dsPIC AD converter input impedance

[Joerg]

The model for an unbuffered switched-capacitor ADC input really does
not look like a simple resistor, so a model with parameters makes a
lot more sense to me.

Here, right from the data:

http://ww1.microchip.com/downloads/en/devicedoc/33023a.pdf

... PDF page 391.

That's the circuit I found in a tutorial. Hoping it's the same on dsPIC.
It would be kind of nice if they had at least a little more meat in
their datasheets.

The details become especially important if you're using more than one
channel.

Probably also how often that C-hold is accessed.

So no I have three pieces of information. Datasheet sez 200ohm, some
other paper sez 5k, and your link (note 3) sez 10k. Wot a mess

 

[Spehro Pefhany]

Spehro Pefhany wrote:

That's the circuit I found in a tutorial. Hoping it's the same on dsPIC.

Almost certainly not- the hold capacitor value varies widely between
families, IIRC. The above reference is for a practically obsolete
8-bit converter on a 16 series chip.

It would be kind of nice if they had at least a little more meat in
their datasheets.

You need to refer to the appropriate reference manual for the dsPIC,
not the midrange reference manual. PIC30 may be different from PIC33
as well.

The total documentation is well in excess of 1000 pages- the data
sheet is just a brief overview, with some chip-specific
characteristics. It's like the ARMs and other micros- not everything
is in one place. Or consider the data sheet the user manual, but as
competent engineers we want to see the service manual..

They've broken the reference manuals up by chapter in the latest
chips, you can't even download it all in one chunk AFAIK.

Probably also how often that C-hold is accessed.

Exactly, on the conversion rate. The Chold cap is _discharged_ prior
to conversion so it's proportional to the conversion rate.

So no I have three pieces of information. Datasheet sez 200ohm, some
other paper sez 5k, and your link (note 3) sez 10k. Wot a mess

<shrug> I don't see a mess- just different chips and situations.

 

[Joerg]

Almost certainly not- the hold capacitor value varies widely between
families, IIRC. The above reference is for a practically obsolete
8-bit converter on a 16 series chip.


You need to refer to the appropriate reference manual for the dsPIC,
not the midrange reference manual. PIC30 may be different from PIC33
as well.

The total documentation is well in excess of 1000 pages- the data
sheet is just a brief overview, with some chip-specific
characteristics. It's like the ARMs and other micros- not everything
is in one place. Or consider the data sheet the user manual, but as
competent engineers we want to see the service manual..

They've broken the reference manuals up by chapter in the latest
chips, you can't even download it all in one chunk AFAIK.

Understood. However, we live in the age of cyberpower, post-Tricorder,
where lots of storage space and bandwidth is available, teens stream
gigabytes of MTV shows, and so on. So I'd have expected that a search
would lead to what you find at TI: Datasheet, family spec, app note
links, all nicely in one place. Or at least a hyperlink in the paltry AC
section of the datasheet linke IEEE does, "to probe further". But no ...

Exactly, on the conversion rate. The Chold cap is _discharged_ prior
to conversion so it's proportional to the conversion rate.

We can be super slow on that, once a second is enough most of the time.
Faster rates a well but that's only for trending where absolute accuracy
is not important. In this application a dsPIC is like driving a 5-ton
truck to bring three letters to the post office. It wasn't my pick. Or
PIC

<shrug> I don't see a mess- just different chips and situations.

Well, my definition of an orderly assortment of technical documents
appears to differ from Microchip's

 

[Tim Williams]

Microcontroller data sheets are very poor on any kind of details. As
soon as you start digging deeper then "hello world", you find out this.
The usual answer from support is "take it from schematics of evaluation
board" or "look it in the source code which comes with evaluation
board".

I've found Atmel datasheets are very thorough and specific. Sometimes too
specific, because all the verbiage for say timer-counters 0 through 2 all
use the same text, with subtle variations appropriate for the section.
But overall, you don't need to read anything that isn't in the section at
hand.

Too bad Altera isn't as specific on their FPGAs, or anyone else for that
matter.

Tim