105
ATtiny26(L)
1477G–AVR–03/05
Scanning Multiple
Channels
Since change of analog channel always is delayed until a conversion is finished, the
Free Running mode can be used to scan multiple channels without interrupting the con-
verter. Typically, the ADC Conversion Complete interrupt will be used to perform the
channel shift. However, the user should take the following fact into consideration:
The interrupt triggers once the result is ready to be read. In Free Running mode, the
next conversioin will start immediately when the interrupt triggers. If ADMUX is
changed after the interrupt triggers, the next conversion has already started, and the
old setting is used.
ADC Noise Canceling
Techniques
Digital circuitry inside and outside the ATtiny26(L) generates EMI which might affect the
accuracy of analog measurements. If conversion accuracy is critical, the noise level can
be reduced by applying the following techniques:
1.
The analog part of the ATtiny26(L) and all analog components in the application
should have a separate analog ground plane on the PCB. This ground plane is
connected to the digital ground plane via a single point on the PCB.
2.
Keep analog signal paths as short as possible. Make sure analog tracks run over
the analog ground plane, and keep them well away from high-speed switching
digital tracks.
3.
The AVCC pin on the ATtiny26(L) should be connected to the digital V
CC
supply
voltage via an LC network as shown in Figure 57.
4.
Use the ADC noise canceler function to reduce induced noise from the CPU.
5.
If some pins are used as digital outputs, it is essential that these do not switch
while a conversion is in progress in that port.
Offset Compensation
Schemes
The gain stage has a built-in offset cancellation circuitry that nulls the offset of differen-
tial measurements as much as possible. The remaining offset in the analog path can be
measured directly by selecting the same channel for both differential inputs. This offset
residue can be then subtracted in software from the measurement results. Using this
kind of software based offset correction, offset on any channel can be reduced below
one LSB.
