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ADC reference higher than Vdd — Parallax Forums

ADC reference higher than Vdd

Bobb FwedBobb Fwed Posts: 1,119
edited 2009-06-17 19:57 in Propeller 1
I haven't had time to look through the datasheet completely, but I didn't see anything on my brief read through. Does anyone know, can you give the MCP3008 (10-bit version of the MCP3208) reference pin a higher voltage than the input (Vdd)? The concept I have is to put a 4.096V reference on the reference pin. I am powering the IC with 3.3V. I could put a 2.5V reference, but I have found that the output voltages is less consistent over wide temperature ranges than the 4.096V reference ICs, not to mention the 4.096V ones are much more common.

Datasheet

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April, 2008: when I discovered the answers to all my micro-computational-botherations!

Comments

  • LeonLeon Posts: 7,620
    edited 2009-06-16 20:36
    If you read the data sheet you will probably see that you can't have any input higher than Vdd.

    Leon

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  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2009-06-16 20:43
    Duh...I was just blind the first time I read through it: "All inputs and outputs w.r.t. VSS .....-0.6V to VDD +0.6V"
    I was already typing "It is usually limited to some diode's forward voltage higher than Vdd"...and there it was.

    ...so probably (at best) limited to 3.9V reference...but that doesn't really exist...so I am stuck with the 2.5V.

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  • Clock LoopClock Loop Posts: 2,069
    edited 2009-06-16 20:50
    Bobb Fwed said...
    Duh...I was just blind the first time I read through it: "All inputs and outputs w.r.t. VSS .....-0.6V to VDD +0.6V"
    I was already typing "It is usually limited to some diode's forward voltage higher than Vdd"...and there it was.

    ...so probably (at best) limited to 3.9V reference...but that doesn't really exist...so I am stuck with the 2.5V.

    Why not just power the chip with your 4.096V, and then use the correct voltage dividers on all the lines that communicate with the 3.3v device.

    I did this when I had to operate a 3.3v lcd with a 5v microcontroller.

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  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2009-06-16 21:01
    The reference IC isn't designed to be a power supply.
    I did consider powering the ADC at 5V (already have a 5V circuit on the board) then reference at 4.096. But I don't have much room left on the board...I'd have to redesign almost the entire board to make room for the resistors between the ADC and the prop.

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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-06-16 21:23
    Bob,

    If it's the same board you posted in the other thread, you'd have plenty of room if you changed your passives to surface-mount.

    -Phil
  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2009-06-16 21:43
    No, it is a smaller board. As of right now, I am doing much by hand and I don't know if I want to mess with a lot of small SMT pieces...

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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-06-16 22:04
    Bob Fwed said...
    I don't know if I want to mess with a lot of small SMT pieces...
    It's definitely a speed bump, but the anticipation is much worse than the reality. With a stencil for applying the solder and an infrared toaster oven to reflow, things can go quite smoothly. Here is a thread that relates my experiences with Zephyrtronics' solder paste and Ryan O'Hara's stencils.

    -Phil
  • TubularTubular Posts: 4,706
    edited 2009-06-17 03:16
    If the voltages you are interested are in the low-middle of that range, you can use a Blue LED as a simple voltage limiter. It kicks in around 3 volts and has the benefit of visual indication. I do this with the MCP3304.
  • Clock LoopClock Loop Posts: 2,069
    edited 2009-06-17 04:28
    Phil Pilgrim (PhiPi) said...
    Bob Fwed said...
    I don't know if I want to mess with a lot of small SMT pieces...
    It's definitely a speed bump, but the anticipation is much worse than the reality.
    -Phil

    I would have to say that smt soldering is not that easy. You do need a precise temp gun, and if you have bad eyes, then you most def need a microscope. smt soldering sucks when you SCREW UP. bridged pins suck. .,,,, they suck.
  • LeonLeon Posts: 7,620
    edited 2009-06-17 04:59
    I don't get bridged pins with drag-soldering. The secret is plenty of jelly flux and a mini-hoof cartridge on my Metcal soldering station.

    Leon

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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-06-17 05:56
    For the first time in my life (and ever since), I did not get any bridged pins by using a solder stencil and the Zephyrtronics solder. Soldering with an iron is definitely not the way to go. Solder paste, a stencil, and a reflow oven (~$99 from Costco) is. It's almost goof-proof. Save the iron for the through-hole stuff.

    -Phil
  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2009-06-17 15:18
    Tubular said...
    If the voltages you are interested are in the low-middle of that range, you can use a Blue LED as a simple voltage limiter. It kicks in around 3 volts and has the benefit of visual indication. I do this with the MCP3304.
    Are LEDs consistent enough of a forward-voltage drop (I mean from one LED to another)?
    I think a better choice would be a 0.1% tolerance resistor voltage divider. Then I could customize exactly what the reference voltage is.

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  • TubularTubular Posts: 4,706
    edited 2009-06-17 19:57
    The LEDs can be a very good match within the same batch. Their advantage is they don't reduce the source impedance around say 2 volts. Thats not always important though.

    The right solution really depends on the nature of your input signal. Is in centred on 2 volts? Is it buffered / driven with a low source impedance? You mention a wide temp range - I assume this is important.

    Also you can run the adc off 5 volts and interface in to the prop via resistors. It can sample faster at 5v.

    tubular
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