Reference Designs for Analogue & Digital Inputs to Microcontroller

Question

I'm not so good when it comes the analogue end of electronics. My microcontroller has 0-3v inputs and I'm trying to get together a range conditioning circuits.

I know this I a reasonably broad question as the analogue design will depend on a lot of other factors, but are there any reference designs around for the following:

Analogue 0-3 volt

Now there'll need to be some over voltage protection and something for voltage spikes, but will that do?

Analogue 0-30 volt

Can I just use a resistor divider with some 1% resistors and hope for the best?

Digital 0-x volts

I guess here I'd be best to use an 'opto' based device that supports the required frequency?

• DarenW: How fast will the signals be varying? What kinds of noise do you expect? What is the impedance of the source of the signals? A simple resistor voltage divider is probably fine for the 0-30V, but good results depend on knowing these things to pick the right resistors.

Answer 1

This is a very general question and there are lots of app notes with reference designs for mixed signal boards. "mixed signal", "digital analog", "design" and "ground" are your keywords. In addition, usually every datasheet of a mixed signal controller or DAC contains a reference design.

A common method for protecting analog inputs is a Z-Diode and a series resistor. You have to realize the overvoltage protection requirements and the DAC leakage current to calculate the series resistor value. As you can see in http://www.atmel.com/dyn/resources/prod_documents/doc2508.pdf a series resistor can protect an analog input even from mains voltage.

Yes, you can use a resistor divider for scaling an analog input. If the input resistance is too low, use an transistor/opamp as a voltage follower.

For digital inputs you can use resistor dividers/series resistors, too, which is the slowest of the mentioned methosd. For up to 15V you can use a 74HC4050 buffer. If it need to be really quick, you're probably better off with a dedicated level shifter IC.