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AMC1200: with Breakout board, RC filter for DC operations, Humidity and related questions

Part Number: AMC1200
Other Parts Discussed in Thread: MSP430FR5994, , AMC1301

Hello,

I am building a digital current sensing solution, for which i am using AMC1200 for amplification of low voltages of kelvin shunt resistor and reading them via MSP430FR5994 differential ADC. Below is my current schematic around AMC1200.

Please note that, all components of this schematic are surface mount, soldered on breakout board. Since rest of my design is through hole, i have converted this design to through hole with the use of DIP breakout board.

I have few questions regarding this design.

1. Is it safe to use the breakout board approach? Will it affect the accuracy or performance of AMC1200? I am placing capacitors and resistors as close to AMC1200 as possible.

2. Since operation of system is purely DC, i plan to use 15Ohm and 10uF input RC filter, so that i get approx 1KHz cutoff frequency. ( fc = 1/(2*pi*R*C) ). Are my calculations correct for filtering AC noise?

3. I have not used RC filter on output side. But datasheet example designs (page # 15,16) have output RC filters also.

Is it necessary to use output RC filter? My microcontroller is less than 80 mm far from AMC1200 breakout board. If yes, then what values of output filter, and how close to microcontroller?

4. I had bought this AMC1200 from mouser.com, and its casing had some info written over it, such as number of hours before which IC should be soldered after opening the case. And some information related to humidity restrictions also. I have opened the case around 4 months back. Does that render my IC useless, as the case mentioned soldering IC within a week. ?

5. I am using same +5V source for high side and low side (converted to +3.3V via AMS1117) supplies, and GND1, GND2 are also taken from same ground plane. Is it okay to do that? I don't need isolation as my load (high power LEDs) and other circuit probably won't generate fluctuations.

Thank you for your time and help in answering my questions.

Regards,
Hard Patel

  • Hi Hard Patel,

    Thanks for your questions about the AMC1200. Sounds like an interesting project!

    1. Breakout boards (in my opinion) are never quite as good as an actual PCB design, but since this is pretty simple setup, you may be fine.
    2. I'm a little concerned about using a 10uF cap on the inputs though, consider 150 ohm and 1uF as a start and let us know how that works out.
    3. Using a filter on the input of the ADC can help improve measurements, so at least add some 0 ohm resistors and space to add caps just in case they are needed.
    4. Opening the package on the chips can have an impact on solderability - the warnings are directed more at solder reflow operations, but hand soldering can also be an issue. You can always bake the parts before you solder - look over this document for some additional information:
    www.ti.com/.../spraby1.pdf

    5. You can use the same GND/Power if you don't need isolation.
  • Hi,

    First of all, thanks for the really quick reply.

    Thanks for your questions about the AMC1200. Sounds like an interesting project!

    Yes, it's an interesting project with lot of challenges. And also lot of learning for me. It's a precision high current digital regulator.


    1. Breakout boards (in my opinion) are never quite as good as an actual PCB design, but since this is pretty simple setup, you may be fine.

    It is actually 150+ components board, with one main board and two breakout boards (One is AMC1200 & the other is AD5272 digital potentiometer). I am using custom made breakout board only, so as to segregate hand solder-able and machine solder-able components. Below is my layout for AMC1200 breakout board.



    2. I'm a little concerned about using a 10uF cap on the inputs though, consider 150 ohm and 1uF as a start and let us know how that works out.

    That can be done. But according to AMC1200 datasheet recommendations, page 16 section 8.2.1.2, using resistor values more than 24 Ohm may cause incomplete settling of input circuitry. Shouldn't that be a problem?  Also, for my learning, may i know why 10uF caps on input are not a good idea.?


    3. Using a filter on the input of the ADC can help improve measurements, so at least add some 0 ohm resistors and space to add caps just in case they are needed.

    That's an excellent idea. I will surely implement it.


    4. Opening the package on the chips can have an impact on solderability - the warnings are directed more at solder reflow operations, but hand soldering can also be an issue. You can always bake the parts before you solder - look over this document for some additional information:
    www.ti.com/.../spraby1.pdf

    5. You can use the same GND/Power if you don't need isolation.

    It's a good document. Thanks again for the help.

    Regards,

    Hard Patel


  • Hi Hard,

    With the AMC1200, you are basically connecting the analog input directly to a switched capacitor network (see Figure 31), the sample cap is only ~3.6 pF which is what drives the recommendation on page 16.  The sampling time is also dependent of the capacitor, which we show as being on the order of a few hundred pico-farads.  You might consider using the AMC1301 instead which has a resistive input and the same pin-out.

  • Hi Tom,

    Yeah, i looked at the reference mentioned by you. So now i am thinking of using 1nF cap with 15 Ohm resistor at input. If that doesn't work as expected, i will have to scale it up to 1uF.

    Also, what value of filter should i use at output? Just before my launchpad's ADC. Can 10uF with 15 Ohm be a good value at the output? Or should i go for 1kOhm with 10nF?

    Regards,
    Hard Patel
  • Hi Hard,

    I don't know the specifics of the ADC on your launchpad, but you will have the same issues with voltage across a sampling capacitor there as well.  You are going to want to keep the capacitor value relatively low, 10nF may be too much for the ADC inputs.