AREF bypass capacitance on ATMega2560?

Folks,

What's the usual capacitance? Any stability issues there? I was planning
on using a 1uF X7R ceramic cap on the AREF pin of an ATMega2560, in
order to be able to use its internal bandgap reference. I saw people
using 0.1uF and 0.47uF. The datasheet is silent about stuff like that,
as usual.

-- 
Regards, Joerg

http://www.analogconsultants.com/

Joerg <invalid@invalid.invalid> writes:

> Folks,
>
> What's the usual capacitance? Any stability issues there? I was planning
> on using a 1uF X7R ceramic cap on the AREF pin of an ATMega2560, in
> order to be able to use its internal bandgap reference. I saw people
> using 0.1uF and 0.47uF. The datasheet is silent about stuff like that,
> as usual.

MCU manufacturers are so generally so clueless with respect to the
analog aspects... I would just experiment with a devkit if at all
worried.


-- 

John Devereux

On 8/19/2013 3:14 PM, Joerg wrote:

 > Folks,
 > What's the usual capacitance?

0.1uF

 > Any stability issues there?

Nothing special.

 > I was planning
 > on using a 1uF X7R ceramic cap on the AREF pin of an ATMega2560, in
 > order to be able to use its internal bandgap reference.

Do not. Use Vcc as reference.
Internal reference is inaccurate.

 > I saw people
 > using 0.1uF and 0.47uF. The datasheet is silent about stuff like that,
 > as usual.

It doesn't really matter.

Vladimir Vassilevsky
DSP and Mixed Signal Designs
www.abvolt.com

Vladimir Vassilevsky wrote:
> On 8/19/2013 3:14 PM, Joerg wrote:
>
>  > Folks,
>  > What's the usual capacitance?
>
> 0.1uF
>
>  > Any stability issues there?
>
> Nothing special.
>
>  > I was planning
>  > on using a 1uF X7R ceramic cap on the AREF pin of an ATMega2560, in
>  > order to be able to use its internal bandgap reference.
>
> Do not. Use Vcc as reference.
> Internal reference is inaccurate.

I noticed that awhile ago.  The specs on my
LDO regulator were about 5x better than the
specs on the internal "reference".

Vladimir Vassilevsky wrote:
> On 8/19/2013 3:14 PM, Joerg wrote:
> 
>> Folks,
>> What's the usual capacitance?
> 
> 0.1uF
> 
>> Any stability issues there?
> 
> Nothing special.
> 
>> I was planning
>> on using a 1uF X7R ceramic cap on the AREF pin of an ATMega2560, in
>> order to be able to use its internal bandgap reference.
> 
> Do not. Use Vcc as reference.
> Internal reference is inaccurate.
> 

Yeah, sure looks ghastly. I provided my own reference from another board
and filtered it with 0.1uF.


>> I saw people
>> using 0.1uF and 0.47uF. The datasheet is silent about stuff like that,
>> as usual.
> 
> It doesn't really matter.
> 

However, not everyone knows that because they don't say much about the
innards of the chip. It's my first ATMega case, or maybe the 2nd.

-- 
Regards, Joerg

http://www.analogconsultants.com/

"Joerg" <invalid@invalid.invalid> wrote in message 
news:b7l1blFlu93U2@mid.individual.net...
>>> I saw people
>>> using 0.1uF and 0.47uF. The datasheet is silent about stuff like that,
>>> as usual.
>>
>> It doesn't really matter.
>>
>
> However, not everyone knows that because they don't say much about the
> innards of the chip. It's my first ATMega case, or maybe the 2nd.

Finally gave up and bit the uC bullet?  ;-)

Tim

-- 
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com 

Tim Williams wrote:
> "Joerg" <invalid@invalid.invalid> wrote in message 
> news:b7l1blFlu93U2@mid.individual.net...
>>>> I saw people
>>>> using 0.1uF and 0.47uF. The datasheet is silent about stuff like that,
>>>> as usual.
>>> It doesn't really matter.
>>>
>> However, not everyone knows that because they don't say much about the
>> innards of the chip. It's my first ATMega case, or maybe the 2nd.
> 
> Finally gave up and bit the uC bullet?  ;-)
> 

Sometimes you need it. A while ago I even had a switcher design that
would have been totally impossible to do without a uC. But it does raise
eyebrows if I request timers and port pins and MIPS, probably because of
my analog background. "YOU want some of the uC resources? What for?"

