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The responce of AD637 when the large signal is came

Dear Sir/Madam,

I just use your AD637 RMS to DC converter.
Input is the sound signal.
CAV is put 100uF capacitor.
When the large sound is inputed to AD637 Vin momentarily,we just found to output pulse-signal.
So the analog meter that is put the RMSOUT-pin was shaked off.
So I replaced the CAV capacitor 100uF to 2.2uF.
Then this trouble was fixed.
I have a question.
Why this trouble was happen? I would like to know the reason why.

Thanks, Kaos

  • Hi Kaos,

    Thanks for using the AD637 and sorry to hear of your difficulty. Unfortunately there could be a great many things happening and without further knowledege of your circuit and the nature of the input signal. I can't be sure of the specific problem. Here are some thoughts, however.

    1. If you used 100uF, it must be a polarized capacitor of some kind. If this is the case, be sure that you connected the capacitor so that the polarith is correct (+ should connect to the output).

    2. 100uF will give you very good low frequency response but will also increase the settling time. If you are sampling the output, there may be a sampling issue of some sort.

    3. When the large signal is applied, is it a momentary sound? Continuous?

    4.Please remember that the averaging capacitor functions to averae a certain number of cyclical period to yield a dc voltage. If you are trying to detect a large burst, the output won't be continuous.

    I hope this helps.



  • Hello jstaley,

    OK, I will confirm our conditions well.

    If I have more question, I will let you know

    Thakns , Yasuda / AVNET JAPAN

  • Hello Jstaley,

    I just enclosed my schematics.

    Pls refer to my attahced.

    "+" pole of 100uF capacitor is put on RMSOUT-pin. So no prolem.

    We put 2 AD637s that are different value CAV capacitors 100uF and 0.1uF.

    Is this no problem?

    2 AD637s are connected by each DENIN-pins.

    And my output explanation in the past means BFOUT not RMSOUT sorry.

    Thanks, Kaos

  • Hello Kaos,


    Thank-you for the schematic.


    If you notice on your schematic, the 100uF averaging capacitor connected to the output of U206 is backwards, meaning that this capacitor will be very low impedance. Please note that the output of the AD637 is always positive, so the output will see an abnormally low impedance. Note also, that the output of U208-2 will be negative going because the input is positive, This is what is causing the negative transition shown in the scope shot and also explains why the circuit no longer exhibits the transition when you remove the capacitor.


    For the first step, you can try reversing the polarity of  of the 100uF capacitor. If this doesn't work, try removing the capacitor and see what the output looks like.


    You don't show either input voltage so I cannot tell what you should be seeing. I've attached sections 2 and 3 from our rms-dc apps guide which explains more about how this circuit should perform. I believe you may find the section entitled misc math computations on pages 43 and 44 of help..


    Finally, can you tell us a little bit about your application? Is this in connection with a research or university experiment? Is it part of a circuit you intend to use in an industrial application?





  • Hi, Jstaley,

    Sorry to my late reply.

    "Section-III" you enclosed had broken, so I could not open this by Adobe reader. Another application also could not open this.

    So can you please enclose "Section-III" again?

    Thanks, Kaos

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