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Old Sep 24, 2012, 02:55 PM
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LM1881 works fine with both PAL and NTSC. The only difference is the number of lines - and LM1881 is just used to detect if it's a new line or frame.

The extra filter is optional. In most cases it shouldn't be necessary, but it removes some high-frequency noise. SimpleOSD doesn't use the extra filter and lot of people use that without problems.
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Old Sep 24, 2012, 03:05 PM
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well then my last LM1881 scheme is correct? And for current sensor I'm looking for ACS758LCB-050U-PFF-T with some additinoal components from datasheet.. Will solder that up..
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Old Sep 24, 2012, 03:10 PM
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well then my last LM1881 scheme is correct? And for current sensor I'm looking for ACS758LCB-050U-PFF-T with some additinoal components from datasheet.. Will solder that up..
Yep, that should be right (just remeber C1, 100nF/0.1uF to make sure the signal is AC-coupled).

Remember not to add too much capacitance at the output of ACS758.
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Old Sep 24, 2012, 03:23 PM
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I will do as in datasheet..

For Rf an Cf I will don't put in anything as I don't know what should I put there and it is optional.. Maybe you can suggest something.. I looked at ebay for ready these type sensors soldered up to pcb and there also is only one capacitor which is 100nF I guess..
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Old Sep 24, 2012, 03:28 PM
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I will do as it is in datasheet..

For Rf an Cf I will don't put in anything as I don't know what should I put there and it is optional..
But that's not gonna work very well

An ESC controls the speed etc. by closing an opening a couple of FET's very fast. That will give you peaks like those seen here:
(Please note, this is an actually measurement of the output from an Allegro current-sensor used in front of an ESC)


You just add the largest capacitance possible (see datasheet or my post a few days ago) - and a matching resistor. You can calculate a descent time-constant or just pick a good resistor-value.

This should give you a lot more stable readings. It will never be "true RMS" - but you should be fine

I don't recommend to use the current-sensor without filtering the output.
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Old Sep 24, 2012, 03:35 PM
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I looked in datasheet and didn't find them but now I found.. R(LOAD MIN)4.7kOhm and C(LOAD MAX)10nF.. are these the right ones?

OK found your previous post also.. These are the right ones.. Thanks for helping me..
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Old Sep 24, 2012, 03:53 PM
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I looked in datasheet and didn't find them but now I found.. R(LOAD MIN)4.7kOhm and C(LOAD MAX)10nF.. are these the right ones?

OK found your previous post also.. These are the right ones.. Thanks for helping me..
Year, but you can ignore the R(load min) 4.7kOhm, as the input-impedance of the ADC on Atmega should be higher than that. 4.7kOhm prop works just fine, but you might wanna go a bit higher. The time-constant with 4.7kohm and 10nF is 47 us.
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Old Sep 24, 2012, 04:06 PM
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Well that information is too complicated for me.. Which is the time-constant to reach for? I see that the formula is 4700 ohms * 0.01 uF? I should go for 10kohms? as result 100us (us is miliseconds?)? I don't know anything about this
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Old Sep 24, 2012, 04:14 PM
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Just a quick graphical illustration.

Green is input-signal. The signal are all with 10nF cap, and the resistors are 4.7K, 10K and 100K. The 100K is just to show the effect.

The higher resistor-value, the more stable output. But too high resistor will give problems with noise and voltage-divider between the resistor and input-impedance.



10K resistor and 10nF should be a fine place to start. If you don't have stable current-readings, you can increase it.
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Old Sep 25, 2012, 01:41 AM
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Hi.. I hope I understand correct..
Green is raw Vout from ACS758, with 4.7k and 10nF it gives blue line, with 10k and 10nF it gives red line and with 100k and 10nF it gives light blue line which is almost straight.. maybe I should use pot to change values from 10k up to value X which is stable..
Thanks for this great explanation and your work..
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Old Sep 25, 2012, 01:46 AM
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Hi.. I hope I understand correct..
Green is raw Vout from ACS758, with 4.7k and 10nF it gives blue line, with 10k and 10nF it gives red line and with 100k and 10nF it gives light blue line which is almost straight.. maybe I should use pot to change values from 10k up to value X which is stable..
Thanks for this great explanation and your work..
You got it right. Don't "overthink it", but I would recommend at a filter.

In worst case, you will get a little unstable current-reading.
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Old Sep 25, 2012, 02:40 AM
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Well I have really dumb questions I am looking for these resistors and in shop there are like 1/4W 1/2W 1W etc resistors.. Which ones should I choose, are there any diference? and also capacitors has some additional parameters as RM some has 2.54 and some 5.. Which ones are better?
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Old Sep 25, 2012, 02:44 AM
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Well I have really dumb questions I am looking for these resistors and in shop there are like 1/4W 1/2W 1W etc resistors.. Which ones should I choose, are there any diference? and also capacitors has some additional parameters as RM some has 2.54 and some 5.. Which ones are better?
The power-rating doesn't matter in this case. Pick the cheapest, smallest, sexist or whatever you prefer

The capacitor is not critical either, a normal 10 nF ceramic should be fine.
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Old Sep 25, 2012, 02:55 AM
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And if there aren't exactly the same resistors and capacitors as needed then I can put resistors in series to mach the needed value and in paralel capacitors?
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Old Sep 25, 2012, 05:22 AM
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And if there aren't exactly the same resistors and capacitors as needed then I can put resistors in series to mach the needed value and in paralel capacitors?
That should do the trick
The resistor-value is not critical, just pick whatever you can get around the desired value. Same goes for the capacitor, except, don't use a capacitor-value above 10 nF.
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