Electronics 2000

Hi Nick,
Yes I saw the non inverting summing amps but I think they introduce gain so I thought I would just get the inverting one working first. I dont understand why its not inverting it definitely is showing positive on the AVO.
I will keep tweaking and come back to you if I have any more queries.
Thanks for your help and feedback once again.
Kind Regards
Nevjc

Yea whhhhwwwwhoo, it will take me 6mts to get my head round that.
Any one want to sum it up and start again... two AD converters and add them together hmmm.

I am trying to set up 4, 3 phase delta systems in a wheat stone bridge configuration, thats one system per arm 12 wires, one is earth one loose, if I double that (24 wires) 2 earth 2 loose ... join the two loose wires together via the headphones, if earth is a variable voltage will variations apear at the head phones, can this be done... I am trying to wire up a chromosome by the way.

Sorry for the interuption achermmm.

Tar&stuff.

What is the purpose of R1 and R2, is it just to pull the voltage closer to 0v?
What happens if you take them out all together?

HI Paul,
R1 and R2 are providing a small load for the output of each inst. amp. As far as I can tell their only real function would be to provide output stability and noise rejection.
You could most probably remove them without too much of an issue, but you may find that the CCT is slightly more noise sensitive.

Although I couldn't find it explicitly mentioned in the datasheet for the INA122, given it can only source 3mA but can sink 30mA I'm guessing it's probably a class A output. I.e. a pullup resistor and a transistor to ground, so forcing a small current on it's output is no bad thing.

Hope that ramble helps.

Kind regards
Nick

Thanks
I'm more used to 741 op amps so am looking at it from that point of view where we have negative feedback into the op amp, the output attempts to zero the input and hence you end up with gain on the output.

From the diagram on page 2 it looks like the wheatstone bridge is going into an amp with unity gain hence it will rocket up or down with any sort of variance between the + and - input, or is the INA122 different to a 741 in this respect?

Cheers

Paul

Hi Paul,
Be a little careful interpreting the schematics in the posts, opamp symbols have been used in place of instrumentation amp symbols in some cases (this is noted in one of the posts). This was done simply for ease of drawing (though I fully admit sloppy and not really good practise on my part). The INA122 is actually an instrumentation amplifier chip, not a straight opamp. It contains most of the feedback resistors etc. inside the chip as they are precision laser matched resistors. It then gives you the option of attaching a single resistor externally to set the gain of your choice.

Sorry if it has caused some confusion.

Kind regards
Nick

Hi Nick,
I saw the last couple of posts. I have attached the circuit as I have built at this dropbox.

https://www.dropbox.com/s/j0pa4m3gj8y72 ... ircuit.pdf

For some reason the instrumentation amps work fine and gives me a nice linear output and I have built a non inverting summing amp with a small gain as I tried it without but for some reason the output is not linear. I have tried to reduce the output with a PD but the scale stops reading. I think it could be to much noise. So then I increase the gain and it reads but becomes non linear. Also it appears to change if you take one of the inputs away.
I have started looking at a velleman board so I can read it on a USB input and then I will attempt to see if I can write some visual basic to read it.
As I am sure you are aware I dont have much experience with electronics but I am learning a lot.
Kind regards
Nevjc


https://www.dropbox.com/s/j0pa4m3gj8y72 ... ircuit.pdf