AD636 rms to dc Operation

Hi,

I have a signal coming from a sensor which is DC and AC, I want to use AD636 as a RMS to DC converter.

I have few questions:

  1. Can AD636 be used for DC and AC signals together? My sensor has a 2.5V offset on which AC and DC signals vary.
  2. Does AD636 work on Single Supply for AC signals? I have an AC signal of the order +/- 10mV, can it sense that on a single supply? What is the operating range on single supply? what is the difference of operation for single and dual supply?

Regards,

Chaitanya

  • 0
    •  Analog Employees 
    on Oct 22, 2014 5:51 PM

    Hi Chaitanya,

    Thank you for using ADi products and sorry for the delay of response.

    About your questions.

    1. Yes, as what the datasheet stated, the device will convert ac voltage (or AC plus DC) to its equivalent DC voltage. The input of AD636 can go up to +/-12Vpp continuous  non destructive input and 200mV rms continuous rms level.
    2. Yes, It can work on single supply. AD636 can be used with unipolar supplies as low as 5 V and up to 24 V. You can refer to the datasheet for proper connection of the supply (page 10 of 16).The device  will work the same with single supply as with dual supply for AC input however it is impractical when you have DC or DC plus AC input due to the resultant dc level shift at the input. I suggest that you power the device in dual configuration if you have ac plus dc input.

    On the other hand, would it be okay if you could tell me your application? Let me know if you have any more concerns.

    Regards,

    Phil

  • Hi Phil,

    Thanks.

    I am planning to use it infront of AD22151 which is magnetic field sensor I am using it for isolated low current measurement. I am planning for a RMS to DC converter and a comparator creating a hall switch which can sense AC and DC low amplitude currents.

    I don't have dual supplies and the sensor gives output which is at a offset of 2.5V. The comparator which I am planning at the output of AD636 will not respond for small amplitude DC, needs to be offseted.

    Basically the concept is AC or DC small amount of change in current of the order 300-500 mA will cause change in flux in turn AD22151 sensor will give me a change of the order of  ~100 mVpp or 100 mV VDC which will be riding on a 2.5V offset, RMS to DC converter independent of AC or DC will give me equivalent DC which will be compared to a steady reference and comparator output will change if the threshold is crossed. Now the challenge is to make it work on single 5V supply.

    Please give me your suggestions.

    Regards,

    Chaitanya

  • 0
    •  Analog Employees 
    on Jan 9, 2015 5:56 PM

    @Chaitanya,

    Thanks for the email, I am the applications engineer for rms-dc converters.

    This looks like a recent application question from another customer. If I may suggest an alternative device, I developed a single-ended solution with provision for driving a follow-on device with a dissimilar common-mode voltage. You can ac couple the response from the 22051, then re-combine the rms result following conversion. I just released an application note that shows how to do this trick. The application is designed around our recently released AD8436 rms-dc converter. Attached are PDFs of the data AD8436 and AN1341.


    Good luck!

    attachments.zip
  • 0
    •  Analog Employees 
    on Jan 9, 2015 5:58 PM

    Before I forget, the application is on page 10 of AN1341.

    Best

  • Hi Jstaley,

    Thanks it is a really good application note.

    My problem is slightly different I have 4 cases:

    1. Pure DC input riding over DC bias In this case I want RMS to DC converter to pass it as it is. So output is DC input+ DC bias.
    2. Pulsating DC riding over the DC bias, in this output should be DC bias + Mean of Pulsating DC.
    3. AC riding over DC bias input, in this case I want the RMS value of AC to be added to the DC bias at the output of RMS to DC
    4. AC + DC input riding over DC bias output should correspond to AC RMS + DC input + DC bias

    Bias coming at the output is not necessary, it can be added externally.

    Assume we are doing it digitally then an ADC and a RMS calculator function in uC will result me correct DC value. It is Root mean and Square so if you feed a DC to that function it will give DC output. Just same.

    I think the RMS to DC converter chips AD636/8436 operates only for AC inputs and can not accept the DC input.

    What I am thinking is RMS to DC AD8436/636 is AC coupled. Output of AD8436 will be given to a OPAMP based summing amplifier (adder) and other input is coming from the DC part of the original input which was not coupled to AD636, now output of adder  (adder with C in feedback so it is a integrating adder to cover pulsating DC ) is finally is given to a comparator.

    Can we use the OPAMP in AD8436 as a adder in my scheme?

    Please suggest what do you think?

    Regards,

    Chaitanya