MCP3008 VDD 3.3 and Vref 5.0

[spuds]

Subject is the general question.

I going to use the MCP3008 in a project with my Rpi. The sensors I have work at 5V mostly to provide a wider scale of output, although the Ax input back to the chip Ax inputs will not exceed 3V.

So the question is can one set the VDD at 3.3V and Vref at 5.0V? Looking at the spec it appears the VDD is the voltage to communicate back to the pi on the SPI pins, and Vref is used in the internal converter to return the scale value from Vref to Ax

If this is not acceptable, then could I run the Vref = Vdd = 3.3, still drive the sensors at 5V knowing the output (and Ax input) will be below 3.3V (due to the range of output that the sensors will actually see). Seems like this would be OK, but maybe some form of clamp on the sensor output to avoid the voltage going > 3.3?

Thanks for your input on this.

[adafruit_support_bill]

The absolute maximum ratings for the chip state:

All Inputs and Outputs w.r.t. VSS ............– 0.6V to VDD + 0.6V

http://www.adafruit.com/datasheets/MCP3008.pdf

That means that no input should ever exceed VDD + 0.6v. That includes Vref as well as any sensor inputs.

[pbrunnen]

Hi Bill,
I was trying the same and have been having trouble and happened upon this post.

I am currently running with Vdd as 3.3v for the RPi and 5V for Vref. Your post might explain the troubles I have been having with my values... But my reading of the spec document didn't lead me to the same conclusion (hence having tried this path too).

Vss is defined as -0.6v to Vdd + 0.6v as you state. However, the electrical specifications table states that "Input Voltage Range for CH0 or CH1 in Single-Ended Mode" is a Min of Vss and a Max of Vref. I am finding this a bit confusing...

Thanks! -Cheers, Peter.

[adafruit_support_bill]

"Absolute Maximum" ratings are not the same as the normal operating range. The Absolute Maximums are the point beyond which permanent damage to the device is likely to occur.

The ranges listed in the "Electrical Specifications" section define the normal operating range. Operation outside of these ranges, but inside the Absolute Maximum ranges may not cause damage, but is not guaranteed to function correctly.

I am currently running with Vdd as 3.3v for the RPi and 5V for Vref.

That exceeds the normal operating range specified in the Electrical Specifications by 1.7v.

Reference Input
Voltage Range 0.25 — VDD V

It also exceeds the Absolute Maximum rating by 1.1v.

All Inputs and Outputs w.r.t. VSS ............– 0.6V to VDD + 0.6V

[pbrunnen]

Oh, I see... I completely missed that "Reference Input" heading since it wasn't bold... I see now what your referring to. This would certainly explain the unpredictable values I was getting...

Thanks! -Cheers, Peter.