Z80 external VREF for ADC, max voltage?

Should be listed in the data sheet for your part under the Absolute Maximum Ratings section. It is probably the same as the maximum for the individual AD channel inputs. Usually it is not a lot higher than your Vcc.

Series resistors will limit the current that could flow into your VREF and AD pins but the real solution is to make sure that the maximum input they will see is less than the value listed for the absolute maximum.

HTH

--
James T. White

I would expect - barring whatevers on the datasheet - it to be in the range

0.6-VCC-0.6V at best. Generally, A/Ds will not digitise outside their voltage ranges. The purpose of the vref pin is not to provide gross adjustment, but a fine reference. If you need to digitise large signals, you've got to reduce the voltage first, so it's in the range acceptable by the A/D.

However. Read the datasheet, and hopefully all will become clear.

Thanks for the help. I had looked for the ratings in the spec sheet, but had not looked hard enough. Found it eventually, it says the External Reference Voltage should be less than VDD.

So, I'll bring the voltage down with a voltage divider, unless someone says that there's a better way to do it. (?)

The spec sheet also says it put a decoupling capacitance from VREF to AVSS. I'll assume that means a capacitor in series, please correct me if not.

In case you're curious, this project is to measure the voltage across a lead acid battery and log it to a PC. I'm building an electrathon (electric endurance racer) and want to test my batteries automatically.

Thanks for the help. Tom.