Re: Current measuring resistor calibration

On Jun 22, 10:22 am, MooseFET <kensm...@xxxxxxxxx> wrote:
On Jun 22, 2:05 am, Petkovic <mpetko...@xxxxxxx> wrote:





MooseFET wrote:
On Jun 21, 4:24 pm, Petkovic <mpetko...@xxxxxxx> wrote:
Hi, I need to measure current in the range of 1mA to 10A.
The precision I need is 0.1% of the current that I'm measuring i.e for
10A current i need 10mA resolution for 1A I need 1mA resolution and so
forth. Potential across resistors are fed to an Opamp that has offset
and gain trimming  networks.
The current is changing within the range limits and I need to measure
current continuously and send the value to computer.
I have built a prototype but I have some problems that I hope you can help.

Because the range is too broad to be measured accurately with only one
resistor, I have used these resistors:

Actually, you could do it with just one resistor assuming that the
current is DC.  With a chopper op-amp and careful design, you can get
down to a micro volt.  You have to beware of the metals involved.

-----------------------------------------------------------------------
I need 0.1uV resolution if I want to measure 1mA with 0.1 Ohm Resistor
with 0.1% accuracy. I never could measure lower than 10uV in practice
and that only happens if I use every trick that I know. for me 100uV
sounds more practical. So if you excuse me I can't do this unless you
teach me practically how.

I went on to answer the other questions because it is hard to do.  I
was only pointing out it was not actually impossible.  Switching
resistors is going to be a lot easier.

---------------------------------------------------------------------------
I have used very wide tracks to ensure there is no IR drops in tracks
and also in switches.

You need to run the connections directly to the resistors.  This
matters the most at the high current end where the resistor is small.
You also want to make sure you aren't making thermocouples.

but even in lower currents this problem exists. I mean 100mA is not much
current for a short(a few Cm) 200 mil track. It should not cause this
problem.

Temperature gradients along the copper make voltage gradients.  (This
is the real way a thermocouple works).  Even at 100mA you are making
some heat in the sense resistor.





---------------------------------------------------------------------------­>> 2- When current is high (>1A), current measurement resistor becomes hot
and after a while current that it reads change e.g. first it reads 2..16A
and after 10 minutes, it reads the same current 2.65A.

Check the tempco first.  Look at the metals you have in the design to
see how many little thermocouples you have made.  Look at you op-amps
last but don't forget to check.

3- If I want to mass produce this, is it an easier way to calibrate
offset and gain of each resistor instead of manually change trim
networks of Opamp?

You can use "electronic pots".  These are sort of like DACs with
EEPROMs built in.

---------------------------------------------------------------------------­-
But how to calibrate these digipots?
calibration takes a lot of time and I hoped to somehow do it >automatically.

If you have something that needs adjusting, it takes time to adjust
it.  Even if you make a computer do the work, it will take some time.
It is better to try to design the need out.- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -

A micro can assist resistor switching and calibrate linear output. It
can also monitor temperature changes for compensations. In
productions, you can build lookup tables while calibrating and
programming the micro. The so called "ASIC" are probably pre-
programmed micro.
.

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