AD8436 current meter

Hello,

Working on a project, where we have to measure currents up to 25Amps on pure sine wave voltage. For this we chose CKSR-15 from LEM. With two primary turns, it is working right between 0A and 25,5A as we need it. For RMS to DC conversion we are going to use AD8436 and here I need schematic approval (I am not strong in OpAmps so help is more than welcome ).

Since current sensor's output is 2,125V at 25,5Amps what comes to some 1,502V in RMS, it would be good to have signal amplified x2 – my ADC reference is 3V3. 

Here I have first question: should I amplify signal at the input or at the output?

In attachment you can find my schematics for which I think it will do 2x amplify at the input together with table of all inputs and outputs. Is this, as it is OK? Will it work as I expect? For C at CAVG I know, it is not orinted as it should be - bug in my schematic...

Thank you for all support and best regards,

Andrej

P.S. Data sheet for AD8346 is full of errors - especially for avaluation board used in single supply mode...

  • Hello Jest62,

    Thanks for your question re the AD8436 applicatiohn. In this instance it makes sense to use the FET-input amplifier because you can achieve the gain of 2x using the internal resostor. The rest of the circuit looks fine.

    As the author of the data sheet I'm always interested when people discover errors. Please list as many as ou have time for as we will be releasing an uprev in 1Q13.

    Best,

    js

  • Hello,

    speaking about data sheet there are pin names that are not always used the same way. For instance pin number 6 is sometimes BUFGAIN and next time IBUFGN. And others...

    But the biggest problem is using evaluation board if you are beginner. There is no word about connecting Vee and GND in case you are having single supply. Another problem is with IBUFF gain setting. It is written, that you should add 0R at R5 to have 2x gain. But there is no word about removing RFBH if you want to achieve this gain...

    I know, you professionals know all this but for OpAmp beginner is hard.

    Thanks for support and best regards,

    Andrej

  • Thanks Jest,

    These are great comments. Our job is to present the material for all skill levels so it is clear and understandable. We sure don’t want to see errors whether by commission or omission so comments such as yours are very helpful.

    We’ll modify accordingly.

    Best,

    js