I have a BeagleBone (BB) and want to connect the analog output of a motor controller to it. The motor controller creates an output in the range of -4V to +4V and the BB anlog inputs only accept voltages between 0V to 1.8V (digital pins run at 3.3V). I only have a comparably small limited space of perfboard available, but 4 motors to monitor. I can provide 5V and 3.3V supply and 1.8V ref Voltage. Here are the options I found so far:
- Make an op-Amp circuit to convert [-4..4] to [0..1.8]. Probably a nice way, because I could use directly the BB-ADCs, but I am not to confident with these circuits (I found this example) and I would need one for every input which seems to be to many parts/space.
- Use an ADC and connect it via I2C: The Max127 seems to provide exactly what I want. Single 5V supply, analog input range between [-5..+5] and I2C interface. I would really like to use this one, the only problem I have is that according to the data sheet digital high is seen as 0.7*Vdd = 3.5V. I do not now if 3.3V would be detected correctly (anyone has experience with that?). Pulling up to 5V is not possible as the BB is not 5V tolerable.
- The Max1270 is basically the same as the Max127 but with SPI-interface. There the data sheet states the digit high threshold is 2.4V. I know that the I2C slave only pulls down. How does it work with spi, because the data sheet mentions digital outputs voltages (which are Vdd)?
- Lose some resolution and only use the output range from [0..4]V. That gives a larger variety of possible unipolar ADCs, but most of them have the same limitation with 0.7*Vdd for digital high signals.
- As in 4. But use ADCs with 3.3V supply voltage which have a range of [0..5], but I could not find any with 4+ channels.
I believe that I somehow going in circles. Does someone know a viable solutions or how to address the mentioned problems of (at least) one of the above described? I would generally prefer through hole parts (as it's perfboard) but I could probably get some small breakout boards for SMD-devices as well.