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[Reply #15 on:]

R3/R4 - yes, this just sets a DC signal for you to multiply.
R1  sensor
R2 - gain control.

Could also put your part in place of R4, use R3 of 100K and set R1/R2 for gain of ~20 (10K, 200K)
Vin then = 5V*(3k/103K) = .145mV, Vout = 2.912
Vin = 5V*(5K/105) = .238mV, Vout = 4.76

Math is  straightforward, find a combination that works with the parts you have.

[Reply #16 on:]

 while I will be needing thos opamps ... I still ended up doing some fun things with my sensors :-D ...
check it out: http://arduino.cc/forum/index.php/topic,53357.0.html

[Reply #17 on:]

In that case, take the voltage out from the original ciruit, offset it down in DC level and then add some more gain for wider response.
What you need is a non-inverting summing amplifier
http://masteringelectronicsdesign.com/how-to-derive-the-summing-amplifier-transfer-function/
but you need a -2.5v source to add in.

maybe something like this
http://www.maxim-ic.com/datasheet/index.mvp/id/1728/t/al

[Reply #18 on:]

Hey CrossRoads

I only saw your last reply right now... I dont understand what your saying - What case are you referring to?

Ok, I sort of understand the content of the first link, however I dont quite understand what benefit that has over this first circuit you suggested.  The second link points to an elaborate "voltage inverter"?

Why would I want to do it this way?

****

Anyway, I got myself some of the opamps & capacitors you originally suggested. I hope to get around testing this soon, though I am sort of swamped with university stuff at the moment. The capacitors are just add in parellel between Voltage in & Ground, correct? Their function is basically a low pass filter and the 0.1mf capacitor filters higher frequency signals, while the 1mf prevents noise with slightly lower frequency?

(sorry if I'm asking really simple/silly questions I just have no knowledge of electronics whatsoever, and I just want to double check that I actually understand what I am doing.)

Thanks for your help so far, and any additional help you can offer.

Cheers

P.

[Reply #19 on:]

I don't know anymore, been too long, need to review what it was you were doing.
I think I was suggesting a way to change the DC level so your ac signal could be amplified more so the ADC could take samples over a wider range.

[Reply #20 on:]

got it. makes sense.

if I want to use a 0.1mf and 1mf capacitor to reduce noise, where would I put them, assuming I am using the set up you suggested?
( http://arduino.cc/forum/index.php?action=dlattach;topic=53242.0;attach=883;image )

To my best understanding, I connect them between +5v and ground and I connect them in parallel - so even before R3. However intuitivly I would connect them after R2.

Could you give me a pointer as to where to add them to your schematic?

Thanks

p.

[Reply #21 on:]

Put them across the noise source going into the op amp, so across the +in or the -in terminal to ground, whichever side your sensor goes in.
Stop the noise before it gets amplified.