hi, i am developing a weigh scale application using Cirrus Logic 5532 24-bit ADC in unipolar mode.the output i am getting is very unstable (flickering on display). Can anybody please tell me how to get a stable count.
Thanks for all the replies. first let me clarify that i am not looking for full 24-bit resolution. i just want 18 bits resolution with manageable noise to get a weight to count ratio of 1:5. please guide me as to how i should average the counts in my software so that the noise in the counts is not reflected in the weight displayed.
an addition to the above: one very often overlooked fact is the resistors on the Op-Amp preceeding the A/D. If you visualize Rin and Rfb drifting 0.1% (yes THAT little) each their way, you get an errot of 0.2%. This corresponds to about 1 bit of 512 i.e. a 9 bit precision error. Another thing: my discussion above may lead to the impression that auto correction of zero and gain will cura all ills. While it is, indeed, required, it will not get you beyond the "practical limits" I posted above. Erik
Analog signalling, noise-free at 24 bit ADC resolution? Sorry, but you're day-dreaming. absolutely. It is easy to read a datasheet and say 24 but precision - WOW. Getting it is impossible (for less than $1.000.000 ?) A few facts from my experience (I have 12 scales in my resume) and recollections from previous posts by others. whatever your resolution you need to average to get much beyond 8 bit resolution you need a 4 layer board with separate analog and digital ground planes to get beyond 10 bits you need a chopper stabilized op-amp at the front end if you get to 12-14 bits you are darn good if you get to 16 bits someone will pay you a lot of money (the highest paid guy at the scale factory where I worked was the front end designer) if you get beyond 16 bits you most likely are lying. Erik PS The Burr-Brown '51 derivative with a 24 bit a/d was analyzed by someone when it came out (I can not find the analysis in the 2 minutes I am willing to spend on that - you are welcome to try at 8052.com if you do not believe me). The designer of the chip (Russell Anderson) posted a "reluctant" agreement to the analysis of the datasheet. Anyhow the analyss proved that if you set all errors to the max and min stated in the datasheet the precision of the chip itself is only 16 bits. Now add to that the errors introduced by the other circuitry involved.
Analog signalling, noise-free at 24 bit ADC resolution? Sorry, but you're day-dreaming. 24 bits corresponds to more than 140 dB of signal/noise separation. That level of noise suppression is hard to achieve even for a cable. For an electronic circuit of non-trivial complexity designed by anybody but a high-level expert on the topic, it's quite impossible. How many digits does that display of yours have, and how many of them are flickering?
24 bits is a lot of resolution. How many digits are you trying to show on the weigh scale? Assuming that there is nothing fundementally wrong with your hardware, noise is still a fact of life. Dealing with noise in software falls into the domain of Digital Signal Processing. There is an excellent, practical on-line book on this vast subject: http://www.dspguide.com/pdfbook.htm You will find most of what you need in chapter 2. One of the best and simplest ways of dealing with noise is to take the arithmetic mean of a number of consecutive samples.
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