SAADC — Successive approximation analog-to-digital converter

The ADC supports up to four external analog inputs. It can be operated in One-shot mode with sampling under software control, or Continuous mode with a programmable sampling rate.

Internally, the ADC is always a differential analog-to-digital converter, but by default it is configured with single-ended input in the MODE field of the CH[n].CONFIG register. In single-ended mode, the negative input will be shorted to ground internally.

The assumption in single-ended mode is that the internal ground of the ADC is the same as the external ground that the measured voltage is referred to. The ADC is thus sensitive to ground bounce on the PCB in single-ended mode. If this is a concern we recommend using differential measurement.

The output result of the ADC depends on the settings in the CH[n].CONFIG and RESOLUTION registers as follows:

RESULT = [V(P) – V(N) ] * 1/REFERENCE * 2(RESOLUTION - m)

where

V(P)

is the voltage at input P

V(N)

is the voltage at input N

m

is the mode setting. Use m=0 if CONFIG.MODE=SE, or m=1 if CONFIG.MODE=Diff

REFERENCE

is the selected reference voltage

The result generated by the ADC will deviate from the expected due to DC errors like offset, gain, differential non-linearity (DNL), and integral non-linearity (INL). See Electrical specification for details on these parameters. The result can also vary due to AC errors like non-linearities in the GAIN block, settling errors due to high source impedance and sampling jitter. For battery measurement, the DC errors are most noticeable.

The ADC has a temperature dependent offset. If the ADC is to operate over a large temperature range, we recommend running CALIBRATEOFFSET at regular intervals. The CALIBRATEDONE event will be generated when the calibration has been completed. Note that the DONE and RESULTDONE events will also be generated.

Up to four analog input channels, CH[n](n=0..3), can be configured.

See Shared resources for shared input with comparators.

Any one of the available channels can be enabled for the ADC to operate in one-shot mode. If more than one CH[n] is configured, the ADC enters scan mode.

An analog input is selected as a positive converter input if CH[n].PSELP is set, setting CH[n].PSELP also enables the particular channel.

An analog input is selected as a negative converter input if CH[n].PSELN is set. The CH[n].PSELN register will have no effect unless differential mode is enabled and CH[n].PSELP is set, see MODE field in CH[n].CONFIG register.

The ADC input configuration supports several modes of sampling.

• One-shot, one channel
• One-shot, scan (one sample for each channel)
• Continuous, one channel
• Continuous, scan

1. The ADC must be enabled and started via the ENABLE register and START task.
2. At least one channel must be enabled (via the CH[n].CONFIG registers).
3. Now the ADC can be sampled, by triggering the SAMPLE task.

The ADC indicates a single ongoing conversion via the register STATUS. During scan, oversampling, or continuous modes, multiple conversions take place in the ADC. As a result, the value in STATUS register toggles at the end of each single conversion.

One-shot operation is configured by enabling only one of the available channels defined by CH[n].PSELP, CH[n].PSELN, and CH[n].CONFIG registers.

Upon a SAMPLE task, the ADC powers up and starts to sample the input voltage. The CH[n].CONFIG.TACQ controls the acquisition time.

The time it takes to perform the first sample is t_PWRUP + t_ACQ + t_CONV, where t_ACQ is the acquisition time, t_CONV is the conversion time, and t_PWRUP is the time it takes to power up the ADC.

A DONE event signals that one sample has been taken.

In this mode, the RESULTDONE event has the same meaning as DONE when no oversampling takes place. Note that both events may occur before the actual value has been transferred into RAM by EasyDMA. For more information, see EasyDMA.

Continuous sampling can be achieved by using the internal timer in the ADC, or triggering the SAMPLE task from one of the general purpose timers through the PPI system.

tSAMPLE > tACQ + tCONV

The SAMPLERATE register can be used as a local timer instead of triggering individual SAMPLE tasks. When SAMPLERATE.MODE is set to Timers, it is sufficient to trigger SAMPLE task only once in order to start the SAADC and triggering the STOP task will stop sampling. The SAMPLERATE.CC field controls the sample rate.

A DONE event signals that one sample has been taken.

In this mode, the RESULTDONE event has the same meaning as DONE when no oversampling takes place. Note that both events may occur before the actual value has been transferred into RAM by EasyDMA.

