External ADC - Polarity?

[LordSputnik]

I'm making a basic digital audio recorder with an ADC, PICAXE microcontroller and EEPROM, and I have a quick question about ADC usage.

I don't want to use the internal ADC on the PICAXE because I'm concerned about speed issues - I'd rather get an external chip to do it, while the PICAXE manages other things.

So, I had a look, and found the . **broken link removed**

Looking at the spec, the input voltage is apparently supposed to be positive. I haven't had much experience with ADCs, so does anyone here, who's used this ADC before, know whether I'd have to bias my audio signal to be encode the full signal, and not just the positive section?

Many Thanks in advance!
Sput

 

[Nigel Goodwin]

As with any ADC, you need to bias it half way if you're going to record both positive and negative signals.

However, I would suggest you reconsider your entire premise - you're wanting to use an external ADC because you want more speed, yet you're using a slow PICAXE as a controller, and trying to write to extremely slow EEPROM.

 

[LordSputnik]

The PICAXE is a necessary part of the design - I'm only at British "A" level and I haven't been taught to use any other controllers. I know of them, but not enough about them to use another...

The EEPROM allows page write with 128 bytes, so I should be able to move the data out of the PICAXE RAM fast enough to keep up with a 4kHz sample speed for the audio.

I just didn't want to use the actual PICAXE ADC, because I didn't know how many cycles it takes to sample the signal. The problem being that if it takes longer than the sample period, I'll miss data, and won't have any time to store the data I do receive.

However, I've read tonight on the PICAXE forums that the time ranges between 25us and 30us with the PICAXE running at 4MHz, which leaves me almost 900 clock cycles to do data processing before another sample is required. So it's looking like I'll be able to use the internal ADC after all.

 

[Nigel Goodwin]

Yes, if you're only sampling at such a low speed the internal ADC will be fine.