Hi Jens,
Well, you calibrated first for 8mA (at 5 ohm that is 40mV).
You noticed an error at 1.7mA, reading as 1.4mA which is .3mA too
low. An error of -.3mA at 5 ohm is -1,5 mV times -15,6 gain factor
you mentioned is about -20mV. This is all very reasonable for
a LM358.
Most opamps have an offset error. You get it for free. Since it turns
out negative here, you would suffer a 'dead' area in your measurements
when you try to fix it in software.
Several 'solutions':
1) Find a better LM358 (requires some luck)
2) Find opamp with positive error and fix in software
3) Add calibration pot for offset
4) Use opamp with ultra low offset (AD708 or something)
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.
All the best,
Frank
----- Original Message -----
From: "jb-electronics" <webmas...@jb-electronics.de>
To: <neonixie-l@googlegroups.com>
Sent: Saturday, March 19, 2011 2:37 PM
Subject: Re: [neonixie-l] Re: Nixie tube amperemeter
Hi Frank,
thanks, how did you do it? I measured and it is -19.5mV. This is freaky.
OK, but now I am confused. Why does this offset occur, and how can I
resolve this issue? It might be due to an input impedance of the ADC that
is too low, so that it loads the circuit when actually nothing should
happen, right?
I could probably subtract the 19.5mV manually in software, but there
should be some way to do this externally?
Jens
Am 19.03.2011 13:47, schrieb Frank Bemelman:
Hi Jens,
You only have a gain adjustment, nothing to compensate for offset.
Your opamp has a small offset and is also multiplied by the gain of the
circuit.
At zero current, what voltage do you have on pin 2 of your DAC.
It should be 0V. From your previous measurements I suspect a
small negative offset there, 20mV perhaps.
Best wishes,
Frank
----- Original Message ----- From: "jb-electronics"
<webmas...@jb-electronics.de>
To: <neonixie-l@googlegroups.com>
Cc: "threeneurons" <threeneur...@yahoo.com>
Sent: Saturday, March 19, 2011 1:09 PM
Subject: Re: [neonixie-l] Re: Nixie tube amperemeter
Hi all,
here is the schematic for my amperemeter.
http://www.jb-electronics.de/tmp/amperemeter_schematic.png
Jens
Am 18.03.2011 02:25, schrieb threeneurons:
On Mar 17, 6:13 am, jb-electronics<webmas...@jb-electronics.de>
wrote:
Got any pictures ? I think in pictures.
A quick and dirty schematic of your setup would be nice.
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