> Anyway, now I'd like to build a small amperemeter. I made > myself a small OpenOffice Calc table and played with the > numbers (wanted to use a similar trick with the amp meter > ... I have: > > - use R = 6Ohms as a shunt resistor. > - use a 10Bit ADC with 2.5V internal ref. > - amplify the voltage drop across the R_shunt with the > factor 13 using an opamp, 20 samples (as above with the > voltmeter, I'd like the two multimeters to by synchronous > as I'll be running them from the same chip) > - divide by 64 in the end, again, as above > > This gives me (example): > > Current: 20mA > U_shunt: 0.12V > U_ampl: 1.56V > ADC-value: 638 > sampled (x20): 12760 > divide by 20: 299 (<- oops, div-64: 199) > > .... > regards, > Jens
199 displaying 19.9mA, for a 20mA input. That's only 0.5%. Pretty good. I wouldn't diddle with gain. Even though a small trimpot for, oh, trimming, is always a good thing. In this case, it'll will be short one count, or 0.1mA, on most measurements. There is a small amount of round off error. For all practical purposes, its spot on. I bet most cheap multimeters have one count round off errors. If only that ! The better meters, most likely have extra internal (non displayed) resolution. If its over, or under, midway, that effects the last displayed digit. Good work ! -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
