Voltage Clipping for ADC

Discussion in 'The Projects Forum' started by Steve_S, Mar 12, 2015.

  1. Steve_S

    Thread Starter New Member

    Mar 12, 2015
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    Prefix: New here and to electronics.

    I'm working on a project that will be run with a raspeberry pi. The area that I'm a bit confused on is where I am interfacing a freescale mpx5010 pressure sensor to an ADC MCP3008.

    The mpx5010 will be driven with 5V from a regulator board, the MCP3008 with 3.3V from the Rpi.

    Although the output of the mpx5010 sensor can be in the 0-4.7V range, in this application, under normal use, it will produce no more than 2.5V. This should provide me with 2.5V on the 3.3Vref scale on the ADC, so thats enough resolution for what I'm after.

    Now my concern is protecting those ADC inputs from the potential max voltage output of the sensor, just in case someone does something unexpected.

    I was considering a 3.3Vz zener diode from the sensor output to ground, which should clamp things to ~3.5V max (I think) on the input which is fine as the MCP3008 datasheet says Vdd +0.6, so anything < 3.9V is safe (thats another I think).

    In looking at zener diode clipping circuits there is always a resistor in series with the input signal, and I have no idea how to size that, or if its even needed?

    I also read about those types of clipping circuits altering the below clip signal / value as well. I'm not sure that is a concern given the max range I'm interested in being 2.5, so as long as the 0-2.5V output of the sensor is not altered and I clip anything above 3.3V (3.9V max) it should be fine.

    I've attached a quick pic of the specifics, input on how this should be done and how to size the components are greatly appreciated!
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    Zener diodes are not all that accurate in their clipping action, but what is worse they add a substantial capacitance to your analog input. For my money an opamp configured as a voltage follower or as an attenuator will give you the protection you need along with the series resistor to limit the current from an external source.
     
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  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    If you use an op amp to protect the input on the micro you still have to come up with a way to protect the input to the op amp.

    With a clamp circuit you size the resistor by looking at the largest voltage input, the zener voltage, and making a decision about how much current you can allow. Also, for an analog input such as a A2d converter there is a max spec on source resistance, so you need be below that. It's not quite an exact science.

    One technique is used when the input has an ESD protection diode: just use that diode as the clamp, and add an appropriate series resistor. Just like your circuit only i made the zener go away.
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    I think I mentioned the series resistor. If the A/D has input clamp diodes then all you need to do is limit the current. The opamp will provide a method of avoiding the affect of a series resistor on the input impedance of the A/D. What's the problem with using one?
     
  5. ebeowulf17

    Active Member

    Aug 12, 2014
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    What about a clamping circuit with a standard diode to 3.3V supply instead of zener to ground? Does that introduce the same capacitance problem, or is it just the zener configuration that does that?
     
  6. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Just use a series resistor, the input has it's own internal clamping diode to the positive rail.
    You need be mindful of the max current level this diode tolerates, choose the resistor value appropriately.

    Zeners suck at this application, the soft knee and poor predictability make this idea more trouble than it's worth.
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    1. Cost.
    2. Reliablity.
    2. Accuracy: the output of any op amp is different than it's input. Yes, you can buy very low offset amps, but see issue #1.

    One should never just toss in parts without a well defined need.
     
  8. Steve_S

    Thread Starter New Member

    Mar 12, 2015
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    Thank you for all the replies.

    Looks like the Zener is not a good plan, especially if it adds capacitance to the analog input, I don't want to alter that in any inconsistent fashion. I was hoping that since I was far enough below the knee point that it would not have any significant input, but thats why I asked :D

    I'm confused by adding the series resistor though. How does that prevent the over voltage to Vdd, is it just that it has lowered the current so much that it will not pass through the return diode or that the current is so low that it does "not matter"?

    I've attached the analog input model from the mcp3008 datasheet for reference.

    The mpx5010 says it has a "Output Source Current at Full Scale Output" of o.1 mA so does that mean I should add a series resistor of 50K for a 5V .1mA max input to the ADC?

    Again thanks for your help, I'm new to this so trying to understand it as best I can.
     
