Improving sampling accuracy

SAADC has multiple options to improve sample accuracy. Noise shaping modes provide the highest performance by enabling a delta-sigma configuration with analog filtering, oversampling, and digital filtering. Pure oversampling is also supported.

Noise shaping

Noise shaping is implemented using the ADC peripheral within a first-order delta-sigma loop. The output is decimated and subsequently filtered with FIR filters. The sampling rate is 1 Msps in noise shaping modes and high-resolution settings (RESOLUTION ≥ 12) are recommended. Enable noise shaping by configuring the register NOISESHAPE. Depending on the selected mode, the input signal must be bandwidth limited. See Electrical specification parameters f_BW,NS for details.

Noise shaping modes are configured using the register NOISESHAPE.

Table 1. Noise shaping modes
Mode	Description
0	Noise shaping is disabled.
NS1	Noise shaping and decimation by 8, giving a sample rate of 125 ksps.
NS2	Noise shaping and decimation by 32, giving a sample rate of 31.25 ksps. Recommended resolution setting is 14 bits.
NS3	Noise shaping with oversampling set by the register OVERSAMPLE.

Oversampling

Oversampling can improve the signal-to-noise ratio (SNR) by approximately 10log(OSR), where OSR is the oversampling ratio. Each oversampled result is the combination of the 2^OVERSAMPLE samples that have gone through an accumulate-and-average filter.

Oversampling and scanning can only be combined when burst mode is enabled. Without burst mode, oversampling and scanning will average across multiple input channels.

The register OVERSAMPLE controls the accumulator. The 2^OVERSAMPLE samples must be taken before the result is written to RAM. This can be achieved by either of the following:

Use the built-in SAADC local timer and the register SAMPLERATE to perform sampling.
Use the TIMER peripheral and DPPI to trigger the SAADC SAMPLE task to sample 2^OVERSAMPLE times at a fixed rate.
Trigger the SAMPLE task 2^OVERSAMPLE times from software.
Enable burst mode and trigger the SAMPLE task once.

Burst mode can be enabled to avoid manually triggering the SAMPLE task 2^OVERSAMPLE times. When burst is enabled, ADC automatically samples the input 2^OVERSAMPLE times consecutively, with an approximate timing of (t_ACQ + t_CONV) × 2^OVERSAMPLE. Besides extending the conversion time, it behaves like one-shot mode.

A DONE event indicates that a single sample has been acquired, while a RESULTDONE event indicates that enough samples have been gathered to transfer an oversampled result to RAM.

Chopping

Chopping is a technique used to cancel offset voltages and other low frequency errors, and is enabled in the register CH[n].CONFIG.CHOPPING. When chopping is enabled, the input to ADC is alternately inverted (chopped) at the input mux, and a conversion is performed for each phase of chop. Chopping is combined with oversampling and ADC output is passed through the accumulate and average filter used in oversampling mode.