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'[PIC] Analogue to Digital converter speed on an 18'
2005\07\11@124557 by

Toying with the idea to make a small datalogger but what is the fastest
speed I can realistically expect to sample signals on an 18F1320 ?

I'd consider using the dsPIC range but my programmer wont support them so
I'm stuck with the 18F series.

Cheers
Dom

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RTFM.  ;-)  Just kidding- I'd never actually JUST post that as a response, but it IS in there, in an obscure way.  Per the datasheet, Tacq = Tamp + Tc + Tcoff, whereTamp = 5 usTcoff = (Temp-25 degC)(.05us/degC) (for temps above 25 degC)Tc = -(CHOLD)(RIC + RSS + RS) ln(1/2047) ìswhere CHOLD = 120 pF (the internal sample-and-hold cap)RIC = 1k ("interconnect resistance")RSS = 7k @ 5V (sampling switch resistance)RS = source resistance- keep below 2.5k! (easy-ish with properbuffering of input)Tamp is the settling time of the amplifier, Tcoff is temperaturecoefficient, and Tc is capacitor charge time.
The example they provide in the manual assumes 5V operationand input impedance of 2.5k, with operating temp of 50 degC.No equation is given for switch resistance (RSS), but a graph of the value is in the manual.  Their calculation gives 15.86 usper sample, or a sampling frequency of 63 kHz.
Note that room temperature operation and lower sourceimpedance will drop that time to 12.32 us, or about 78 kHz.
You can do better with an external ADC, of course, but thenyou'd be using an external ADC!
Mike H.
PS- My copy of the datasheet may be a bit out of date.Double check all facts with the latest version!
> Toying with the idea to make a small datalogger but what is the fastest> speed I can realistically expect to sample signals on an 18F1320 ?> > I'd consider using the dsPIC range but my programmer wont support them so> I'm stuck with the 18F series.> > Cheers> Dom> > --> No virus found in this outgoing message.> Checked by AVG Anti-Virus.> Version: 7.0.323 / Virus Database: 267.8.11/45 - Rellease Date: 09/07/2005> >
Why not use external logic for strobing hte A/D and memory,and a fast counter for adress, bypassing the PIC during sampling?
Have a look at http://www.bitscope.com/ the schematic is there somewhere... but you could make a much simpler version of course

/Morgan

Dominic Stratten 18:44 2005-07-11:
{Quote hidden}

>-
Dominic Stratten wrote:
> I'd consider using the dsPIC range but my programmer wont support them
> so I'm stuck with the 18F series.

You could always get a programmer that does support the dsPICs.  If this is
a commercial project, then the cost of a dsPIC programmer is negligeable.
My ProProg (http://www.embedinc.com/products) can do dsPICs for \$295.  (\$200
if you can catch one of us at Masters before the limited quantity at that
price is sold out.)

*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com
Thanks Olin - this is a personal project though.

My "bread and butter" products use standard 16F627 and 18F1320 which my good
old trusty PS+ supports.

When the money comes rolling in (eventually) I'll probably upgrade to a PM4
and/or ICD2

I had a feeling I'd reliably get around 50k samples per second - I had a
look at the datasheet but its always reassuring when people confirm it.

Thanks guys

Dom

{Original Message removed}
> My "bread and butter" products use standard
> 16F627 and 18F1320 which my good old trusty
> PS+ supports

Does the PS+ lack the necessaries to program
the dSPICs ? Ever ? I haven't considered them
for any projects yet, but presumably I'll have
to get another programmer when I do ?

next to the Picstart Plus when I select the dsPICs.

It would be a shame for it not to support them in the future but I suppose
they are trying to push the ICD2 kit more now.

Dom

{Original Message removed}

> no-go telltale next to the Picstart Plus when I select the dsPICs.

I looked in the readme for 7.20 at Microchip (still using 6.4 and 7.0
here) and there's no mention of the dsPICs

> It would be a shame for it not to support them in the future but I
> suppose they are trying to push the ICD2 kit more now.

I'd have hoped it could be done through the PUM but presumably
there's a h/w reason why the PS+ simply isn't capable. Not that I
wouldn't buy a second programmer (it would certainly come in
handy when working with different PICs in the same overall project
rather than swapping cables) but I can get by without an ICD2 for
now

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