[sdiy] Possibly useful reference data
Magnus Danielson
magnus at rubidium.dyndns.org
Thu Dec 31 12:38:41 CET 2009
Antti Huovilainen wrote:
> On Thu, 31 Dec 2009, Veronica Merryfield wrote:
>
>> I recently had to take account of input RC settling time of an ADC and
>> calculated the number of time constants to wait to allow the RC to
>> settle to half an LSB. I did this for a couple of bit widths whilst I
>> had the spreadsheet up.
>
> The equation to calculate those is RC_n = ln(2^-bits)
> vice versa, bits = log2(exp(-RC_n))
Very strange values appears using those formulas:
bits RC_n
8 -5.545177444
9 -6.238324625
10 -6.931471806
11 -7.624618986
12 -8.317766167
I think the negation should not be there and there should be a multiply
with 2 for even approaching Veronicas numbers:
RC_n = 2*ln(2^bits)
Using fairly standard reduction a much simpler form turns out:
RC_n = 2*ln(2)*bits
ln(2) is a table item but 2*ln(2) is about 1.386294361
Using this I get:
bits RC_n
8 11.090354889
9 12.47664925
10 13.862943611
12 16.635532333
14 19.408121056
16 22.180709778
I still haven't made any propper analysis thought. What is the assumed
analysis case?
Cheers,
Magnus
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