-- 
Regards, Joerg

http://www.analogconsultants.com/

Vladimir Vassilevsky skrev 2013-08-21 21:47:
> On 8/19/2013 3:14 PM, Joerg wrote:
>
>  > Folks,
>  > What's the usual capacitance?
>
> 0.1uF
>
>  > Any stability issues there?
>
> Nothing special.
>
>  > I was planning
>  > on using a 1uF X7R ceramic cap on the AREF pin of an ATMega2560, in
>  > order to be able to use its internal bandgap reference.
>
> Do not. Use Vcc as reference.
> Internal reference is inaccurate.
>

I remember a customer design where the Vcc rippled between 3 and 5 Volt.
Was cheap, but then using Vcc as a reference is NO GOOD.


BR
Ulf Samuelsson

>  > I saw people
>  > using 0.1uF and 0.47uF. The datasheet is silent about stuff like that,
>  > as usual.
>
> It doesn't really matter.
>
> Vladimir Vassilevsky
> DSP and Mixed Signal Designs
> www.abvolt.com
>
>

On 8/22/2013 1:51 PM, Joerg wrote:
> Tim Williams wrote:
>> "Joerg"<invalid@invalid.invalid>  wrote in message
>> news:b7l1blFlu93U2@mid.individual.net...
>>>>> I saw people
>>>>> using 0.1uF and 0.47uF. The datasheet is silent about stuff like that,
>>>>> as usual.
>>>> It doesn't really matter.
>>>>
>>> However, not everyone knows that because they don't say much about the
>>> innards of the chip. It's my first ATMega case, or maybe the 2nd.
>>
>> Finally gave up and bit the uC bullet?  ;-)
>>
>
> Sometimes you need it. A while ago I even had a switcher design that
> would have been totally impossible to do without a uC. But it does raise
> eyebrows if I request timers and port pins and MIPS, probably because of
> my analog background. "YOU want some of the uC resources? What for?"

Sounds to me like you need FPGA resources moreso than an MCU.  Are you 
using the ADC or DAC, guess so or are you using the reference for the 
brownout or something else?  DACs are easy in FPGAs, ADCs not as easy. 
Otherwise FPGAs do everything an MCU does plus...

-- 

Rick

rickman wrote:
> On 8/22/2013 1:51 PM, Joerg wrote:
>> Tim Williams wrote:
>>> "Joerg"<invalid@invalid.invalid>  wrote in message
>>> news:b7l1blFlu93U2@mid.individual.net...
>>>>>> I saw people
>>>>>> using 0.1uF and 0.47uF. The datasheet is silent about stuff like
>>>>>> that,
>>>>>> as usual.
>>>>> It doesn't really matter.
>>>>>
>>>> However, not everyone knows that because they don't say much about the
>>>> innards of the chip. It's my first ATMega case, or maybe the 2nd.
>>>
>>> Finally gave up and bit the uC bullet?  ;-)
>>>
>>
>> Sometimes you need it. A while ago I even had a switcher design that
>> would have been totally impossible to do without a uC. But it does raise
>> eyebrows if I request timers and port pins and MIPS, probably because of
>> my analog background. "YOU want some of the uC resources? What for?"
> 
> Sounds to me like you need FPGA resources moreso than an MCU.  Are you
> using the ADC or DAC, guess so or are you using the reference for the
> brownout or something else?  DACs are easy in FPGAs, ADCs not as easy.
> Otherwise FPGAs do everything an MCU does plus...
> 

I nearly always need ADC capability. Not for the brownout, I never trust
anything on those chips for that, the POR/BOR/WDT goes external. FPGA
have some downsides, they tend to become large, expensive and sometimes
power-hungry when you need math capability or an MCU core. But the main
issue for many projects is people. If the client has the experts in
house like in this case, fine. But if not then one is better off using a
uC where one can find a programmer in nearly every town. This is one
reason I am often partial to the 8051.

Of course the beauty of FPGA is that the availability of very fast glue
logic. Most uC are sorely lacking in that domain.

-- 
Regards, Joerg

http://www.analogconsultants.com/