Re: ADC
2004-05-31 by Ned Konz
--- In AVR-Chat@yahoogroups.com, jay marante <jaythesis@y...> wrote: > im sorry for asking this question but i really have a hard time understanding bout the ADC. > > first, what should be connected to the AGND? why its not just the GND of the MCU? how bout the AVCC? can anyone explain me the basics of these pins? i already understand the operation, i just need the reasons for having these pins different from the VCC and AGND. thanks in advance. They are different because that way it gives a chance to reduce noise. By connecting the AGND to GND back at a single point (usually at the output filter of the regulator), you won't see ground noise in your measured signals. Likewise, the AVCC gives you a chance to reduce the noise. Read the datasheet, and search for AVCC. In the Tiny26L datasheet, it says: ------ AVCC is the supply voltage pin for Port A and the A/D Converter (ADC). It should be externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter. See page 77 for details on operating of the ADC. [p77] The ADC has an analog supply voltage pin, AVCC. The voltage on AVCC must not differ more than ±0.3V from VCC. See the paragraph ADC Noise Canceling Techniques on page 88 on how to connect these pins. [p88] 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 VCC supply voltage via an LC network as shown in Figure 52. 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. ------ Hope this helped.