A/D Conversion Clock Select bits confussion ..

[gunayburak]

The only part I cant understand is the line he somehow calculated 40 + 38.4 us = 78.4 uS .. where did that 40 come from ?

[aratti]

40 us is the value the author arbitrarely gives as acquisition time "sampleus parameter".

Remember:

A too high acquisition time will give issues when the analog input approch the V(ref+)

A too low acquisition time will give issues when the analog input approch the zero

So, if you do not see any issues in your reading at the extreme of the reading field then the acquisition time used is correct!

Ciao, complimenti per il tuo italiano!

Al.

[tumbleweed]

A too high acquisition time will give issues when the analog input approch the V(ref+)
A too low acquisition time will give issues when the analog input approch the zero

Where do you get this from?

The aquisition time is the MINIMUM amount of time you have to wait for the holding capacitor (and it's RC network) to fully charge/discharge to the voltage level you're sampling.

If you set the acquisition time too low then you won't have allowed enough time for the charge on the capacitor to reach the desired level, and you'll get an incorrect conversion. It doesn't matter so much if the input is near Vref or zero, it'll be wrong.

If you're measuring a DC voltage, there is no "too high" an aquisition time.

[aratti]

Where do you get this from?

From the charging function of a capacitor!

Al.

[tumbleweed]

So you're saying that if you wait too long the voltage on your sampling capacitor will be wrong?

If you put a small RC network across a voltage source, what will the voltage across the capacitor be if you wait a second? A minute? An hour?

[aratti]

You cannot use the full function because it is foundamentally a logaritmic function. For the ADC convertion you need a linear function. Once time, resistence and capacity are fixed than the charging function depend only from the voltage applied. If during the charging time you do not pass the 1T point of the function, than the charging function can be considered "quasi liner" and the system work. In other words you can deduce the value of the voltage applied. If you pass the 1T point then thing become more complicated since from that point on the function is purely logaritmic.

Cheers
Al.

[tumbleweed]

You're somehow mixing Acquisition time with the linearity of the ADC conversion function.

For the Acquisition time, you need the sample cap to charge to whatever value you want the ADC to convert. Period.

The time constant of an RC network is T=RC, and after one T the voltage across the cap is roughly 66% of the input voltage.
If you look at T vs percent, you get:

1    63.2% 
2    86.5% 
3    95.0% 
4    98.2% 
5    99.3% 
10   99.995%
20   99.9999998%

If you set the Acquisition time to 1T and then convert the value, you could be VERY wrong... 37% wrong. Of course, that depends on the new input voltage you want to convert and any existing charge on the sample cap, but T gives you a worst-case number.

For example, an 8-bit ADC (1 part in 256, or 0.39%) would need an acquisition time of better than 5T, but there's nothing wrong with giving it 10T or 20T or 100T, as long as nothing else changes in that time.

You do not have to hit some magic 1T window trying to get it into a "quasi-linear" range, or guess at things to "see if it's good enough".