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Help me offset the output of an amplifier

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markdem

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Hi all, I am tiring to design a circuit to amplify the output form a PH probe so I can then read it using a ADC. I am using a TL081 with +/-5 supply. I have set the gain to 10 with a 9k1 and 1k resister, and that part works ok. I also need to offset the output so I only get positive results. I have tied to put a 10k pot on pin 1 and -5v rain with the wipe going to pin 5, but that is not doing anything, eg the output will not offset when I short the inputs. Can someone please help me offset the output?

Thanks, Mark
 

Re: opamp offset

markdem said:
Hi all, I am tiring to design a circuit to amplify the output form a PH probe so I can then read it using a ADC. I am using a TL081 with +/-5 supply. I have set the gain to 10 with a 9k1 and 1k resister, and that part works ok. I also need to offset the output so I only get positive results. I have tied to put a 10k pot on pin 1 and -5v rain with the wipe going to pin 5, but that is not doing anything, eg the output will not offset when I short the inputs. Can someone please help me offset the output?

Thanks, Mark

With Pin1 & 5 you can only compensate the offset of the opamp itself!
(the inputvoltage you need to get zero Volt at the output)

If you need an offset at the output you have to modify your external circuit.
You have to connect the 1k resistor to a negativ Voltage. (and not ground)
 

Re: opamp offset

OK, i have now attached a 100k pot to -5 and ground with the wipe going to the negative probe on my multimeter, and it works. What i want to know now is why does it work. Why is the offset been set like that. Also, where to i attached the wipe when i use a ADC?

Thanks, Mark
 

Re: opamp offset

markdem said:
OK, i have now attached a 100k pot to -5 and ground with the wipe going to the negative probe on my multimeter, and it works. What i want to know now is why does it work. Why is the offset been set like that.

???, now you only measure the voltage with an offset....

You can look here (sorry this page is in german, i only know this one):
https://www.elektronik-kompendium.de/public/schaerer/opa1.htm

take a look at picture 5.3 (in german it is called BILD 5 and then the right circuit)

In this circuit there is an input voltage of 0.666V is transferd to 0V at the output!
(therefore you have to connect R1 to 1V)

The same you can calculate fpr your neg. Offset ...
 

Re: opamp offset

I just read my question back to myself, and it does not make sence, even to me :(

What i have built is the following circuit.

**broken link removed**

It work OK, but i can only read the output when i put my multimeter accross the two outputs. How can i now feed this output into a ADC port of a PIC to read it?? In other words, what do i do witht he negitive output coming from the 10k reference??

I have tried to use a second opamp, but without feedback, it just saturates and outputs 4.5v.

What do i need to do??


Thanks, Mark
 

Re: opamp offset

If you want to do it with one OP you have to build a circuit like this:
**broken link removed**
on page 4 the circuit in the right upper corner:"Offset Voltage Adjustment for non inverting OP...."
With this circuit you can select the gain & offset seperatly.

Or you can use two OPs.
With the first one you amplify & with the secend one you add the offset.
e.g. the first OP is used as an inverting OP and the secend OP as inverting adder.
(this all are common basic circuits)
 

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