-bit conversions, the ADC performs four 10-bit conversions using four different reference voltages and combines
the results into a single 12-bit value. Unlike simple averaging techniques, this method provides true 12-bit resolution of ac or dc input
signals without depending on noise to provide dithering. The converter also employs a hardware dynamic element matching algorithm
that reconfigures the largest elements of the internal DAC for each of the four 10-bit conversions. This reconfiguration cancels any
matching errors and enables the converter to achieve 12-bit linearity performance to go along with its 12-bit resolution.
The 12-bit mode is enabled by setting the AD12BE bit in register ADC0AC to logic 1 and configuring the ADC in burst mode (ADBMEN
= 1) for four or more conversions. The conversion can be initiated using any of the conversion start sources, and the 12-bit result will
appear in the ADC0H and ADC0L registers. Since the 12-bit result is formed from a combination of four 10-bit results, the maximum
output value is 4 x (1023) = 4092, rather than the max value of (2^12 – 1) = 4095 that is produced by a traditional 12-bit converter. To
further increase resolution, the burst mode repeat value may be configured to any multiple of four conversions. For example, if a repeat
value of 16 is selected, the ADC0 output will be a 14-bit number (sum of four 12-bit numbers) with 13 effective bits of resolution.
The AD12SM bit in register ADC0TK controls when the ADC will track and sample the input signal. When AD12SM is set to 1, the
selected input signal will be tracked before the first conversion of a set and held internally during all four conversions. When AD12SM is
cleared to 0, the ADC will track and sample the selected input before each of the four conversions in a set. When maximum throughput
(180-200 ksps) is needed, it is recommended that AD12SM be set to 1 and ADTK to 0x3F, and that the ADC be placed in always-on
mode (ADEN = 1). For sample rates under 180 ksps, or when accumulating multiple samples, AD12SM should normally be cleared to
0, and ADTK should be configured to provide the appropriate settling time for the subsequent conversions.
EFM8UB3 Reference Manual
Analog-to-Digital Converter (ADC0)
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