AVCC low pass filter suggestions?

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#1
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I'm designing an ATMega128 board w/ the TQFP package. I have no plans to use the ADC but figure I might as well set things up so I can in the future. The only discussion of layout considerations in the datasheet (nothing in the app notes I can find) is the suggestion that AVCC should be connected to VCC via a low-pass filter. I'm pretty lame when it comes to analog stuff so, while I know in general what a low-pass filter is, I have no idea what's called for in this application. What have other people done or seen? Are there other considerations? I'm planning on just a 2-layer board. Thanks.

Karl

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Hello Karl Lautman,

Quote:
I'm pretty lame when it comes to analog stuff

I was going to write "you're also pretty lame when it comes to reading datasheets" but that would be too cruel.

Look at Atmel mega128 Datasheet (DOC2467.PDF). Figure 114 "ADC Power Connections" is right up your alley. Atmel say 10uH / 100nF if you can't find it.

Nigel

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Use a 10uH chip inductor between DVcc and AVcc with a 100nF capacitor to ground from AVcc, as described in the Mega8535 data sheet. There should be something similar in the Mega128 data sheet. Also use an analogue ground plane under the ADC pins.

Leon

Leon Heller G1HSM

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Nigel - I deserve the withheld comment. Thanks for the pointer.
Leon - Since I'm trying to keep this to 2 layers, would a copper fill around the analog pins on one of the layers work? If so, what's the minimum size I can get away with? Can I connect it directly to the digital ground? Thanks.

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An RC filter to the AREF will work too. If the aref is to VCC and its good and solid, that will work pretty good too. In other words, if you only need 7 or 8 or 9 bits, maybe the trouble of getting the aref noise down below 10bits isnt worth the effort. I use an RC filter and my 10 bit readings seem pretty solid.

Imagecraft compiler user

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Thanks, Bob. What values do you use?

Karl

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Anything for the R and anything for the C is better than nuthin and nuthin. Try 1K and 10uF. That gives f=1/2piRC hz... 16Hz? Thats less than 120 hz ripple

Imagecraft compiler user

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Hate to offend any purists here but if I do 10 bit readings then a bit of old stripboard works fine and the readings are rock solid with AVcc connected to Vcc. and just a little 100nF for bypass.

Ralph Hilton

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If you use a resistor in the low pass filter, remember that it is the power supply for PORTF. If you plan to use PORTF as outputs, then the resistance needs to be very low. (10 ohms?). Or you could leave 1K in there and change it to 10 uH when (if) you decide to use it for analog.