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12bit ADC MCP3208 Vdd 3.3V and Vref 5V possible? — Parallax Forums

12bit ADC MCP3208 Vdd 3.3V and Vref 5V possible?

StefanL38StefanL38 Posts: 2,292
edited 2012-04-18 17:06 in General Discussion
Hi,

I would like to know if it is possible to use the MCP3208-chip with powersupply-voltage 3.3V and Vref 5V.

The datasheet says All inputs and outputs w.r.t. VSS ............... -0.6V to VDD +0.6V. I interpret this as

If Vdd is 3.3V the maximum voltage to VRef and any ADC-input-pins is 3.3V + 0.6V = 3.9V.
Or does it mean maximum-input-voltage is Vdd-max + 0.6V which would mean 7+0.6 = 7.6V?

Of course I can set Vdd to 5V and then add current-limiting resistors of 4.7k for connecting to the propeller-chip.
But I would like to avoid this.
best regards
Stefan

Comments

  • Duane DegnDuane Degn Posts: 10,588
    edited 2012-04-18 11:48
    I'm betting it means you can't use more that 3.9V when using 3.3V as Vdd.

    You should only need one current limiting resistor when powering the chip with 5V. The Dout pin is the only pin connected to the Prop with 5V on it.
  • Erik FriesenErik Friesen Posts: 1,071
    edited 2012-04-18 14:40
    Why not run both at 5v? I have been doing this for years now, with 1k resistors between the 3.3v device and the 3208. The internal diodes won't allow 5v with 3.3v supply.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2012-04-18 14:55
    Erik Friesen wrote: »
    with 1k resistors between the 3.3v device and the 3208.

    Technically 1K isn't enough to limit the current to acceptable levels (according to datasheet). I think a lot of people use 1K resistors but the official recommendation is to use a higher value.
  • Erik FriesenErik Friesen Posts: 1,071
    edited 2012-04-18 15:09
    You are right. 500ua means it needs at least a 2.8k.

    Really? Only 500ua?
  • Duane DegnDuane Degn Posts: 10,588
    edited 2012-04-18 15:17
    Erik Friesen wrote: »
    Really? Only 500ua?

    That's what I've heard (and read).
  • kwinnkwinn Posts: 8,697
    edited 2012-04-18 16:52
    StefanL38 wrote: »
    Hi,

    I would like to know if it is possible to use the MCP3208-chip with powersupply-voltage 3.3V and Vref 5V.

    The datasheet says All inputs and outputs w.r.t. VSS ............... -0.6V to VDD +0.6V. I interpret this as

    If Vdd is 3.3V the maximum voltage to VRef and any ADC-input-pins is 3.3V + 0.6V = 3.9V.
    Or does it mean maximum-input-voltage is Vdd-max + 0.6V which would mean 7+0.6 = 7.6V?

    Of course I can set Vdd to 5V and then add current-limiting resistors of 4.7k for connecting to the propeller-chip.
    But I would like to avoid this.
    best regards
    Stefan

    Definitely not a good idea to have Vref any higher than Vdd. From the absolute maximum ratings "All inputs and outputs w.r.t. VSS ..... -0.6V to VDD +0.6V". I got curious and tested a chip up to VDD + 0.7V. The readings became noticeably non linear after Vdd + 0.4 and the input was damaged at Vdd + 0.7 (constant 4093).
  • Mark_TMark_T Posts: 1,981
    edited 2012-04-18 16:59
    StefanL38 wrote: »
    Hi,

    I would like to know if it is possible to use the MCP3208-chip with powersupply-voltage 3.3V and Vref 5V.
    No, Vref can range from 0.25V to Vdd (and not above it at all).

    The datasheet says All inputs and outputs w.r.t. VSS ............... -0.6V to VDD +0.6V. I interpret this as

    If Vdd is 3.3V the maximum voltage to VRef and any ADC-input-pins is 3.3V + 0.6V = 3.9V.
    Or does it mean maximum-input-voltage is Vdd-max + 0.6V which would mean 7+0.6 = 7.6V?

    Of course I can set Vdd to 5V and then add current-limiting resistors of 4.7k for connecting to the propeller-chip.
    But I would like to avoid this.
    best regards
    Stefan

    You can't even do that, its guaranteed input high value is 3.5V (0.7Vdd) so the Prop cannot be guaranteed to talk to a 5V MCP3208 without full level-shifting on the SPI bus.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2012-04-18 17:06
    Mark_T wrote: »
    You can't even do that, its guaranteed input high value is 3.5V (0.7Vdd) so the Prop cannot be guaranteed to talk to a 5V MCP3208 without full level-shifting on the SPI bus.

    I guess it's not guaranteed, but it's done that way a lot.
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