Posted on:

Hello, I am looking for a circuit that forms the average of a current over a given time (like as long as someone presses a pushbutton). Does anyone knows such a circuit? Please tell me the advantages and disadvantages, too. The reason why I am asking is because I have to develop a device that can measure a very low current and therefore it needs a very small resolution (0,001uA) but also has got a wide measuring range (10uA)  actually it should be able to measure up to 100mA but I divided the range in smaller pieces. Usually I could choose a clockfrequency for the uC independently from the resolution of the ADC but building a average joins these two separate aspects together. So, I am now thinking about using a ADC with a very high resolution, which means that I can do all the calculating within the uC, or building the average analog and using a ADC with a ordinary resolution of (10 or 12bit). Of course I have considered the solution to integrate first and then diving within a uCprogram. But as I have to develop this device for my thesis I want to know something about every of the three solutions. And thats why I am especially asking now for the complete analog solution. Many thanks in advance! Gabriel
Posted on:

Hi Gabriel, I think your're looking for what we in German call "Meßgleichrichter". It consists of two OpAmps and two diodes  purely analog ;) One of the OpAmps compensates the forward voltage drop of the diodes so that you can theoretically measure voltages in the scale of a few hundret of milivolts. The second OpAmp amplifies the result with an adjustable magnitude. I think, you could search the internet using key words like "measuring rectifier opamp" or something like that, as I don't know the exact translation of "Meßgleichrichter". Cheers, Jochen
Posted on:

The analog solution would indeed be an integration. The advantage over the digital solution is that you are actually averaging the signal continuously, where the digital solution samples it at certain points of time  any change between two sample points will be missed (like, a spike). The disadvantage of the analog solution is that any offset error in the integrator will eventually sum up as well.
Posted on:

Hello Jörg, what about using a low pass? And how to stop measuring then? Whould you prefer to use a potentiometer instead of a switch? Thanks for your reply!
Posted on:

And isn't a low pass the less complicated system regarding to noise and temperature changes instead of using a opamp for (math.) integration? Thanks again in advance!
Posted on:

Gabriel wrote: > what about using a low pass? Essentially, an integrator is nothing different than a lowpass filter.
Posted on:

  Gabriel schrieb: > I am looking for a circuit that forms the average of a current over a > > given time (like as long as someone presses a pushbutton). Does anyone > > knows such a circuit? Do a search for: ramp generator. That's what does the nondigital solution. > Please tell me the advantages and disadvantages, > > too. It does work like a charm, but it seems to be oldfashioned.