Question for AD7730 Filter

Hi

I'm testing AD7730 for weighscale application

From the datasheet, I could find Sinc3 and FIR filter.

FIR filter looks like good performance at 20Hz ~ 60hz noise rejection with slow settling.

S0, I tried to use Sinc3 Filter to reject noise at 20Hz ~ 60Hz.

Question :

1. It seems like Sinc3 Filter is not appropriate to reject noise at 20Hz ~ 60Hz. Am I right?

When I saw sinc filter of AD719x series, seems like it can reject 20Hz ~ 60Hz noise.

(Because of output data rate constraint) See the below!

2. Is it possible to apply oversampling technique for AD7730?

e.g) When I set the ODR to 200Hz, Output noise is bigger than 400Hz ODR.(Fast = 1, CHOP enabled)

If so, can you give me data to understand it?

3. If Sinc3 filter is not stand for rejection noise of 20Hz ~ 60Hz, What is its function?

Parents
  • 0
    •  Analog Employees 
    on Dec 18, 2016 7:40 PM over 4 years ago

    Hi, Jake.

    The first stage sinc3 filter primary function is to remove the quantization noise introduced at the modulator. The AD7730 has a 22-tap FIR filter which follows the sinc3 filter. This filter was added as, being an FIR filter, it has a steep roll off and good stop band attenuation. However, care must be taken. Example, for filter response above. Chopping is on. This filter provides good rejection from 20 to 60 Hz. However, at 100 Hz and 150 Hz, there is not much rejection. So, harmonics of a 50Hz mains power supply may not be attenuated enough with this filter profile. Chopping is useful to minimize offset and offset drift. Chopping also changes the filter response. With chopping enabled, the filter response is ‘softer’ i.e. the roll off is not as quick and the stop band attenuation is less when compared to a standard sinc3 or sinc4 filter. So, chopping can be a disadvantage for weigh scale design where the signal of interest needs to be processed but the mechanical vibration tones may not be attenuated enough as the stop band attenuation is not good. When you take a look at the filter response below. The chopping is turned off and the data rate of the sinc filter is adjusted to 150 sps, you will see that the first image of the FIR filter occurs at 150 Hz. The overall rejection is now better as the sinc3 filter has notches at the images of the FIR filter, leading to a good overall response. Chopping is now off so the offset and offset drift will be higher.

     

    So, the above are some things to consider when dealing with weighscale designs. Trade-offs are needed between the filter and the offset/offset drift.

     

    For new weigh scale designs, the AD7190 or AD7195 may be better choices. These parts have higher performance and lower current consumption than the AD7730. Also, weigh scales normally require a gain of 128. At this gain, the AD7190 and AD7195 have low offset and low offset drift when chopping is disabled. So, a customer could use these parts with chop off and still have better offset drift than the AD7730. The standard sinc3/sinc4 filters on these parts with chop off will provide sufficient rejection of the mechanical vibration tones also. So, for new weigh scale designs, these parts should be recommended.

    Thanks,

    Jellenie

     

  • How do I get the file in excel ?. I could email it edward.udep@gmail.com.Thank you.

  • 0
    •  Analog Employees 
    on May 6, 2020 1:00 AM 8 months ago in reply to Edward-Yamunaque
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