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beagle bone black current meter

Former Member

I'm going to try and make a current monitor on the beagle bone, but a single adc dosn't have enough resolution for me so I want to use 4 of the beagles adc pins with a quad opamp to amplify the voltage to separate bands and a couple of transistors to keep the max value to the beagles 1.8V adc reference voltage. the range of current i want to measure would be 0-100mA at 3.6V. I have the programming skills but not the faith in my hardware knowledge so if anyone wants to help me with tips or component lists, It would be appreciated.

  • 0

    Need more detail !

    What current (AC/DC, min, max, resolution, accuracy) are you trying to measure and how will you sense it. How many samples per second.

     

    Using amplifiers will work to give you several current ranges but can't (usually) help with extending the resolution of an ADC.

     

    MK

  • 0 in reply to michaelkellett

    ok the device that I will monitor peeks about 70mA so i want to measure 0-100mA, I believe the standard way is to place a resistor in series with the device and measure the voltage across the resistor.

     

    I will be using the beagle bones ADC to measure that voltage but they have a range of 0-1.8V across 256 points allowing a resolution of 7mV, id like to try and increase that but as you said I cant change the ADC resolution but what if I can stack it so the first ADC measures 0-25mA, the second 25-50mA, third 50-75mA and forth 75-100mA then I can program it to take the reading that isnt maxed or minimum values,

     

    Does that make sense?

  • 0 in reply to Former Member

    It might make sense - the problem is that your extra parts still leave you with a feeble 8 bit ADC - this might do for your purpose.  Better ADCs are quite cheap. Using an external 12 bit ADC would get you 4x the resolution of your amplifier plan. You didn't answer about sample rates so no suggestions as to which part are possible.

    If you use the multiple amplifier idea be careful about overdiriving the ADC inputs - some ADC and multiplexor systems suffer from errors on ALL inputs if only one is overdriven.

     

    MK

  • 0 in reply to michaelkellett

    I'm not that concerned about sample rates, but i am concerned about gpio pins, most ADCs will interface using i2c or scp and they will be needed, and i know that the i2c is stackable. but i have no plans for the adc so i want to use it for this (I know im awkward)

     

    as of the 8 bit i wouldn't be 8 bit id get 10bit.

     

    I wanted to protect the ADCs from overdrive by using a transistor or similar to vary from the reference voltage of 1.8V with the signal

  • 0 in reply to Former Member

    I've just looked at the BBB's ADC spec and it's a 12 bit converter. Unless you are willing to use 0.025% resistors to set the gain of your amplifiers (or trim them this well) just use one channel. If you set it up for 100mA full scale then the resolution will be 100/4096 = 0.024mA = 24uA.

    If you use a 1R current sense resistor the voltage drop will be 100mV full scale and you'll need an amplifier with offset drift of less than 10uV over temperature to get the best from the ADC. (Don't worry about the basic offset because you can trim that out in calibration.)

    Transistor protection doesn't work very well (either affects the gain near full scale or doesn't limit the voltage exactly). Power the amplifier from the 3.3V supply and set the amplifier gain to 33. (You are going to need an amplifier with rail to rail IO). Use a resistor attenuator from the ampliifer to the ADC so that 3.3V on the amplifier gives you 1.8V on the ADC (use 833 R from the amplifier and 1K to ground - ideal values - you can make the 833 bigger and the gain will be  a little bit low).

    If you really want you can use two amplifiers with different gains to get two ranges but I can't see that you need 4.

    The amp needs to be quite good, 3.3V supply, RRIO (rail to rail input and output), low offset, low offset drift.

     

    AD8551ARZ would be  a good amp to use and the data sheet (page 21) shows you how to measure low side current. It only comes in surface mount packages which may be a problem for you.

    Something cheap like MCP6231-E/P comes in DIL but has a huge offset(> 5mV) and not very good drift.


    I couldn't find a reasonable low drift part in DIL package.


    MK

  • 0

    Why not use the very simple devices by Zetex? They're quite small, already have a built-in gain, or a current output (so you can set the gain by adding an external resistor). Take a look at ZXCT1041 (bidirectional, gives absolute value of current) or ZXCT1109 (1-directional, is supplied by sense voltage. Ideal for monitoring a power line).

    Touchstone also provides these sensors, and ships free samples (Current Sense Amplifiers | Touchstone Semiconductor)

  • 0 in reply to vsluiter

    There might be some problems with the '1041 because its output impedance is 15k but the suggestion in the BBB docs is that the source impedance be less than 1k. It's in surface mount as well but the OP hasn't said if that's a problem.

    The high side sensing ability might be useful.

     

    MK

  • 0

    thanks all for your input, package isn't that much of a problem as-long as is big enough to solder by hand though Id prefer through hole components,

    to Michael Kellett, I read that it was an 8bit but further reading has shown me that too, so 1 ADC may well be enough but your comment on a second amp wouldn't give me any advantage as it will still have the same 0 and thus id only get increased resolution on the low end but I did find a possibility if I add a reference voltage to the signal and put them in a negative of an opamp then the wave is shifted down allowing me to start the second from 1/2 the hight of the wave and thanks for the heads up on the ZXCT1041 I'm not sure what the problem is with impedance if you could give me a point in the right direction regarding the docs.

     

    Thanks again all