Operational Amplifier (need help)

[Kaylia]

I promise to not make fun of engineering for the rest of the year if someone can answer this.


I'm going to skip the detail, but basically, I need to build something to amplify a 10-50mV signal for an experiment. I'm not exactly sure how weak it is, but it can't be higher or I would be able to detect it normally. The only data acquisition card I have go up to 10V, so I was aiming to for a x100 amplification


I only have one type of amplifier around but it should be enough afaik since I use these in the past
http://www.datasheetcatalog.org/data...0/53989_DS.pdf

The source I use is a +/- 12V power supply. Other sources around can't be used since they create a lot of noise,making them useless to some extent.

I went for a non inverting amp since it's the easiest one that I could remember, but it doesn't really matter.
http://upload.wikimedia.org/wikipedi...erting.svg.png

Rf =100 kiloohms Rg=1 kiloohms

The input signal is attached to the +/-, and the data acquisition card is linked between Vout and the ground.



Now..my questions are:

#1 Why am I getting a fucking 10V offset at the output, shouldn't it be around 0? If it's normal, what can I do to bring it back to 0?

#2 Are the resistor correct, or am I forgetting something obvious?


#3 Why electronics never works like it should? It's pissing me off, I wasted the whole day on something that I have done before, and shouldn't have to do anyway if we had equipments that works.

[Eliseos]

I can answer #1 and 2 when I get home from work, but #3 I like to blame on my stupid labs that never taught me anything except how to do it wrong.

[Kaylia]

I would love you forever if it's not too much trouble.



[edit]

I can answer #1 and 2 when I get home from work, but #3 I like to blame on my stupid labs that never taught me anything except how to do it wrong.

I can't say they taught me how to do it wrong, but they were holding our hand all the time, and it help when you saw the theory the week before.


In physics lab, I need to build everything from scratch all the time, including a computer program to read the data, electronics circuit to read whatever measurement you're doing and the fucking experiment itself. Worst part is the equipment that would put Macgyver to shame.

[Eliseos]

The only thing I can think of off the top of my head is to make sure all your connections are correct. For the op-amps I used in my lab this semester, we had 6 connections we had to make, the +/- input, the +/i power source for the op-amp (different than the source you use for your actual circuit), 1 for the output and 1 for the ground. You could also try hooking up a 10V source to your offset to see if that helps out with #1.

[Kaylia]

I'm almost certain connections are correct. The circuits is simple, and it's not the first time I build one. I also started it from scratch at least 3 times "just in case".


Sorry for the shitty paint skill, but this is how everything was plugged


http://img213.imageshack.us/img213/305/electronic.jpg
Both ground are the same (didnt link them on the pick)

[edit] I also grounded the +/- 12v power supply on the common ground, should i do this?

[Eliseos]

The circuit you have is correct according to my spice simulation:

       --- Operating Point ---

V(+v):	 15	 voltage
V(n001):	 0.0100192	 voltage
V(vs):	 0.01	 voltage
V(-v):	 -15	 voltage
V(vo):	 1.01202	 voltage
I(R2):	 -1.00192e-005	 device_current
I(R1):	 -1.002e-005	 device_current
I(+v):	 -0.00159987	 device_current
I(-v):	 0.0015999	 device_current
I(Vs):	 -4.84162e-010	 device_current
Ix(u1:3):	 4.84162e-010	 subckt_current
Ix(u1:2):	 8.82965e-010	 subckt_current
Ix(u1:7):	 0.00159987	 subckt_current
Ix(u1:4):	 -0.0015999	 subckt_current
Ix(u1:6):	 -1.002e-005	 subckt_current

Try following the diagram of my schematic. I think you may be connecting the negative terminal of your source to your op-amp, which isn't what you want. Connect the negative terminal to ground, that may fix things.

[Kaylia]

What's the difference between both diagram (I dont see any differences ;/)?

http://www.datasheetcatalog.org/data...0/53989_DS.pdf
The -12V is plugged in Vee (#4) and the +12 is plugged in Vcc (#7)
They are linked directly to the +12 and -12 of the source, so I doubt I made a mistake here (I also tried doing the opposite just in case the +/- werent labeled correctly.)


Maybe it's the ampli itself that is broken...wouldnt surprised me considering the state of everything else.


Thanks for verifying it in spice, at least I know the result im getting isnt normal.

[Eliseos]

Stealing your paint schematic, where I circled on your diagram is where I think you are having issues. You're connecting the negative terminal of your source to in between your two resistors. Try connecting the negative terminal of your input source to the ground, see if that fixes everything. When I connect the circuit to how you have it in your diagram, I get about a 10V offset from what it is supposed to be:

--- Operating Point ---

V(+v): 15 voltage
V(n001): 0.138598 voltage
V(vs): 0.148598 voltage
V(-v): -15 voltage
V(vo): 13.9985 voltage
I(R2): -0.000138598 device_current
I(R1): -0.000138599 device_current
I(+v): 0.0213721 device_current
I(-v): 0.0015999 device_current
I(Vs): -5.60328e-010 device_current
Ix(u1:3): 5.60328e-010 subckt_current
Ix(u1:2): 7.47475e-010 subckt_current
Ix(u1:7): -0.0213721 subckt_current
Ix(u1:4): -0.0015999 subckt_current
Ix(u1:6): -0.000138597 subckt_current

[Kaylia]

I cant try until tomorrow, but I doubt it's the issue because even without any signal connected to the circuit Voutput was around 10V

[Korietsu]

Download Orcad P-Spice. Build amp in p-spice, simulate and profit. If you have to hand calculate, use the equation for the inverting amplifier.

[Kaylia]

Download Orcad P-Spice. Build amp in p-spice, simulate and profit. If you have to hand calculate, use the equation for the inverting amplifier.

I swore to never touch p-spice again after my last and only electronics engineering class!


Anyway, I went back today, and couldnt find the fucking issue with it. Turn out it's amplifying despite the 10V offset, but it's acting like R2 = infinity ohms instead of 100kOhms.

I figured the datasheet I had was wrong, and tried nearly every combination, but nothing. Oh well, I gave up