Differential Gain Channels

When using differential gain channels, certain aspects of the conversion need to be taken into consideration. Note that the differential channels should not be used with an AREF < 2V.

Differential conversions are synchronized to the internal clock CKADC2 equal to half the ADC clock. This synchronization is done automatically by the ADC interface in such a way that the sample-and-hold occurs at a specific phase of CKADC2. A conversion initiated by the user (that is, all single conversions, and the first free running conversion) when CKADC2 is low will take the same amount of time as a single ended conversion (13 ADC clock cycles from the next prescaled clock cycle). A conversion initiated by the user when CKADC2 is high will take 14 ADC clock cycles due to the synchronization mechanism. In Free Running mode, a new conversion is initiated immediately after the previous conversion completes, and since CKADC2 is high at this time, all automatically started (that is, all but the first) free running conversions will take 14 ADC clock cycles.

The gain stage is optimized for a bandwidth of 4kHz at all gain settings. Higher frequencies may be subjected to non-linear amplification. An external low-pass filter should be used if the input signal contains higher frequency components than the gain stage bandwidth. Note that the ADC clock frequency is independent of the gain stage bandwidth limitation. For example, the ADC clock period may be 6 μs, allowing a channel to be sampled at 12kSPS, regardless of the bandwidth of this channel.

If differential gain channels are used and conversions are started by Auto Triggering, the ADC must be switched off between conversions. When Auto Triggering is used, the ADC prescaler is reset before the conversion is started. Since the gain stage is dependent of a stable ADC clock prior to the conversion, this conversion will not be valid. By disabling and then re-enabling the ADC between each conversion (writing ADEN in ADCSRA to “0” then to “1”), only extended conversions are performed. The result from the extended conversions will be valid. See Prescaling and Conversion Timing section