Converting logic and sign

Question

I have an interesting project and I could use a little help. I'm using a microcontroller who's input to the ADC and output on the GPIO and DAC pins are all within a 3.3V range, and I have to interface with a 15V logic controller.

Can anyone tell me what would be the best way to convert down to 3.3V (assuming there's a better way than a resistive divider with a Zener to clamp) from 15V for the simple I/O?

How to scale down properly for the ADC and scale up properly for the DAC? At least, does anyone know of an industry standard or commonly used driver chip for this?

Answer 1

Digital IO can be scaled by any number of means. Some basic research around the terms "level shifter" will yield dozens of solutions. There are passive or active, integrated or discrete solutions.

To scale analog voltages down for the ADC, a resistive voltage divider optionally followed by a buffer is a good solution. Depending on the maximum source impedance imposed by your ADC, the buffer may not be necessary.

To scale up, try an op-amp. Since you are working with 3.3V you will probably be interested in the rail-to-rail variety, which are able to accept inputs and drive outputs nearer to the supply rails than traditional op-amps. I'd suggest you start by playing with TI's parametric search but it appears to be broken at the moment.

You can also make a quick and dirty DAC by PWM switching a transistor with its collector connected to a higher voltage and low-pass filtering the output, depending on the speed, accuracy, and noise requirements of your application.