I am working with an unknown 8 pin op amp that is rated for a min of 5 volts.
I am only using 3 volts. Would this cause me to NOT be able to adjust it to zero?
It has a UV sensor hooked to it, and in total darkness I still get over 700 mv output, even with the zero adjust pot maxed out.

Bob

 

Hello,

Using opamps outside the specifications can lead to unwanted effects.

Take a suitable opamp.
Have a look at page B-2 of the following PDF for more info:
slod006b Op Amps For Everyone.pdf

Bertus

 

Thanks for the reference PDF.
Will have to find/make a 5 volt supply and see if I can zero it with that.
I am using this board: http://www.thanksbuyer.com/a24b-uvm...ut-w-signal-amplification-linear-output-29218
They painted over the op amp on it so I have no idea what they used.
will have to play with it a bit more.
Bob

 

Hello,

That is a nasty practice.
They probably want to protect their schematic, so it can not be reproduced.

Bertus

 

With Vcc below the minimum operational level you don't really have an opamp. You have something, but parts of the internal workings are probably non-functional. Trying to get zero volts out of a single supply opamp can be challenging under the best of circumstances unless you have a part the operates to the negative power supply rail. The best way to guarantee this is to use a bipolar supply or create a vitual ground at Vcc/2. Then you can define "0" output as mid scale and play away to your hearts content.

 

Found the info on the Op AMPhttp://www.100y.com.tw/product_jpg_original/A046627.jpg
3140A also has P3340AB on it
I am thinking this is a 3140A OP AMP, looking at mouser data sheets now.

 

Its a CA3140 op amp
can you draw the circuit out, and we can help you to mod it.

http://www.ing.unlp.edu.ar/electrotecnia/circelec/CA3140.pdf

 

Hello,

Dodgydave, are you sure it is CA3240?
ca3240-a.pdf

Bob says it is likely an 3140:
CA3140.pdf

The 3140 has some schematics in the datasheet for offset compensation:



Bertus

 

Can be used in single supply mode with diode to limit output swing. No indication it can go below 5 V though.

 

ca3140 has dc off set

 

The board you see in the picture I posted is what I have. I have no schematic of the unit. Only data sheet I can find is in Chinese.
http://us.100y.com.tw/pdf_file/57-A24B-UVM-30A.pdf Took me forever to translate which trim pot was which.
Tried changing the zero offset pot from 1K to 10K dropped the output a few MV.
Being that I do not have a schematic, nor can I see the traces on the board to reverse engineer it, I am kind of stuck.
Will have to try and add some resistors to either side of the Zero adjust pot and see what that does for me.
While I understand the basics of an Op Amp, my expertise is VERY limited. I also intend to hook this up to a 5 volt supply and see if that is what is making it
not read a zero output.
May have to abandon what I have and build my own from scratch.
Thanks everyone for all the input.

Bob

 

dc off set is pins 1-5, with th wiper to pin 4, what happens when you short pins 2 and 3 together, does the output on pin 6 go to zero..

 

Dave,
Will give that a try tomorrow.
May have to rebuild the amplifier from scratch and keep the sensor part.

 

Here are some test results.
Running from a 6 volt supply
Shorting pins 2-3 drops to 1.5mv, may not have had the best contact on the pins, hard to do with surface mounted stuff,
Lowest I can drop the in the dark voltage to is 38.9mv on the out put of the board.
Got the same results using a 3 volt power supply.
Right now there is a 10K multi turn pot on the zero adjust. Have 2 thoughts,
1) Try a 1 meg trim pot.
2) build my own amplifier for this thing and just use the sensor from the existing one.
Only thing I can add is that the amplifier is amplifying voltage, not current to voltage. The sensor part has a small current to voltage amp of some sort on it already.
Course I could spend the $300 that Davis wants for their uv sensor but what fun is that.

Bob

 

Do you know what the sensor dark current is?

 

Try a 1nF cap across pins 2,3

 

the actual Detector chip data is here: http://www.dfrobot.com/image/data/TOY0044/guva-s12sd.pdf
I had a page that had the data for the sensor head (small circuit with the chip no amp) that I can not relocate.
Will give a small cap a try. I will post the data for the sensor head if I can find it again.
EDIT:
Found the sensor head.http://detail.1688.com/offer/794476306.html?spm=0.0.0.0.w2PbLB
you will have to Google translate it to English.