Negative voltage on LPC1114 ADC input

[lpcware]

Content originally posted in LPCWare by rbohl on Thu Jan 12 08:34:41 MST 2012
Hello,

The LPC111x datasheet indicates a maximum negative voltage for all inputs of -0.5V.

Without introducing a level-shifting circuit, can a LPC1114 ADC input tolerate a temporary input of a -1.0V to +1.0V audio frequency signal for the purpose of a system loop-back test without damage to the LPC1114?

Thanks

[lpcware]

Content originally posted in LPCWare by gbm on Fri Jan 13 04:13:42 MST 2012
They are not ESD protection diodes. They are ordinary diodes with specified maximum current guaranteed not to cause latchup. They may be used safely if the maximum allowed current is not exceeded.
There are, however, some quirks. First, many manufacturers do not specify the max. current. Second, in 3.3 V powered 5 V IO tolerant devices, the structure of IO pad may be not that obvious - this affects the positive voltage clipping.

[lpcware]

Content originally posted in LPCWare by ArneB on Fri Jan 13 01:32:58 MST 2012
A better way would be to use a decoupling capacitor (AC signals can pass through), which is connected to the AC signal and on the other side to a 1:1 voltage divider. You are shifting the +-1V AC signal to a VCC/2 +-1V signal with this approach.

I am always a bit scared to rely on built-in ESD protection diodes. They were not designed for continuous operation.

[lpcware]

Content originally posted in LPCWare by gbm on Thu Jan 12 13:01:48 MST 2012
If you use a series resistor of 2..10 kOhm, you can feed the input with out-of-range voltage. Parasitic diodes will happily sink the small current for you. Note however tat ADC will only measure positive voltage and it will return 0 for negative values.