I'm trying to design a power supply for 8 hobby servos which senses the current consumed by each of them. Size and cost are the main drivers, much more so than absolute accuracy.
I'm designing for 1A per channel, although it might be nice to have a couple of 2A channels.
I have an MCU with enough single-ended analog inputs. If the best way to do this is differentially, which I suppose might happen if the sense resistor is on the high side of the load (is there a reason to put it there), then I might need more pins and would probably add something like a MAX11609 or MAX11611 I2C ADC. Apart from knowing whether it needs to be single-ended or differential, I've got the ADC part under control.
My questions are:
- Where do I put the sense resistor?
- How do I do sufficient low-pass filtering that I can sample these current measurements slowly (say 10Hz) and smooth out whatever is going on at frequencies higher than that?
- Does it make sense to look at something like a current-mirror FET? I can't seem to find any with current ratings remotely as low as 1-2A, they all seem to be rated for 40A+ and sized accordingly. It would be nice because then there would be over-current protection, but I can certainly live without it if its going to be a 300% price difference and a 100% size difference.
- How do I amplify the signal? Does it make sense to amplify before or after filtering, and before or after multiplexing?
- Is there something I'm not thinking of which is going to cause a major problem. ;)