Picaxe 18X ADC stability?

andrewpro

New Member
Ok, I'm at odds here. I'm building a radio telescope (a few different ones for different purposes actually) and I'm going to use an 18X to do most of the controlling. The high speed readings are going to be done over a parallel interface not involving the picaxe, but for slower (much slower) integrated readings, I'd like to skip a second "discrete" ADC as it just adds complexity, cost, and noise that I'm trying to avoid.

So...after all that...how is the stability of the ADC on the 18X. I was overlooking the microchip datasheet, and found data error information, conversion time info, etc, but nothing about the general noise stability. As in if I have 2.5 volts steady into it, how many readings out of say, 100, or 1000, would it read the same?

Also on that note, has anyone actually measured, or can toss out an educated guess, as to the tracking capability of the ADC? What kind of frequencies can I expect to get reliable results without skew during the conversion process?

Sorry if I'm asking questions that are in the datasheets or other literature, but for the life of me I could not find it! Thanks in advance for any help.

--Andy P
 

BeanieBots

Moderator
As with all the PICAXE range, the ADC is only as good as your power supply. If you can keep the supply good to one bit then the ADC will also be good to one bit. As for tracking, the PICAXE itself can only take a reading once every mS or so once it is in a program loop so take tracking out of the equation by applying a low pass filter on the input. A simple RC will do the job.
Don't forget, the biggest issue with PICAXE ADC errors is when the source impedance is too high (should be < 2k5) and if negative current can enter the device substrate. Use the enhanced download circuit to avoid this.
 

womai

Senior Member
In my Picaxe circuits I found the ADC readings to be very stable, i.e. repeated readings of a constant level yield always the same value or at worst jump between two adjacent values (which can happen whenever there is even the slightest amount of noise and the signal is close to the threshold between the values). Of course your power supply should be very stable (use ample capacitive decoupling close to the Picaxe).

As for speed, I remember having read somewhere in the Microchip data sheet that the analog bandwidth of the ADC is around 50 kHz. The input load schematics (resistances, load capacitances) in the data sheet would indicate a much higher -3dB bandwidth, so I suspect that number (50 KHz) is the fastest you can go and still have the ADC settle to within 1 LSB. (-3dB on the other hand that you would only settle to within 30+ percent of the full change in value). There definitely should not be any issue with acquisition speed for you since the Picaxe's operation is rather slow anyway.

Wolfgang


Edited by - womai on 10/24/2005 11:05:50 PM
 
Top