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ADS7830: Assess the risk of ferrite bead for ADS7830IPWR input channel

YEN-RU LAI
Prodigy 10 points
Part Number: ADS7830

Dear TI specialist,

Recently, our client had some questions about the filter on the ADS7830 channel when they reviewed our 2nd source component. The schematic is shown in the figure below. After the power supply passes through the filter, it is directly connected to the ADS7830 as a voltage detection device.

 

The Main source ferrite bead (BLM18PG300SN1D) specification:

And the impedance-frequency chart:

The Second source ferrite bead (GMLB-160808-0030S-N8 ) specification:

And the impedance-frequency chart:

 

Regarding our circuit design, it is a low-pass RC filter.

Thus, according to the low-pass RC filter equation, the cutoff frequency can be demonstrated as below:

R=30ohm, C=47pF in our schematic.

Then we can get fc around 112.88Mhz.

Under 112.88Mhz condition, we may find out our 2nd source ferrite bead is closer to 30ohm (the impedance-frequency chart I provided) than the main source.

So personally speaking, I think GMLB-160808-0030S-N8 can be qualified as 2nd source because it has better performance under this frequency. And I believe GMLB-160808-0030S-N8 won’t cause any problem for ADS7830. I and our client would like to know What is TI's opinion on this?

We are looking forward to your reply and thanks for any help you may provide.

  • +1
    TI__Mastermind 24278 points

    Yen-Ru Lai,

    In general, we do not recommend using a ferrite in series with the ADC input.  Typically, a fixed resistor is used with a filter capacitor.  For example, a 50 ohm resistor with a 1nF capacitor is a common input filter for a SAR converter.  This filter is often tuned depending on the sampling rate and resolution of the converter. When using a ferrite, my concern is resonant peaking.  Also, the RC filer will have a much lower cutoff frequency and act as a more effective filter for noise.  To directly answer your question, I don't think you will see any difference with the substitute ferrite, but we don't normally recommend this approach.  In your case,  I think you have been using this approach for some time and it works ok for you so the substitute should be fine.  Alternatively, you could test the design with a small value fixed resistor (e.g. 30 to 50 ohms).  This should work fine as well, and more closely matches the normal recommended approach.  I hope this information helps!

    Art

  • 0 in reply to Art Kay
    Prodigy 10 points

    Hi Art,

    I really appreciate your reply and the explanation!

    We've implemented this design for several years, and we are not considering changing the design for now.

    Regarding the difference between main and substitute ferrite, we also care about impedance tolerance.

    According to your reply, it seems tolerance is less impact on this matter, so it should be fine if we use GMLB-160808-0030S-N8 as the substitute ferrite. Am I right?

  • +1 in reply to YEN-RU LAI
    TI__Mastermind 24278 points

    Yen-Ru Lai,

    You are correct.  I don't expect any issues with your substitution.

    Art

  • 0 in reply to Art Kay
    Prodigy 10 points

    Hi Art,

    I really appreciate your reply! It is clear and this thread is closed.