Some clarification regarding datasheets

[arunb]

Hi,

How much -ve supply can be given to the ADC pins of a 18F4620 mcu.

In the datasheet, page 325, the following is given

Voltage on any pin with respect to VSS
(except VDD and MCLR) .................................. -0.3V to (VDD + 0.3V)

Does this mean -0.3V is maxmum possible ??

I have a op-amp supply that generates -0.5 V will this damage the ADC port??

thanks
arunb

[FvM]

You can answer this question yourself by considering another absolute maximum ratings specification: |Input clamp current| < 20 mA. That means, the input voltage won't damage the chip, as long as the current is limited suitably.

But, specified operation, particularly of the analog part isn't guaranteed under this conditions. Driving an input into clamp can e.g. cause an exessive input current of some other pin or additional ADC errors. You won't find a specification, but similar effects are known from other CMOS parts as CD405x analog multiplexers.

So if you expect the input voltage to cause conduction of internal clamp diodes (e.g. at 0.5V) in normal operation, you should add external schottky clamps.

[SherpaDoug]

To add to FvM's comments, there are two fault conditions to consider: a) faults that will permanently damage the hardware, and b) faults that will cause temporary malfunction of the hardware or at lease temporary failure to meet spec.

If you connect the A/D input to a -0.5V source through a resistor of say 1k, the resistor will limit the current flowing through the PIC to keep the current below the 20mA limit. The voltage seem on the actual PIC pin will probably be close to -0.3V. In this case nothing will be permanently damaged. But having this large current flowing through the A/D pin is likely to disrupt the operation of the A/D such that reading on other channels will be inaccurate. If this is just a start-up condition then everything should be fine once things stabilize. But if you have multiple sensors on the A/D and one sensor going off scale or suffering a broken wire might cause the -0.5V condition, you can expect that other sensor reading will go bad as well.

If you use the 1k resistor and add a schottky clamp to keep the PIC pin voltage away from -0.3V, then the PIC may suffer the loss of one sensor without disrupting the others.
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