Stephen Rynn

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8 years, 10 months ago.

Will inserting a negative supply into the mbed analog inputs damage the hardware?

Hi all

I have is a -0.7V to 3.3V input (-0.7 diode breakdown voltage on a negative supply). Id like the mbed to read 0V to 3.3V as an analog input. Would inserting the -0.7V into an analog IO pin cause any issue with the mbed NXP LPC1768 hardware?

Many thanks Stephen

2 Answers

Sanjiv Bhatia
8 years, 10 months ago.

I would NOT feed -0.7v onto the analog input pins of this micro. From the datasheet for the LPC1768, Section 8. Limiting values, page 37:

-0.5 volts is the min value to be applied onto the analog input pins (without damage).

Consider to apply in series a low Vfs schotkky diode to block negative voltages yet allow positive voltages with a very low voltage drop. Always recommend that you test the diode operation out of circuit to confirm indeed the component will block your negative voltage but it should work fine.

Reference: http://www.electro-tech-online.com/threads/blocking-negative-voltages-to-adc.124892/

Accepted Answer

Thank you for your answers. I have got the min input voltage down to -0.3V which should be safe now.

posted by Stephen Rynn 13 Jan 2016
David Thedens
8 years, 10 months ago.

Probably will cause issues. The Electrical Specs specify that the analog input can be 0v Min to Vdda Max. You could condition the input with an opamp(s) to move the range from -0.7 to 3.3v to 0v to 3.3v.