Interfacing hall-effect sensor to embedded CPU

Discussion in 'General Electronics Chat' started by stube40, May 30, 2011.

  1. stube40

    Thread Starter Member

    Feb 3, 2010
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    I'm trying to interface an Allegro A1360 half-effect sensor to a TI MSP430 CPU.

    The A1360 is a 5V device whilst the MSP430 operates from a 1.8V to 3.6V supply. The MSP430 has onboard ADC, but the analog input pins have an absolute max rating of VCC + 0.3V.

    I have both 3.3V and 5V regulators on my PCB design and I'm assuming that I'll need some opamps or something to interface between the MSP430 CPU and the A1360 sensor.

    I'm still fairly new to electronics design and I'm looking for advice as to a good way to interface these two devices.
     
    Last edited: May 30, 2011
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    It depends on your application of the hall effect output.

    If you just need to sense presence and nothing else, a simple resistive divider should suffice. More info please.
     
  3. stube40

    Thread Starter Member

    Feb 3, 2010
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    I will be measuring the voltage output of the A1360 using the MSP430's internal ADC in order to evaluate the current flowing through a cable - the A1360 slots into a ring concentrator that encircles a cable that will be carrying several 100s of amps.

    I'm not sure what the maximum output of the A1360 will be at this stage as I'm not sure what the ring concentrator looks like yet, so I need to assume that I may have to read the full 5V output.

    I'm only making a handful of these and they will be individually calibrated.
     
  4. jaclement

    Active Member

    Apr 15, 2009
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    Try running the A1360 on the 3.6 volt supply. the output will be not go above its supply
     
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Doesn't the hall sensor already have an open-drain output?
     
  6. stube40

    Thread Starter Member

    Feb 3, 2010
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    I don't know..........

    "open-drain" is a term I've heard alot since I've been working with embedded CPUs, but I've never fully understood the meaning of it. I'm just about to pull out Horrowitz and Hill to see if it can enlighten me!

    In the meantime, the data sheet shows an internal functional block diagram which shows two series op-amps on the output. It also describes "output source current 3mA" and "output sink current 10mA" in the absolute max ratings section.

    Does this make sense in terms of working out whether its open-drain or not?
     
  7. stube40

    Thread Starter Member

    Feb 3, 2010
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    OK - Horrowitz and Hill pg 488:

    " The predecessor of three-stage logic was 'open-collector' logic, which allows you to share a single line among the outputs of several drivers. An open-collector (or open-drain) output simply omits the active pullup transistor of the output stage. The name 'open-collector' is a good one. When you use such gates you must supply and external pullup resistor somewhere. "

    Hmmm, not sure I'm any clearer after that (imagines the many experts on this forum rolling their eyes..............)
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Sorry Stube40 I could have been more descriptive with that term. Yes I meant "open drain" as an output that can pull low to connect to ground, but will not output a positive (source) voltage. Normally you can consider "open drain output" sensors as the equivalent of a simple switch to ground.

    The Allegro page shows the internals of the A1360 and it appears to be a linear voltage output, so that is not open-drain, sorry for the confusion;
    [​IMG]

    Does the datasheet list the max output voltage of the sensor? Is there a spec for Vout?

    Or (as Jaclement suggested) you could just run the sensro from 3.3v like your micro, but check the datasheet to see that the sensor will operate properly at 3.3v.
     
  9. Jaguarjoe

    Active Member

    Apr 7, 2010
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    The datasheet sez 4.5 to 5.5 Vcc
    They describe it as a linear sensor yet give values for pullup and pulldown resistors.
    If it can source 3ma it is not an open drain/collector device.
     
  10. stube40

    Thread Starter Member

    Feb 3, 2010
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    Thanks for all your help folks!

    I've manged to establish that the magnetic fields I will be measuring equate to a maximum of around 280mV from the sensors output, whilst expecting normal usage to be in the 0 to 59mV range.

    Also, since I will be needing to measure both mag field polarities, I need to make it bi-directional. The sensor comes setup with a DC offset of 2.0V, so this would mean that 2.280V equates to my maximum positive mag field measurement and 1.720V is the max negative field.

    So, I'm not thinking I will need a to put together a circuit with at least one opamp (possibly two??) to get this working. Best dust off my copy of Horrowitz and Hill and see what I can come up with.............
     
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