Logarithm in PBP

[savnik]

I have one voltage which vary logarithmical.
How change the code below to show voltage on LCD in logarithm.(not linearity)

loop: 	
    ADCIN 0, adval					' Read channel 0 to adval (0-1023)

    adval = (adval */ 500)>>2		' equates to: (adval * 500)/1024

    LCDOut $fe,1,"DC Volts= "
    LCDOut DEC adval/100,".", DEC2 adval	 ' Display the decimal value
    Pause 500       				' Wait .5 second

    GoTo loop       				' Do it forever

[mackrackit]

I searched the form for logarithm and found this,
http://www.picbasic.co.uk/forum/show...ight=Logarithm

[savnik]

anyone to know to help me

[Jerson]

Build a table containing log values derived from a HLL running on a PC. Embed this table into your program. You could use maybe 10, maybe 50 points in the table and interpolate between them to get a log reading. This method is not precise but workable since I assume you only need to visualise the log reading on a display. For precise log values, you have to do it the hard way.

Jerson

[Acetronics2]

Hi, Savnik

Jerson is right ... the simplest Pic way is to build a retrieving table.

IN PbP that's called LOOKDOWN2 ... ( catches the closest values, sup and inf ... just think to use a clever index !!! )

Then just have to add a linear approx. between table values ...


Buuuuuut, My old brain tells me it is very easy to do it with 3or4 couples of linearising resistors and diodes ...

or to use a logarithmic AOP !!!

Last and simplest solution: The App Notes of ICL 7106 or 7136 show a logarithmic scale design ...

Which is closest to your requirements ??? ...

Alain

[savnik]

Hi, Savnik

Jerson is right ... the simplest Pic way is to build a retrieving table.

IN PbP that's called LOOKDOWN2 ... ( catches the closest values, sup and inf ... just think to use a clever index !!! )

Then just have to add a linear approx. between table values ...


Buuuuuut, My old brain tells me it is very easy to do it with 3or4 couples of linearising resistors and diodes ...

or to use a logarithmic AOP !!!

Last and simplest solution: The App Notes of ICL 7106 or 7136 show a logarithmic scale design ...

Which is closest to your requirements ??? ...

Alain

I have make a table.
I want to measure the volts at the input of pic , and show on LCD the power

POWER	VOLT		POWER	VOLT
1	7,1		26	36,1
2	10,0		27	36,7
3	12,2		28	37,4
4	14,1		29	38,1
5	15,8		30	38,7
6	17,3		31	39,4
7	18,7		32	40,0
8	20,0		33	40,6
9	21,2		34	41,6
10	22,4		35	41,8
11	23,5		36	42,4
12	24,5		37	43,0
13	25,5		38	43,6
14	26,5		39	44,2
15	27,4		40	44,7
16	28,3		41	45,3
17	29,2		42	45,8
18	30,0		43	46,4
19	30,8		44	46,9
20	31,6		45	47,4
21	32,4		46	48,0
22	33,2		47	48,5
23	33,9		48	49,0
24	34,6		49	49,5
25	35,4		50	50,0