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Kai Klaas
11/12/05 07:31
Read: 2036 times

#103621 - Filter providing two time constants
Responding to: Thakkar Bhaskar's previous message
Thakkar said:
I am designing a weight scale using ic7135.The samples very between three to four, or sometimes five values. What would be the best method to avarage out the samples, so that result seems to be steady?

There can be some reasons for noisy readings: Noisy reference voltage, poor ground routing intoducing hum loops, injected hum due to a mains transformer in close distance to ICL7135, signal noise at input, etc.
Mostly the noise comes from input signal noise, which can be rejected to some degree by a suited low pass filter. Standardly you should use a low pass filter consisting of 100kOhm and 100nF, as it's suggested in datasheet. But if this does not present enough rejection, then corner frequency should be lowered. Unfortunately, settling time increases then, means it takes longer to get a stable reading and for big changes of input signal settling time can become unacceptably high.
The following circuit offers both, fast settling time for big changes of input signal and at the same time improved noise filtering due to lowered corner frequency:

The circuit works as follows: If a big change of input signal arrives, then one of the Schottky diodes becomes conducting whenever the difference between input and output voltage of the filter is higher than about 0.2V. In this case only the 1kOhm resistor forms the time constant of low pass filter. So, the capacitor is charged up much faster, than with the 100kOhm resistor alone.
If the difference between input and output voltage of filter falls under 0.2V, the Schottky stops conducting and now the 100kOhm resistor forms the much bigger time constant of low pass filter, resulting in improved noise filtering.

Take care, only polypropylene capacitors should be used for the caps at ICL7135!!


List of 16 messages in thread
Weight Scale Avarging method      Thakkar Bhaskar      11/12/05 02:20      
   Filter providing two time constants      Kai Klaas      11/12/05 07:31      
      median filter, simulate algo on PC      Frieder Ferlemann      11/12/05 07:47      
         Periodic interference can make them fail      Kai Klaas      11/15/05 08:32      
            Acknowlegded.      Frieder Ferlemann      11/16/05 08:24      
               Agreed      Kai Klaas      11/16/05 09:24      
                  50Hz sampling      David Smith      11/17/05 03:54      
                     Anti-aliasing filter will help      Kai Klaas      11/21/05 06:27      
                        what this is all about      Erik Malund      11/21/05 06:37      
   Moving average window.      Graham Cole      11/14/05 01:50      
      Average on condition      Kai Klaas      11/15/05 07:29      
         is that not SOP (standard operating proc      Erik Malund      11/15/05 07:41      
            Hallelujah!      Kai Klaas      11/15/05 07:55      
   meaningless      Erik Malund      11/14/05 06:34      
   What are you using that one for in a wei      Erik Malund      11/14/05 09:14      
   hello from chhatraliamd for weight scale      Chhatralia Mansukhbhai Dh      12/27/05 01:01      

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