Thanks, Harvey, that's an awesome explanation!
> So firstly, let's feed all the sensors from 1.8 volts. Now this > works, and can work well. The question is how to get the 1.8 volts. A > switching converter, while efficient, generates noise, is relatively > complex, and is moderately large. I was planning to use a 3-pin Recom device (R781.8-1.0) which supposedly has very low ripple voltage. The idea was to smooth the ripples down to an acceptable magnitude using an adequately sized capacitor. An easier way to get 1.8 volts is > to use a simple 3 terminal regulator. There are some very small ones > available. They generate little electrical noise. A typical current > limit is about 100 ma, which should power a number of sensors. > Yeah, I thought about that too, but 100 mA is a little too skimpy for my application, unfortunately. Also, I'm concerned about efficiency since at some point it would be cool to run the whole thing on batteries. With a regulator I would churn out 2/3rds of the power used in the measuring circuit into heat. Another thing that I should probably mention is that I really want the measurements to allow for a (more or less) precise estimation of the sensor resistance. Hence it's not so much about just protecting the input from overvoltage - I'd rather avoid any nonlinear attenuation in the signal path, e.g. by a diode that starts conducting when the voltage approaches the upper limit. > > Putting an op amp in allows several possibilities. Op amps can be > inverting or non-inverting. By running the op-amp from 1.8 volts and > ground, the maximum output voltage that the op-amp can put out is > limited to these values. You will need a chip that outputs rail to > rail (ground to positive power supply voltage). > I'd personally recommend the op-amp for the most general solution, and > the resistive divider ONLY if the application can be guaranteed to > never exceed the voltage limits of the chip under any reasonable > conditions. > After reading up a bit on rail-to-rail capable op-amps I have the impression that they'll be outside their linear range once the signal approaches either rail. Since linear behavior is important in my case, an op-amp is probably not the best way to do things in my case, is it? -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/beagleboard/4807ad2a-726e-4b6c-9270-4c6bbc4ff772%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
