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TIDA-01471: Some questions about reference design

Victor Ilien
Prodigy 60 points
Part Number: TIDA-01471
Other Parts Discussed in Thread: REF3425, REF6225

Dear Ahmed,

First of all, thanks for your work on this reference design and for answering questions on this forum. It's difficult to overestimate you help.

I have some more questions related to this reference design.

1. The first question is a bit amateurish, but still: Why do we need several attenuation ratios? Most of IEPE sensors have output range equals 10 V (+ -5 V). In this case, we only need 1/16 settings for matching with amplifiers of the amplification stage. Why we need another settings (1/2, 1/4, 1/8)? Should we use them only with sensors, whose output range respectively 1.25V, 2.5V, 5V? Or is there another explanation?

2. The second questions about multiplexing inputs. As you said earlier, the best option is to position the multiplexer after the HPF stage. What about common-mode supply? Should it be after multiplexer, one item for whole scheme? Or we need several common-mode supplies before multiplexer?

3. The third question about coupling capacitor (C35) at HPF stage. In this reference design you have used ceramic capacitor (1 μF, 50 V, +/- 10%, X7R, 0603). Is it sufficient for production use? Or the special audio capacitor can be used? I saw Metallized Polyester Capacitors (PET) in ADC's equipment for IEPE sensors like WIMA MKS 4. In other words, is the capacitor an important element of the circuit that makes sense to improve?

Best wishes,

Victor

  • 0
    TI__Expert 3500 points

    Hello Victor,

    1. The multiple attenuation settings are actually to maximize dynamic range in case of lower input swing. it might be because the sensor output range is limited (e.g. 5V only which is common), or if the application has a low level vibration that needs to be resolved (so some physical/mechanical limits keep the output of the sensor at low level)

    2. CM supply should be after the HPF. one CM supply could be used, but simulation shows quite large current surges passing through the CM buffer. I would recommend having single reference with multiple buffers, one per each input channel. that way cross-talk can be kept at minimum. simulation files (on the TIDA page) can be used to evaluate different solutions, and selecting appropriate buffer.

    3. I'm sorry I can't comment on this topic, as I didn't do extensive comparison or related measurements. but if this is the first production I would play safe and keep cost improvement for next phase unless there is time and capacity to do characterization and validation for both capacitor solutions.

    Best Regards,

    Ahmed

  • 0 in reply to Ahmed Noeman
    Prodigy 60 points

    Thank you!

    You recommend REF3425 instead of REF6225 as single reference?

  • 0 in reply to Victor Ilien
    TI__Expert 3500 points

    Hello!

    I didn't try REF3425 in this application. It's obviously lower performance than REF6225. using it really depends on the level of performance you are targeting.

    Cheers,

    Ahmed