Electronics > Metrology

Null Detector based on Conrad Hoffman design check

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hozone:
Hello,

I would like to build a null detector based on the Conrad Hoffman design.
I've read the Conrad Hoffman article, find it here: http://conradhoffman.com/mini_metro_lab.html
And this interesting post here: https://www.eevblog.com/forum/projects/conrad-hoffman-null-detector/

The power stage it's built by using 4 AAA battery (6V), then a 5V LDO, then an op-amp as a rail splitter, so I'm going to use +-2.5V for the main circuit.
The op-amp I've selected is an AD8628, based on this @Kleinstein comment https://www.eevblog.com/forum/projects/conrad-hoffman-null-detector/msg3026766/#msg3026766
The only accurate resistor will be the 0.1% on the input divider.
Before going to test this circuit, do you think there is something wrong here?

Thanks

Kleinstein:
The amplifier needs accurate resistors for both the input divider and the resistors setting the gain.
A Nullmeter is normally not about high accuracy in the scale factor, but good accuracy of the zero. So the resistor accuracy may not be that critical.

One may get away with fewer steps in the input divider (e.g. only 1:100 and maybe 1:1000). E.g. divide by 10 could be realized as divide by 100 and x 10.

I would make the input divider higher impedance, like 1 M if not 10 M.

The OP-amp for the rail splitter is not critical and should be a non AZ type.

hozone:
Thank you!

I've just realized I've not attached the schematic. I've now edit my post, you can now find the schematic.

iMo:
I would put a small resistor at the U2's and U3's output (like 33ohm or something like that).
Why such big 470nF in the feedback?
BTW., I had similar null meter in past with only one range (x1000 I think) and it covered all my use cases at that time..

hozone:
Thanks @iMo
I get the reason after the U3, but even after the U2?

Another question is about the resistor accuracy, I've put 0.1% but maybe I can use less accuracy, cause on negative input of the op amp there are 1%, and they are trimmed by RV1, right?

I also would like to add an analog millivolt meter +-100mV on front on my case, switchable with banana plug.

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