Current measuring resistor calibration

[Petkovic]

I doubt that your contact resistances are that low, nor are they
repeatable enough to get the results you are looking for. Redo the
circuit so that you are not measuring the voltage across the current
path relay/switch contacts. The inconsistent contact resistance will
introduce nearly 1 % errors with the lowest current shunt value
(highest range).

------------------------------------------------------------------------------------------
Ok,I will do it, If I want to remove switches then I have to place one
Diff. Opamp for each range and use an analog multiplexer. and then I
have different Offset/Gain/CMRR errors for each range that have to be
calibrated. While I doubt that this problem goes away with just
removing switches but it worth a try. this problem of reading different
values for each range exists even when current is just 100mA. I think
when current is high switches may create significant non-linearity but
because I have this problem even in lower currents and even when I used
0.1% metal film resistors I think I more need a good calibration procedure.
I also tended to try linnix's suggestion of creating look up tables but
interestingly no one else here endorsed his suggestion.
------------------------------------------------------------------------------------------

 

[Baron]

I also tended to try linnix's suggestion
of creating look up tables but interestingly no one else here endorsed
his suggestion.

Lookup tables are commonly used in industrial instrumentation !

 

[SoothSayer]

John Fields wrote:


John,

this is my setup.
I have used OP27.
---
This is how it came out on this end:
---

-----------------------------. .-----------.
| | | | | | |
=== \ o \ o \ o | | |
PGND \ \ \ | | |\| |
\. \. \. | '-----|-\ | ___ ___
o o o | |
> --o--|___|---|---------|___|-|
| | | '-----------|+/ |
|
| | | |/| | |\|
|
.-. .-. .-. |---|-\
|
| | | | | | | >-
------|
| | | | | | |-------- | |-----|+/
'-' '-' '-' | | | |/|
| | | | |\| | |
| | | |---|-\ | ___ |
\ o \ o \ o | >- | -|___|- |
\ \ \ .---|+/ |
\. \. \. | |/| .-.
o o o | | |
| | | | | |
'-----------------------------' '-'
|
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

----------------
Ok, John,
There are 3 resistors that only one of them is connected via 2 switches
and three Op27 are configured
as a Differential amplifier.
Nothing complex, really. I hope it is clear enough.
----------------

I think it is, but let's go slowly here in order to eke out a proper
solution for your problem.

I've taken the liberty of redrawing your circuit in order to make it
easier to visualize and to remove any existing ambiguities, and this is
what I think you have:


.VIN>--O--------O--------O------|+\
. |S1A |S2A |S3A | >-+-[R4]-+---+
. O-> | O-> | O-> | +-|-/U1 | | | +V
. | | | | | [R5] | |
. [R1] [R2] [R3] +------+ | +-|+\
. | | | GND | >--+
. O-> | O-> | O-> | +-|-/ U3 |
. |S1B |S2B |S3B | | |
.GND>--O--------O--------O------|+\ | -V |
. | >-+-[R6]-----+--[R7--+
. +-|-/U2 |
. | |
. +------+

Am I right?

-----------------------------------------------------------------------------------
Yes JF,
That's what I meant. Thanks for the effort.
I use AACuircuit and when I post it, It appeared right.
-----------------------------------------------------------------------------------

It screwed up because of total line length.

 

[Capt. Cave Man]

Also parts like the OP-27 are not a good choice here. It is likely to
add 0.1mV of offset voltage. You need more like "none"

So, a "zero crossing" op amp?