Hello all,
I have a small follow-up problem with my amperemeter. I have ordered a
lower offset-type, the fossil OP07.
But now my amperemeter has a lot of noise in the range of +-0.1mA. Here
is the schematic:
http://www.jb-electronics.de/tmp/nixie_amperemeter.png
Any ideas?
Best regards,
Jens
Am 19.03.2011 18:42, schrieb Frank Bemelman:
----- Original Message -----
I have had luck! I had ordered 10 LM358 (since they are only like 16
cents each) and I actually found one out of these 10 with an offset
error of -0.5mV in the above calculation. So this works fine in this
0..9.9mA interval, but in larger intervals (the next amperemeter will
be able to measure up to 50mA) this should not suffice, either.
I would think, if you are going to build an amp meter with larger range,
the offset problem would be less noticeable. If you can use a lower
gain, it would be less of a problem. If you keep the gain the same,
but lower the shunt resistance, the problem will be the same as with
your 0-9.9mA meter.
I like that. But how would I correct an offset with a trim pot? I
think trim pots only work for scaling, and not for adding/subtracting
a constant value. Or are you suggesting setting the ADC's ground to
that negative/positive offset voltage so that it is effectively zero?
Many ways to do it, see figure 2:
http://www.maxim-ic.com/app-notes/index.mvp/id/803
Use a pot + high value resistor, 1Mohm or something.
I would not recommend fiddling with the ADC ground. It would require a
low ohm resistor in series with ground, to get a point where you can
mix-in
your offset. But if current consumption of this adc chip varies, you'd
get all
sorts of tough to explain behaviour.
4) Use opamp with ultra low offset (AD708 or something)
This lists at 6EUR at my electronics dealer, quite pricy I think if
it can be avoided by including a pot. I don't mind having a zillion
pots in my circuits if that means they can be calibrated nicely.
Expensive indeed. It's been so long that I've been working with opamps
that I don't have a list anymore of the usual affordable candidates.
Personally
I don't like pots, always looked at them as sort of proof of bad design.
I would rather die than using a pot ;-)
With a bit of luck you may be able to find a nice rail-to-rail opamp
(RRIO type) which
does not need a -5V supply.
Might this offset error be due to a non-symmetric supply? I.e. +5.0V,
-4.99V ?
No, that's not a problem at all.
Cheers, Frank
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