Mark, thanks for your info. In my case I already know what the problem is but I want to mask it because of there is no way I can convince the guy here to use 10bit ADC instead of 13 bit. I already check with sensor makers they do not recommend ADC higher then 10 bit but no thing I can do. The extra noise comes from the sensor Andre -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]On Behalf Of Rich Sent: Tuesday, June 19, 2007 9:43 PM To: Microcontroller discussion list - Public. Subject: Re: [pic:] Software low pass filter any idea? Using A/D converters can sometimes be tricky. Much depends on the A/D converter itself but there are some things to consider in its application. Sometimes the reading such as you are experiencing is because the signal driving the analog input to the converter is not at a low enough impedance. Just any old op amp won't do. Check the specs out on the op amp driving the A/D and make sure that is has a low output impedance. The lower the better actually. It does, of course, depend on the A/D you are using. The LTC1605, for example has a series resistance in the input and so does the 7107 (3 and a half digit integrating converter). So without knowing what converter you are actually using it may be as I suggest or it may not. Check out your grounding and power source. Look for some spurious oscillation in the circuit that could contribute to the error. Simply taking the mean my just cover up the problem and not solve it. If you have a scope, you may want to check out the power terminal of the converter and the analog grounds. How did you handle the analog grounds and the digital grounds. Typically, the ap notes that come from the manufacturer are very explicit and following the manufacturers recommendations exactly is a good practice. Even a low bandwidth scope, like 30 MHz, can be helpful and one can be picked up (used) for around 50 bucks. I suspect you will find that you have either a noise problem, a grounding problem or an input conditioning problem, or some combination. I sure hope that it is not a board layout problem. ----- Original Message ----- From: "Mark Rages" To: "Microcontroller discussion list - Public." Sent: Tuesday, June 19, 2007 10:49 PM Subject: Re: [pic:] Software low pass filter any idea? > On 6/19/07, Andre Abelian wrote: >> Hi to all, >> >> I am using external 13 bit ADC witch originally won't necessary >> to go that high resolution "not under my control" now the result >> I am getting is noisy numbers are going high and low. what I did is >> reading it 10 times then I add them together then divide it by 10. >> It helps but one of our engineer asked me to add software low pass >> filter witch I am familiar with it. Reading 10 times values are up to 15 >> point >> different. >> My question is do you have any experience or know how software LPF work? >> any link or info will appreciate. I searched piclist.com not enough info >> I found to understand what to do. >> > > What kind of anti-aliasing filter is on the ADC? > > Regards, > Mark > markrages@gmail > -- > Mark Rages, Engineer > Midwest Telecine LLC > markrages@midwesttelecine.com > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist