lpc214x ADC voltage tolerance

Development Boards MicroMod
1

I’m working out an idea to use the lpc2148 as a volt-meter. I designend an op-amp network that that can shift and scale the difference of an input voltage and send it to an ADC pin. The problem is that in order to minimize distortion I want to supply the op-amps with a +5V rail and a -5V rail. This means the voltage at the ADC pin may be as high as +4V or as low as -4V if the input voltage is too high or low. I carefully read the ADC info in the user manual and I understand that a voltage higher than 3.3V + 10% may cause innacuracies although it’s tolerant up to +5V. This is ok for my application. What I’m worried about is the -4V voltage. Could this damage the pin?

I was thinking that I could try implementing a clipping circuit but I don’t know if it’s even necessary.

Thanks in advance for you help

2

You shouldn’t apply a negative voltage to an input, check the data sheet.

Leon

3

Connect the output of the op-amp to the input of the LPC2148 using a resistor (in the 1K…10K range) and put two clamping diodes to ground and +3.3V. The ADC will still only read values between 0 and 3.3V, but if the voltage becomes negative at least you won’t damage the micro :slight_smile: