Re: Automatic gain control

I know your point! How can the "MAGIC BOX" know its 0,01V(full scale of
a 0-10mV) or a 50% of a 0,02V.
But I would calibrate it(make it go to full scale) and then have a 5V
at the output.
For exemple. A 0-10mV input signal with gain=500 would give me a 5V at
the output. Now, with the same Gain=500 I can have a 0-20mV input
signal and force it go to full scale(for exemple, compressing my strain
gauge all the way so that it goes to full scale), my output would be
10V, right? But isn´t there anything that can make this go to 5V and
record this proper gain(in this case, 250) to use afterwards?
This way, when I enter this "calibration mode" , whatever is the input,
the "MAGIC BOX" would know it is the full scale input"! Understand?
Maybe it is too hard to implement, maybe not... That´s why Im asking
you guys because (many of you) are professionals and have a lot of
experience. If you say it´s something for a doctor thesis, I give up!
:-)
Regards!
Victor

John Fields escreveu:

On Sat, 08 Jul 2006 15:43:35 -0700, Don Bowey <dbowey@xxxxxxxxxxx>
wrote:

On 7/8/06 2:50 PM, in article gs90b29gtdurhtq8vak2uduep4qka2uvt4@xxxxxxx,
"John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:


Can't you read?

That's not what the guy's asking for. He wants to plug in a strain
gage with 10mV full scale out and then later plug in another one
with, say, 20mV full scale out and have the magic AGC know the
difference and put out 5V when the first one's at 10mV and 5V when
the second one's at 20mV.


So you measure the feedback signal.

---
Good luck with that.

Here's what the OP's got:


Vs>----------+-------+
| |
[R1] [R3]
| |
| +-----E1
| |
+-------|-----E2
| |
[R2} [R4]
| |
GND>---------+-------+


Where the resistors are the legs of a Wheatstone bridge and E1 and
E2 are the outputs from the dividers.

In one case he has a strain gage where E1-E2 = 0.01V full scale, and
in another, with the same Vs, he has a strain gage where E1-E2 =
0.02V full scale, and what he wants is this: (view in Courier)

Vs>----------+-------+--------------+
| | |
[R1] [R3] |
| | +---+---+
| +-----E1---| +|--->+5V
| | | |
+-------|-----E2---| -|--->0V
| | +---+---+
[R2} [R4] |MAGIC
| | |BOX
GND>---------+-------+--------------+

Where, in the first case the magic box has a gain of:

Eout 5V
Av = ------ = ------- = 500
Ein 0.01V

and in the second case, a gain of:

Eout 5V
Av = ------ = ------- = 250
Ein 0.02V

Now, with only E1 and E2 available as inputs to the magic box, how's
the box going to be able to tell the difference between the
full-scale output of the first strain gage and the half-scale output
of the second one?

That is, how is this "feedback signal" you allude to being
generated, and where's it coming from?


--
John Fields
Professional Circuit Designer

.

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