A channel is considered enabled if CH[n].PSELP is set. If more than one channel, CH[n], is enabled, the ADC enters scan mode.

In scan mode, one SAMPLE task will trigger one conversion per enabled channel. The time it takes to sample all channels is:

Total time < Sum(CH[x].tACQ+tCONV), x=0..enabled channels

A DONE event signals that one channel has been sampled.

In this mode, the RESULTDONE event comes after each sample, that is, once for each channel.

When the high-speed option is enabled, the the AIN0 (positive) and AIN1 (negative) pins can be used to sample signals with bandwidth up to 500 kHz.

High-speed mode is enabled by the HIGHSPEED field of the CH[n].CONFIG register.

The other channels (AIN2 and AIN3 ) cannot be used in high-speed mode.

CHOPPING is not available in high-speed mode.

After configuring RESULT.PTR and RESULT.MAXCNT, the ADC resources are started by triggering the START task. The ADC is using EasyDMA to store results in a Result buffer in RAM.

The Result buffer is located at the address specified in the RESULT.PTR register. The RESULT.PTR register is double-buffered and it can be updated and prepared for the next START task immediately after the STARTED event is generated. The size of the Result buffer (in bytes) is specified in the RESULT.MAXCNT register and the ADC will generate an END event when it has filled up the Result buffer, see ADC. Results are stored in little-endian byte order in Data RAM. Every sample will be sign extended to 16 bit before stored in the Result buffer.

The ADC is stopped by triggering the STOP task. The STOP task will terminate an ongoing sampling. The ADC will generate a STOPPED event when it has stopped. If the ADC is already stopped when the STOP task is triggered, the STOPPED event will still be generated.

If the RESULT.PTR is not pointing to a RAM region accessible from the peripheral, an EasyDMA transfer may result in a HardFault and/or memory corruption. See Memory for more information about the different memory regions.

The EasyDMA will have finished accessing the RAM when the END or STOPPED event has been generated.

The RESULT.AMOUNT register can be read following an END event or a STOPPED event to see how many bytes have been transferred to the Result buffer in RAM since the START task was triggered.

In scan mode, SAMPLE tasks can be triggered once the START task is triggered. The END event is generated when the number of samples transferred to memory reaches the value specified by RESULT.MAXCNT. After an END event, the START task needs to be triggered again before new samples can be taken.For more information about the scan mode, see Scan mode.

The ADC can use different reference voltages VREF, controlled in the REFSEL field of the CH[n].CONFIG register.

These are:

• Internal reference, VREF = 1.024V
• External reference (0.4 V up to VDD) , VREF provided by the EXTREF pin
• VDD

The AIN0-AIN3 inputs cannot exceed VDD, or be lower than VSS. The input ranges are also limited by the REFERENCE used. The condition

 [V(P) - V(N) ] * 1/REFERENCE <= 1 

must always hold true for valid measurements, otherwise the ADC will saturate and report the max value determined by the RESOLUTION.

To sample the input voltage, the ADC connects a capacitor to the input.

For illustration, see the following figure. The acquisition time indicates how long the capacitor is connected, see TACQ field in CH[n].CONFIG register. The required acquisition time depends on the source (R_source) resistance. For high source resistance the acquisition time should be increased, see Acquisition time.

A channel can be event monitored by configuring limit register CH[n].LIMIT.

If the conversion result is higher than the defined high limit, or lower than the defined low limit, the appropriate event will be generated.

Note that when setting the limits, CH[n].LIMIT.HIGH shall always be higher than or equal to CH[n].LIMIT.LOW. In other words, an event can be generated only when the input signal has been sampled outside of the defined limits. It is not possible to fire an event when the input signal is inside a defined range by swapping high and low limits.

The comparison to limits always takes place, there is no need to enable it. If comparison is not required on a channel, the software shall simply ignore the related events. In that situation, the value of the limits registers is irrelevant, so it does not matter if CH[n].LIMIT.LOW is lower than CH[n].LIMIT.HIGH or not.

Clock jitter, affecting sample timing accuracy, and circuit noise can affect ADC performance.

Jitter can be between START tasks or from START task to acquisition. START timer accuracy and startup times of regulators and references will contribute to variability. Sources of circuit noise may include CPU activity and the DC/DC regulator. Best ADC performance is achieved using START timing based on the TIMER module, HFXO clock source, and Constant Latency mode.