Pin restrictions in AD conversion

godzich

Hi,

Without still digging too deep into the propeller, and learning about the possibilities and limitations, here is one (stupid) question:

What are the pin restrictions (if any) when you build an A/D conversion using the DeltaSigma princilpe (one input pin, one pwm output pin)? I need at least three such A/D channels and wonder how to layout the design, still without programming a line myself...

Can such an ad conversion run on any pins? If not, wich pins (or pin pairs) are usable?

Sorry for an obvious question (for most of you - that is).

Cheers

Christian

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The future does not exist - we must invent it!

evanh

Any pins works. But keep the pair close together and, more importantly, keep the tracks short and fat between the Prop and the two capacitors. And that includes the Prop's power pins.


Evan

godzich

Hi,

Thanks Evan for your fast reply. As a mainly harware guy I realized immediately that this kind of AD conversion is sensivite for disturbances. You should also avoid any transitions on adjacent pins (to the input) during the conversion.

Wonder how stable this AD converision is? Probably it is also a function of how much hf-noise is present on the VDD line.

Would the somewhere presented additional resistor (adding a small hysteresis to the input signal) help? I need a fairly quiet input with no signal (at least 11 bits stable on an 12 bit conversion). I was not able to find any data about the achievable SNR on the AD-conversion. Must probably just experiment with this if there is no "white paper" published on this.

Would a better (fast) external comparator help, not relying on the logic treshold in the digital input stage within the chip? Anyone that has experimented with this before?

Cheers,

Christian



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The future does not exist - we must invent it!

macrofuse

Check this:

http://forums.parallax.com/showthread.php?p=576575

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If the propeller stops - the pilot starts sweating...

Post Edited (macrofuse) : 9/11/2008 2:10:36 PM GMT