Re: Strain Gauges - Signal Conditioning with programmable gain

What I mean by +-10mV is that when we compress the object we should
read a negative value, when we pull the object we should read a
positive value. But the overall is 20mV as you mentioned.

Answer:- This is an important bit of additional information.
You want the output to be 0-10V. But as you are indeed applying force
in both directions to your gauge assembly, the output with no load will
be +5V. At full positive load the output will be +10V and at full
negative load the output will be 0V.
Or do you want the output to go negative for negative applied loads? I
don't think you do, as your ADC is probably not able to accept this.

Unfortunately I won't be able to tell you an approximate error. It
should be small but I know "small" depends on the application. I'll
find out as soon as I can and I'll tell you.

Answer:- In my circuit I have assumed that the bridge supply is 10V (a
common supply voltage). The bridge output is commonly specified as so
many volts per volt of bridge supply. So if your assembly has an output
of +/-10mV with a 10V bridge supply, it will have an output of 1mV/V.
The 'Null error' is needed so that I can set the amount of null
adjustment needed. I will allow a reasonable amount of adjustment (say
20% (4mV null error) of the full scale output. Then we can refine it
later.

What I'm most worried about is the filter part. One of the projects,
we apply the load and read the result 1 second later. The other one,
the whole thing lasts milliseconds, so, we should have to read 1 result

and then read the other one 10microseconds (100 kHz frequency) later.
As I need to input the signal in a A/D I need to project an
anti-aliasing
filtering. 40khz LP filter should be good for anti-aliasing, but I
would still have lots of noise because of the high frequency,
wouldn't I?

Answer:- For the application requiring a 1 second response time,
filtering should not pose a problem. Much more than 4 samples a second
and the human eye/brain is not capable of following what's going on
anyway.
Your other application is more of a challenge. If the equipment is
properly built, the noise should not really be as much of a problem as
you anticipate. But in order to capture a fast moving event (the
breaking of a rod under tension?), your ADC needs to be able to capture
samples at the rate you mention, and ideally much faster than 10uS.
Then you can do software averaging on a number of separate samples.
I will configure the filter as a 40kHz low pass.

As time goes by things seem to get more complicated instead of getting
easier! :-) That's because I'm learning new concepts which are
important for the project!

As the saying goes, "the devil is in the detail"
Robert

.

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