I do hope this isn't considered to be hi-jacking..... I've just read through the postings, I'm doing something similarish but with clamp diodes on a opamp, my signal comes from a lc tank driven by a pic, I want to amplify a very small voltage, there is quite a large voltage present at the input of the opamp before the small voltage I want to amplify. I'm using clamp diodes to 'dump' the large voltage to to power rails, it can get as high as 200v p to p @ 134kHz but low power, everything works fine but do I need to add some more capacitance on the power rail? I'm concerned it may interfere with other stuff being powered (pic, memory etc) I'm right at the start now!
Cheers Geoff

 

It depends not on the voltage but on the current, and are there other elements common to the power rails that will eat that current up for lunch.

How much current? Is it a fraction of the rest of the circuit? Then you probably have no worries.

I'd still stick a scope on your power line and see if any noise is due to the 200VP/P signal. If so, then add caps. If not, sleep well and move onto the next task.

 

It really depends about the voltage

 

Hi Ernie,
Thanks for the advice, I hope its relavent to the op's circuit too, just been playing on google, would it be against the tos to post the names of the microchip app notes for this? I found them quite quickly and have a project that needs to do something I can't discuss here.
Cheers Geoff

 

AN521 (dated 1997) shows how to use the ESD diodes to clamp large signals:

http://ww1.microchip.com/downloads/en/AppNotes/00521c.pdf

TB3013 (dated 2008) shows why not to do this with analog capable inputs:

http://ww1.microchip.com/downloads/en/AppNotes/93013a.pdf

There is another app note discussing this I could not find today.

 

This one?

http://www.microchip.com/downloads/en/AppNotes/00236a.pdf

Cheers Geoff

 

Which one is that?

 

Not up to date but...

http://pic-rosa.blogspot.ca/2007/09/source-code-for-pic.html
Max.

 

Hy. I have succeeded to filter DC signal, and now I'm able to measure my max amplitude

Inductive sensor can measure between 0-100 mm and throught some formulas, I have managed to connect relationship between max amplitude and lenght displacement.
Voltage, according to sensor datasheet, should change linearly. My formula also describes linearity and according to that I get mm displacement on LCD.
The problem is that at the output of instrumentation amplifier, signal, while changing the sensor, does not change linearly.

I can mesuare voltage every 5 mm, then make an interpolation and decribe this curve (voltage change) with appropriate formula, and use it in program for PIC. Then I'll have exact relationship between max voltage and sensor displacement.

But I want to know why I'm not getting linear change at the instrumentation amp output?

Here are sensor specs:

PS: Every time I turn on the power, I must make additional adjustement on Wien oscillator 'cause I'm not getting the output that I had before turning the power off (though, frequency is always the same, voltage is every time different) Also, voltage jumps 100 mV up and down. It's not so stable. What can I do to solve this problem?

 

Did you post what sensor this is? (Not that I have any expierence with them anyway...)

Same with the oscillator: beyond the obvious (stable supply voltages and good quality pots) I would have no idea how to go about keeping the voltage stable.

Perhaps the "General" or "Projects" forums would get better eyeballs on this problem of yours. I'd try posting this problem there.

 

Ok.this is an inductive sensor wl -100 but you will not find anything about it on internet. I only have scanned data which I have alredy posted.
I will try to post my problem somewhere else.
thanks anyway!