  9. MCU88

    Member

    Mar 12, 2015
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    If you have a read through the datasheets you will discover that many modern chips have built-in protection already. Which is basically two clamping diodes. One diode goes to the positive rail and the other to the negative.
     
  10. ebeowulf17

    Active Member

    Aug 12, 2014
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    But my understanding is that the internal clamping diodes are really tiny and can't handle much current, which is why series resistors are being recommended. Without the series resistor, the internal clamping diode will clamp the input to protect the adc... until it overheats and fails, at which point there's no protection.

    As for sizing the resistor, someone else should verify, but I think the current through the clamping diode will be calculated from the voltage difference between input voltage and clamping voltage. If vdd is 3.3, that makes clamping voltage 3.9ish? So if the max input you're worried about is 5v, there's only 1.1v of overage dumping through the clamping diode. I don't know how much power the internal clamping diodes can dissipate, but I think it's going to be in the input specs, and shouldn't have anything to do with the output abilities you mentioned.
     
  11. Steve_S

    Thread Starter New Member

    Mar 12, 2015
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    *nods* ...

    Its the section under absolute maximum ratings that states "All Inputs and Outputs w.r.t. VSS............– 0.6V to VDD+0.6V" Since I'm driving Vdd with 3.3 and have potential on an input to be 4.9V, its above the rating ... could be nothing to be concerned about but wanted to be safe.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    If the maximum output voltage is 5V and you want to keep the input current through the input protection diode at no more than 0.1mA with a 3.3V supply voltage then the added series resistance would be (5V - 0.6V - 3.3V) / 0.1mA = 11kΩ minimum (0.6V is the protection diode voltage drop).
     
  13. ebeowulf17

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    Aug 12, 2014
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    Do we know that the MCP3008 actually has the built in protection diodes? I read through the datasheet and saw no mention of them.

    Assuming they are there, how do we know how much power they can handle? The 0.1mA spec was for the output from the mpx5010, not any input spec on the MCP3008.

    Really, I just want to learn this stuff. I'm not saying any comments above are wrong, but I'd like to be able to figure this out on my own next time, and right now I'm still confused.

    Thanks!
     
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  14. crutschow

    Expert

    Mar 14, 2008
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    If you aren't sure about the input protection then just add a diode, such as a 1N4148, from the input (diode anode) to the plus ADC supply along with the series resistor.
     
  15. MCU88

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    Mar 12, 2015
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    Two diodes...

    [​IMG]
     
  16. Steve_S

    Thread Starter New Member

    Mar 12, 2015
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    I've not been unable to find any information on the protection diodes in the MCP3008 so sizing the series resistor is a bit of a best guess.

    I was looking in to this a bit more, appears to be called a positive bias clipping circuit. If I tie it to the 3.3V rail, and choose a diode with a Vf < .6v then once the voltage climes above 3.9 its clipped. Is that better than the zener arrangement, seems the Schottky diodes are a bit better then the zener in terms of the specs.

    I put together a quick diagram to make sure I've got this idea correct. One downside I guess is that the 3.3V rail is always conducting a small amount of current to keep things biased. Well at least as I have it drawn, not sure thats correct anyway so input is welcome !
     
  17. jjw

    Member

    Dec 24, 2013
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    Figure 4-1 in the datasheet is the input model and there are protection diodes to Vdd and Gnd.
     
  18. Steve_S

    Thread Starter New Member

    Mar 12, 2015
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    The way I understand that is its not going to pass voltage through to Vdd until the input voltage reaches Vdd plus the Vf of the diode.

    So what happens when Vdd is 3.3Vdc and the input is 5Vdc? Would that not pass 5-(3.3+.6) = 1.1Vdc additional on to Vdd? Sorry if thats a dumb question, but I'm trying to understand how that protects the over voltage.
     
  19. ebeowulf17

    Active Member

    Aug 12, 2014
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    :rolleyes: Don't know how I missed that. Thanks!

    The other thing I'd still like to know is how much power those can handle. Don't you need at least a rough idea of that in order to size series resistors appropriately? Is there a number for that in the specs that I overlooked? Or is there a somewhat "standard" number for what internal clamping diodes can safely dissipate?
     
  20. crutschow

    Expert

    Mar 14, 2008
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    You only need two diodes if you want to also protect against negative transients, which is not the case here.
     
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