AD8436: several questions about deployment

I have read the data sheet and AN 1341 but still have some questions: 

1- Data sheet graphs suggest one can use this IC with up to 1Vrms (with 4.8V single supply) but performance data is specified for 300mVrms. I assume this is the input voltage level it is fully characterized. So my question is if I use with a different (higher) input voltage level, what is the impact over the performance, if any?

2- I want to use it to monitor a current source operating at 125kHz (the IC will be preceeded by a conditioning circuit). The output will feed a control circuit that will disable the current source output should output current exceed a limit. I understand it is usable at this frequency. However it is not clear to me how much is the error at this frequency (preferably with 5V single supply or +/-5V otherwise).

3- From the formulae I learned 3.3nF for CAVG would do fine for 125kHz. I understand CCF should be 1nF to comply with the requirement the second pole frequency to be at least three times as set by CAVG. Is that right?

4- C0G is OK for these capacitors?

5- I would like to use OBUF with 5V, single supply. The data sheet specifies performance for +/-5V. Is it useable with single supply? Output apparently is rail-to-rail but no information is provided about input range.

Thank you very much in advance for the support.

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  • 0
    •  Analog Employees 
    on Nov 24, 2017 2:38 PM

    Hello EVC,

    1. The AD8436 error does vary with input level. These types of rms-dc are based on a log-antilog approach, which is constrained by the supply voltage and input level. The 300mV for this part is the test voltage as you noted above.

    2. The error sources for the this type of converter are complex to model, especially since they are interactive. We recommend direct measurement using the eval board we provide. We do offer the LTC1968 which is quite linear with input level and supply voltage but with less dynamic range. The part does operate quite well at 125kHz as well, I've attached a data sheet.

    3. Correct.

    4. COG or NPO are the best.

    5. OBUF (and IBUF for that matter) work well at 5V single supply. Both these buffers incorporate all the opamp technology we have within ADI.

    Depending on your needs, the AD8436 is a pretty good all-around part, especially with the onboard buffers single supply capabilities all in one device.

    Good luck!

Reply
  • 0
    •  Analog Employees 
    on Nov 24, 2017 2:38 PM

    Hello EVC,

    1. The AD8436 error does vary with input level. These types of rms-dc are based on a log-antilog approach, which is constrained by the supply voltage and input level. The 300mV for this part is the test voltage as you noted above.

    2. The error sources for the this type of converter are complex to model, especially since they are interactive. We recommend direct measurement using the eval board we provide. We do offer the LTC1968 which is quite linear with input level and supply voltage but with less dynamic range. The part does operate quite well at 125kHz as well, I've attached a data sheet.

    3. Correct.

    4. COG or NPO are the best.

    5. OBUF (and IBUF for that matter) work well at 5V single supply. Both these buffers incorporate all the opamp technology we have within ADI.

    Depending on your needs, the AD8436 is a pretty good all-around part, especially with the onboard buffers single supply capabilities all in one device.

    Good luck!

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